IMAGING & THERAPEUTIC TECHNOLOGY
1769
The AAPM/RSNA Physics
Tutorial for Residents
Fluoroscopy: Optical Coupling and the
Video System1
Michael S. Van Lysel, PhD
In fluoroscopic/fluorographic systems, an image intensifier is optically
coupled to recording cameras. The optical distributor is responsible
for transmitting a focused image from the output phosphor of the image intensifier to the focal planes of the cameras. Each camera has an
aperture, which is used to control the level of light reaching its focal
plane. The aperture setting determines the patient x-ray exposure level
and the image noise level. Increasing the x-ray exposure reduces image
noise; reducing the x-ray exposure increases image noise. Fluoroscopic/
fluorographic systems always include a video camera. The functions of
the video system are to provide for multiple observers and to facilitate
image recording. The camera head contains an image sensor, which
converts the light image from the image intensifier into a voltage signal. The device used to generate the video signal is a pickup tube or a
charge-coupled device sensor. The method used is raster scanning, of
which there are two types: progressive and interlaced. The vertical
resolution of the system is primarily determined by the number of scan
lines; the horizontal resolution is primarily determined by the bandwidth. Frame rate reduction can be a powerful tool for exposure reduction.
Abbreviations: AEC = automatic exposure control, CCD = charge-coupled device, DSA = digital subtraction angiography, II = image intensifier, MTF = modulation transfer function
Index terms: Fluoroscopy • Physics • Video systems
RadioGraphics 2000; 20:1769–1786
1From the Departments of Medicine (Cardiovascular Medicine Section) and Medical Physics, University of Wisconsin, H6/333 Clinical Science
Center, 600 N Highland Ave, Madison, WI 53792. From the AAPM/RSNA Physics Tutorial at the 1999 RSNA scientific assembly. Received June
29, 2000; revision requested July 18 and received August 14; accepted August 16. Address correspondence to the author (e-mail: vanlysel@facstaff
.wisc.edu).
©RSNA,
2000
1770 November-December 2000
Introduction
The fluoroscopic/fluorographic imaging chain is
used in the performance of many diagnostic and
interventional radiographic procedures. In this system, an image intensifier (II) is coupled to film
cameras (cine, photospot) and a video camera.
Video applications have become increasingly important. Combined with a digital image processor,
video is replacing film in many applications.
The purpose of this article is to discuss two
components of this imaging chain: the optical
distributor and the video system. After reading
this article, the reader will be able to (a) describe
how the optical distributor forms an image for the
cameras to record; (b) describe how the camera
aperture sets the patient x-ray exposure level and
the image noise level; (c) describe the operation
of tube-based and charge-coupled device (CCD)–
based video cameras; (d) calculate the resolution
limitations imposed by the video camera; and
(e) explain the video terms lag, gamma, progressive
scanning, interlaced scanning, fields, frames, standardline, high-line, and upscanning.
Optical Coupling
Overview
The optical distributor is responsible for coupling
the light image on the output phosphor of the II
to the recording cameras. The cameras present on
a fluoroscopic/fluorographic system can include a
cine camera (35-mm motion picture film) or a
photospot camera (100-mm cut film or 105-mm
roll film). Systems always include a video camera.
Initially, video cameras were present merely to
assist in and monitor a procedure, which ultimately was recorded on one of the film cameras
(or on a spot film or film changer placed at the
entrance to the II). However, the quality and
flexibility of video images have improved dramatically over the past 2 decades. Of particular
note is the introduction of digital image processors, which rely on the video camera to provide
an input signal. The result is that film cameras
are becoming less common. This trend is especially true in cardiac imaging, in which most new
systems are cineless; the recording and archival
functions of the cine camera are replaced by the
video/digital system.
RG ■ Volume 20 • Number 6
The optical distributor is responsible for three
functions: (a) transmission of a focused image
from the II output phosphor to the focal planes
of all cameras present on the system; (b) ensuring
that the intensity (ie, brightness) of the images at
the camera focal planes is correct for each camera; and (c) sampling the light level of the II output phosphor and transmitting this information
to the automatic exposure control (AEC) circuitry
of the x-ray generator.
Optical Distributor Components
Let us look at the components of an optical distributor that has both a video camera and a cine
film camera (Fig 1). During cine operation, a
semisilvered, beam-splitting mirror simultaneously
provides images to both cameras. During coronary
angiography, for example, this mirror allows realtime monitoring of the injection with the video
camera while images are recorded on film. (It is
typical to also videotape the video signal as backup
against loss of the film during processing.) During
fluoroscopy, the mirror is removed from the beam
path so that all of the light passes to the video
camera.
The beam path from the II output to each camera contains two lenses. The collimating lens (also
referred to as the objective lens), which collects
light emitted from the II output phosphor, is common to all paths. Each camera has its own camera
lens. Each camera also has its own aperture, which
is used to control the light level reaching the focal
plane of the camera. Other names used for the aperture include f-stop, diaphragm, and iris.
Also included in the beam path is a small mirror or prism for directing light to the AEC sensor. The sensor can be either a photodiode or a
scintillating crystal coupled to a photomultiplier
tube. A lens is used to form an image, then a field
stop is used to prevent light from the periphery of
that image from reaching the sensor. It is not uncommon for the edge of the image to consist of
unattenuated radiation. If the AEC were to include this region in its determination of exposure
level, the central portion of the image, which is
presumably the area of interest, would be underexposed (ie, dark and noisy). If only the central
50% (for example) of the image is sent to the AEC
sensor, this problem is avoided.
Optical Considerations
The optical elements in the distributor are always
arranged in a particular manner. The distance between the II output phosphor and the collimating
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Figure 1. Components of an optical distributor servicing a video camera, a cine camera, and the AEC of
the generator. FL = focal length.
lens is equal to the focal length of the collimating
lens (ie, the phosphor is in the focal plane of the
collimating lens) (Fig 1). As a result, the light
leaving the lens is collimated (ie, the light rays
that were emitted from a single point on the phosphor are parallel after they leave the lens). The image is said to be “focused at infinity.” The video
camera target is placed in the focal plane of the
video camera lens, and the film in the cine camera is placed in the focal plane of the cine camera
lens (Fig 1). Because the light received by each
camera lens is collimated, the image formed by
the lens is at the focal plane. Cameras can go out
of focus over time as vibrations cause the distance
from the lens to the target or film to change. Periodic quality control measurements are necessary
to detect this problem.
A property of collimated light is that every region within the beam path contains light from all
points on the output phosphor (however, there are
limits to this statement that, if not heeded by the
optical designer, will lead to vignetting, or darkening of the image periphery). As a result, when objects such as an aperture or the small mirror for
the AEC sensor are placed in the beam path, these
objects do not show up in the image. The intensity
of the image is reduced due to the reduction in
light reaching the image, but this reduction is uniform across the entire image. This fact is why apertures are always placed between the two lenses.
In contrast, the field stop in the beam path for the
AEC sensor is placed after the second lens. At this
location, after the image has been formed, the
field stop serves to occlude light from the image
periphery; as described earlier in this article, such
occlusion of light is desirable for operation of this
device.
The most important practical matter to understand regarding the optical distributor is the function of the camera apertures. To understand that,
we need to discuss the concept of the speed of
the optical system. The term speed is used here in
the same manner that it is used for a screen in
screen-film radiography. For a fixed dose rate,
the faster the optical system, the less total x-ray
exposure is required to properly expose the camera. The f-number of the optical system (often
written f/#) designates the system’s speed. Amateur photographers will recognize that it is the
lens f/# that they vary to adjust for the brightness
of the scene they are photographing. The f/# of a
lens is defined as the ratio of its focal length to its
diameter (technically, the diameter of the entrance
pupil):
f /# =
focal length
.
diameter
(1)
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RG ■ Volume 20 • Number 6
Figure 2. Relationships governing the
speed of a lens. FL = focal length.
These two properties determine the brightness of
the image formed by the lens (Fig 2). The size of
the image is in direct proportion to the focal
length; longer focal lengths form larger images.
Because the lens collects a fixed amount of light,
larger images must necessarily be dimmer, since
the same number of light photons are spread over
a larger area. Doubling the focal length quadruples the area of the image, which is then only onefourth times as bright.
The tandem lens system forms an image at the
image receptor plane (ie, the video camera target
or the film plane). The size of the image is a function of the focal lengths of the collimating and
camera lenses. The system designer chooses the
focal lengths of the two lenses to match the image
size with the size of the receptor. Image size selection is not a straightforward issue because the image of the II output phosphor is round, but the
film frame and video scanning pattern are rectangular (or square). Thus, the image can be made
large, improving resolution but truncating the
edges, or it can be made small, allowing the entire
II image to be viewed but significantly limiting
resolution. Usually, a medium position is chosen.
This issue of framing, in the context of cine, is
discussed in detail in reference 1.
Although the focal lengths of the lenses are
fixed and thus can be ignored during everyday
Figure 3. Two different aperture styles: a fixed aperture (left) and a variable, motor-driven, “iris-style” aperture (right). Also included with the variable aperture
is a neutral density filter.
operation of the system, the aperture can be adjusted to change the speed of the system. Introduction of an aperture that is smaller than the
lens diameter reduces the brightness of the resulting image (Fig 2). The reduction in brightness is in direct proportion to the reduction in
the effective area of the lens (thus it is also in
proportion to the square of the diameter). Apertures allow the system to operate over a wide
range of x-ray exposure levels, as described later
in this article.
Two different aperture styles are used in fluoroscopic/fluorographic systems (Fig 3). Generally
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Figure 4. Sample calculation of
aperture f/# versus II entrance exposure (Exp). D = aperture diameter, TV = television camera.
speaking, clinical procedures that make use of a
film camera are performed at the same “detected”
x-ray exposure level for all patients (eg, 16 mR [4.1
´ 10-9 C/kg] per frame incident on the II for coronary angiography in the 17-cm mode); thus, a
fixed aperture is used for these cameras. When
the angiographic unit is installed, the service engineer determines the aperture size that will produce a properly exposed film at the desired x-ray
exposure level. Periodically, this aperture may
need to be replaced with one of a different size as
the II gain falls with age, if the II is replaced, or if
the film type or the desired x-ray exposure level
changes.
In contrast to film cameras, video cameras
must operate over a wide range of x-ray exposures: from a low of 2 mR (5.2 ´ 10-10 C/kg) per
frame for fluoroscopy to potentially 1,000 mR
(2.6 ´ 10-7 C/kg) per frame for digital subtraction
angiography (DSA). This large range in x-ray exposure is accompanied by an equivalent range in
II output brightness. This range is too great for
the video camera, so the video camera aperture is
typically configured as a motor-driven iris. The
iris is opened up for fluoroscopy, when the II
output phosphor is dim; it is “stopped down”
during photospot imaging or DSA, when the
phosphor is bright. At very high exposures, it
may be necessary to stop down the aperture to
such a small opening that it is difficult to precisely control. In this case, a neutral density filter
(Fig 3) can be swung into the opening to provide
additional light attenuation.
A sample calculation illustrates the relationship between f/# and the x-ray exposure incident
on the II (Fig 4): If the f/# of the video camera
lens is f/2 during fluoroscopy performed at 2 mR
per frame (Exp1), what is the proper f/# for the
lens during cine angiography performed at 16 mR
per frame (Exp2)? We answer this question by
noting that an eightfold increase in entrance exposure results in an eightfold increase in II output brightness. Therefore, to keep the light intensity reaching the video camera constant, the area
of the camera aperture must be decreased by eight
times. Working through the calculation (Fig 4),
we see that to decrease the light transmitted by
the aperture by eight times, we need to increase
the f/# (note the reciprocal relationship) by Ö8, to
f/5.6.
The reason we increase x-ray exposure, of
course, is to reduce the x-ray noise amplitude in
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1774 November-December 2000
a.
b.
Figure 5. Comparison of x-ray noise amplitudes in coronary angiograms acquired at fluoroscopic (2 mR per
frame) (a) and angiographic (16 mR per frame) (b) exposure levels.
Figure 6. Major components of a fluoroscopic/fluorographic video system. ADC =
analog-to-digital converter,
DAC = digital-to-analog converter, OD = optical distributor, VCR = videocassette recorder.
our images. The noise amplitude is inversely proportional to the square root of the II entrance exposure. Thus, in the example given in the preceding paragraph, an eightfold exposure increase
produces a 0.35 times noise amplitude. When
coronary angiograms are obtained at fluoroscopic
(2 mR per frame) and cine angiographic (16 mR per
frame) exposure levels, the higher level of noise in
the fluoroscopic image is apparent (Fig 5).
Video System
Overview
When IIs were introduced in the early 1950s, the
operator viewed the output phosphor “directly”
through a system of lenses and mirrors. Video
systems were soon introduced. The functions of
the video system are to provide for multiple observers and to facilitate image recording. In addition, the video system provides the input to the
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Van Lysel 1775
Figure 7. Operation of a
video camera pickup tube.
E = electric field, G4 = electrode for removing excess
electrons, Ibeam = focused electron beam, Isig = signal current, VOUT(t) = video signal
output, +VT = target voltage.
digital image processor, which is now present in
virtually all fluoroscopic systems. Digital imaging
has driven major improvements in video system
image quality to the point where digital video is
displacing both photospot and cine cameras in
most new installations.
The components of a closed-circuit video system used for fluoroscopy/fluorography are as follows (Fig 6): The camera head is attached to the
optical distributor, allowing it to view the II output phosphor. The camera head contains an image sensor that converts the light image from the
II into a voltage signal that can be transmitted to
other components of the video system, where it
can be modified, displayed, and recorded. The
camera control unit, which is usually located in
one of the generator’s electronics racks, processes
the video signal from the camera head so that it is
suitable for display and digitization. The camera
control unit also synchronizes the scanning of all
of the components of the system by generating
and transmitting horizontal (H sync) and vertical
(V sync) synchronization signals.
Signal Generation
Two distinctly different devices are now in use
for generation of the video signal voltage from
the incident light image. The traditional device is
a vacuum tube sensor referred to as a pickup
tube. A pickup tube operates as follows (Fig 7):
Light photons absorbed by the photoconductive
target generate electron-hole pairs in the target
material. The holes travel to the inside surface of
the target under the influence of an electric field
(E), which is placed across the target by the target voltage (+VT). These holes collect on the inside target surface, where they represent a latent
image. Hole density is in proportion to the intensity of the light image falling on the target at
that location. To read the charge-density spatial
distribution, a focused electron beam (Ibeam) is
scanned across the inside surface of the target under control of the deflection coils. Electrons from
the beam land on the target and combine with the
holes. The number of electrons that land on the
target is equal to the number of holes at that location. Excess electrons are removed by an electrode (G4), which is located just above the target
surface. Electrons landing on the target cause a
proportional current to flow in an external load
resistor (Isig). The resulting voltage drop across
this resistor constitutes the video signal output
from the pickup tube (VOUT[t]).
The other technology now used to generate a
video signal replaces the pickup tube with a CCD
sensor. A CCD is a solid-state device. As with
the target of a pickup tube, absorption of light
photons in the CCD semiconductor material
generates electron-hole pairs. In this case, the
electrons are held at the location where they are
generated by a two-dimensional array of potential
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Figure 8. Readout operation of a CCD video camera.
VOUT(t) = video signal output.
Figure 9. Video camera lag. Because of lag, the video camera output
(VOUT[t]) does not respond instantaneously to an increase or decrease in light
level.
wells set up in the material. Electrons collect in
the potential wells in proportion to the number
of light photons incident at that location. The
frame-transfer method (Fig 8) is a common
method used to read out the CCD chip. (Although the example shows pulsed x-ray generation, continuous x rays can also be used.) After
termination of the x-ray pulse, the charge in the
wells is rapidly transferred to an adjacent storage
region. The storage region consists of a second
array of potential wells similar to the sensor region. However, the storage region is shielded
from the incident light falling on the sensor region. Charge is transferred in a method that has
been referred to as a “bucket brigade.” Each well
Figure 10. Effect of camera lag.
Angiogram of a rapidly moving
coronary artery shows a trailing
“ghost” due to excessive camera
lag (the direction of travel is from
right to left).
transfers charge to an adjacent well. This charge
transfer operation continues until all of the charge
resides in the storage array. The sensor region is
now ready to accept the next image, whereas readout of the storage region can extend over the entire frame period.
CCD cameras have been available for generalpurpose fluoroscopy systems for several years. In
general, they have not been of sufficient quality
to allow their use in high-resolution angiography
suites, but recently they have begun to appear in
these systems as well. The advantages of CCDs
over pickup tubes include small size, low power
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Van Lysel 1777
Figure 11. Signal transfer curve. Graph
shows video signal as a function of light input to the camera for gamma values of 0.7
and 1 (linear response). For a gamma of
less than 1, small signals in the dark parts
of the image are amplified more than
equivalently sized signals in the bright
parts of the image.
consumption, ruggedness, no geometric distortions, no lag or burn-in phenomenon, long lifetime, and no setup or maintenance tuning. Pickup tube performance degrades over time. For example, the expected lifetime of an $8,000 Plumbicon pickup tube in an angiography suite is 18
months. Proper pickup tube operation requires
careful setup by an experienced service technician.
Two operating characteristics of a video system
that affect image quality and that users should understand are lag and linearity. Video camera lag is
described as follows (Fig 9): When the light level
falling on any region of the video sensor changes
(either increasing or decreasing), it is desirable for
the video signal out of the sensor to change instantaneously to its new value. The term lag refers to
the condition in which the video signal output
rises or falls to the new value more slowly than the
changing light input. It is sometimes said that the
camera is “sticky.” The magnitude of lag varies
greatly between different types of video cameras,
depending on the details of target construction
and materials. CCD cameras have no lag. At the
other extreme, a photoconductive pickup tube
called a vidicon has a very high degree of lag. The
vidicon uses SbS3 as the target material. In the
middle are several pickup tubes (eg, Plumbicon,
Saticon, Chalnicon, Newvicon) that have different
target configurations but all deliver about the
same amount of lag. These “other” tubes are referred to as “low-lag” cameras but actually are low
lag only when compared with the vidicon. In fact,
the amount of lag for these cameras is appreciable.
Lag is signal dependent; it is much higher in the
dark parts of the image than in the bright parts.
Lag increases as a pickup tube ages. Tube replacement is often prompted because the tube
has become excessively “laggy.”
One clinically relevant effect of camera lag is
blurring of objects in motion, since a moving ob-
ject leaves behind a trail of decaying signal (Fig
10). As a result, the vidicon pickup tube cannot
be used in angiography suites, where motion
blurring would seriously degrade the spatial resolution of moving vessels. On the beneficial side,
lag provides a degree of noise integration, reducing x-ray noise amplitudes in the resulting images. The increase in noise levels on a generalpurpose fluoroscopy system when a vidicon is replaced by a CCD camera is profound. Artificial
lag can be provided in these cases by a recursive
filter implemented in the digital image processor.
The second video characteristic that should be
understood by users is the video signal transfer
curve. Transfer curves describe the relationship
between light input and video signal output. Because transfer curves can be described by the following expression:
V
Vo
æ ö
= ççç L ÷÷÷÷
èL ø
g
,
(2)
o
where V is signal voltage and L is input light intensity, curves are usually characterized by their
gamma value (2). The vidicon has a gamma of
about 0.7, whereas all of the other pickup tubes
and CCD cameras have a gamma of unity ( g = 1
indicates a linear response). However, because a
gamma of less than 1 has desirable properties,
many systems with linear cameras impose a nonlinearity on the video signal. Imposition of nonlinearity can be done by either the camera control
unit or the digital image processor. Small signals
in the dark parts of an image are amplified by a
transfer curve with a gamma of less than 1, allowing better visualization of these difficult-to-see
signals (Fig 11).
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1778 November-December 2000
Figure 12. Horizontal line in raster
scanning. Each horizontal scan line is
commenced and terminated by a horizontal synchronization pulse (H sync)
generated by the camera control unit.
TH = horizontal line period.
Video Scanning
The light image falling onto the video camera
target is two-dimensional. The camera transforms this two-dimensional image into a one-dimensional time-varying voltage by raster scanning. The pickup tube scans the target with an
electron beam. The CCD camera scans by shifting the accumulated charge in shift-register fashion out of the sensor region. The fundamental
unit of raster scanning is the horizontal line (Fig
12). Each line is terminated (and each new line is
commenced) by a horizontal synchronization (H
sync) pulse generated by the camera control unit.
Video images are scanned in the same manner
in which we read a book. A line is slowly read
from left to right. When we reach the end of the
line, we rapidly shift our eyes back to the far left,
and a little bit down, to commence reading the
next line. When we’ve read the last line on the
page, we rapidly shift our gaze to the upper left
corner of the next page. In a video system, the H
sync pulse instructs the defection circuitry in both
the pickup tube and the monitor to rapidly retrace
to the beginning of the next line. In a pickup tube,
the electron beam is turned off (the beam is said
to be “blanked”) while this retrace occurs, so that
the yet-to-be-read charge on the target is not disturbed. Retrace results in dead time each horizontal line during which no image information is
transmitted. This dead time, which amounts to
about 15% of the total horizontal period, will be
important when we calculate video bandwidths
later in this article. Similarly, the camera control
unit transmits a vertical synchronization (V sync)
pulse to signal all of the components in the system that it is time to perform vertical retrace back
up to the top of the image. Vertical retrace results
in dead time equal to about 7% of the total vertical period.
The book-reading analogy suffers a bit when we
consider vertical scanning in more detail. There
are two ways in which vertical scanning can be
performed. Progressive scanning (also known as
sequential scanning) does conform to the analogy.
In progressive scanning, lines are scanned in consecutive order. Line 2 is scanned after line 1. Line
3 is scanned after line 2. When we get to the bottom of the image, we will have scanned all of the
lines. Interlaced scanning is a bit more complicated. After scanning line 1 we scan line 3, skipping line 2. After scanning line 3 we scan line 5,
skipping line 4. We scan only the odd lines,
reaching the bottom of the image in one-half the
time required for progressive scanning. However,
having read only half of the lines, we must go back
to the top of the image and read the even lines in
succession (lines 2, 4, 6, etc). The odd and even
fields are interlaced to produce one frame. In this
discussion, it is important to carefully note the
difference between fields and frames.
Interlaced scanning was devised in the early
days of broadcast television as a means to reduce
the required bandwidth of video transmissions,
allowing more channels to be broadcast within a
market. Successful video transmission relies on
the fact that the eye-brain system of the observer
perceives the scanned image as continuous. In reality, the monitor phosphor begins to decay in
brightness as soon as the electron beam moves on
(see the discussion of monitor operation later in
this article). After a few hundred microseconds,
the phosphor is dark again. If your visual system
were fast enough, you would see a bright bar, representing the image, sweeping down the face of a
predominantly dark monitor. The monitor refresh rate is set so that your visual system blends
the scanning bright bar into a continuous, bright
image. The “flicker-fusion frequency” at which
this blending occurs is a variable that depends on
image brightness but generally requires a refresh
rate of at least 50 images per second.
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Van Lysel 1779
Figure 13. Scan conversion. Progressive-to-interlaced scan conversion writes a progressively scanned
image into one memory while reading the previous frame, stored in a
second memory, in interlaced fashion for display. ADC = analog-todigital converter, TV = television
camera.
The standard scanning mode employed by
conventional broadcast video is described as 30
frames per second, 525 lines per frame, 2:1 interlaced video. The 2:1 interlace process was described earlier; two fields are interlaced to produce one frame. The 525 lines per frame means
that there are 525 horizontal lines making up one
frame (one “image”). Thirty of these 525-line
images are transmitted every second. We know
that 30 frames per second is not fast enough to
avoid a flickering image. However, when viewed
from a sufficient distance, the individual fields,
presented at 60 fields per second, blend together
enough (because of the limited resolution of the
human eye) to greatly attenuate the perception of
flicker.
For many years, the video standard used in
fluoroscopic/fluorographic systems was the same
as that used in conventional broadcast television,
30 frames per second, 525 lines per frame, 2:1
interlaced. This system is still used today for inexpensive systems. Substantial cost savings are
enjoyed by using off-the-shelf video components
(eg, cameras, monitors, tape recorders). However, as the use of video has shifted from merely
monitoring a procedure to being the primary diagnostic medium, and also the means to perform
sophisticated interventions, the quality of standard video is no longer sufficient. One improvement is greater use of progressive scanning. Although interfield flicker is usually not apparent
under casual observation of an interlaced display,
close inspection of the image (as is likely to occur
during medical procedures) does reveal an annoying flicker, especially at a horizontally oriented
boundary between bright and dark regions. This
flicker is due to the fact that the 30 frames per
second display rate is, in fact, below the flickerfusion frequency. The best systems now employ
progressive scanning monitors with display rates
of 60 frames per second or above (as opposed to
a 60 fields per second, 30 frames per second, interlaced scanning monitor).
More important than display considerations,
interlaced scanning by the video camera during
acquisition can cause significant image artifacts.
The nature of the artifact depends on the manner
of x-ray production. The most common artifacts
are distortions of moving objects, such as coronary arteries and guide wires. For both continuous fluoroscopy and pulsed fluoroscopy/fluorography at a rate of 60 pulses per second, the moving object is in two different locations for the
even and the odd fields. The resulting image of
the artery in the interlaced frame will have a serrated appearance (3). For pulsed operation, reduction of the x-ray pulse rate to 30 pulses per
second eliminates the serration artifact, but a
more severe artifact is produced. In this case, the
intensity of the two fields is very different, resulting in severe flicker. Cardiac angiographers who
were practicing before the advent of digital imaging are familiar with the flashing that occurred on
the video monitor during 30 frames per second
cine-film imaging. This flashing occurs with a
pickup tube system because, while the scanning
electron beam is reading the odd field, it inevitably reads a great deal of charge that properly belongs to the even field, depressing the intensity of
the even field when it is read next if a new x-ray
pulse does not replenish the lost charge.
A very common method used in modern fluoroscopic video systems to provide both the superior performance of progressive scanning of the
video camera during acquisition and the economy of an interlaced display is scan conversion
(Fig 13). Scan conversion requires a digital image processor to buffer the data received from
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Figure 14. MTFs of the video
system, the II and x-ray tube focal
spot (“everything else”), and the total system for two clinically relevant
examples. Assumed II mode, focal
spot (fs) size, and number of video
lines are indicated. In the peripheral
DSA case, the focal spot size (0.3
mm) is appropriate for fluoroscopy
during the DSA procedure; the angiographic images acquired during
this procedure would be performed
with a larger focal spot.
the camera until it is time for display. In an analog
system without a buffer, the signal read from the
camera is transmitted immediately to the display
monitor. Without a short-term storage method,
both the camera and the monitor must scan in
lockstep. Therefore, if the monitor is a conventional interlaced scanning model, the camera target must be read in interlaced fashion. However,
with a digital buffer, acquisition and display are
decoupled. The camera reads progressively, and
the data are stored in a digital memory. Once one
frame has been read, the camera begins to fill a
second memory, freeing the first memory to be
read out in interlaced fashion for display.
Spatial Resolution
In this section, we will calculate the spatial resolution limitations placed on the radiographic image by the video system. However, it is important
to understand that there are many factors that
can contribute to the final resolution in a fluoroscopic image. These include the video system,
the digital system, the II, the x-ray tube focal
spot, and patient motion. Under some circumstances, the video system is the major limiting
component to resolution. Under other circumstances, the video system may impose no significant limitations because one or more of the other
components are limiting. These two cases can be
demonstrated by plotting the modulation transfer
functions (MTFs) of a video system, the other
components (eg, the II and x-ray tube focal
spot), and the total system (Fig 14). The total
MTF is the product of the video and “everything
else” MTFs:
MTFtotal (u) = MTFvideo (u) ´ MTFeverything else (u) , (3)
where u is the spatial frequency in cycles per millimeter. In the coronary angiography example,
the MTF of the high-line video system is significantly better than the MTF of the II and 1-mm
focal spot. The result is that the additional resolution loss imposed on the total imaging system by
the video component is small. In contrast, in the
peripheral DSA example, MTFvideo and MTFtotal
are virtually identical, indicating that the resolution limitation of this system is dominated by the
video system. In such a case (specifically, when a
large-II format is used), a standard 525-line video
system is insufficient to provide adequate resolution. The reader should understand this point after the following discussion.
The spatial resolution of the video system is
determined by separate considerations in the vertical and horizontal directions (Fig 15). Vertical
resolution is primarily determined by the number
of horizontal lines the system uses. Horizontal
resolution is primarily determined by the bandwidth of the video system. Unlike IIs and screenfilm systems, which tend to have rotationally symmetric resolution properties, the resolution of the
video system can potentially be very different in
the vertical and horizontal directions. However,
the video system designer usually chooses to
make the resolution in the two directions approximately equal.
Vertical Resolution.—Let us perform a simple
calculation of the limiting vertical resolution of a
conventional 525-line video system, which we
might measure using a line-pair gauge. First, we
must understand a peculiarity of video terminology. There are not 525 visible lines in a 525-line
system. Remember that approximately 7% of the
vertical period is spent retracing from the bottom
of the image back up to the top. As a result, there
are 525(1 - 0.07) . 490 active scan lines available
to transmit image data. The remaining 35 lines
RG ■ Volume 20 • Number 6
a.
b.
Figure 15. Primary determinants of video spatial resolution: number of scan lines (a) and
video system bandwidth (b). Vertical resolution
is the resolution you would measure when a linepair gauge is positioned as in a. Horizontal resolution is the resolution you would measure when
a line-pair gauge is positioned as in b.
Van Lysel 1781
Figure 16. Degradation of
vertical resolution by vertical
sampling.
Figure 17. Calculation of the
video resolution referenced to
the object plane. mag = magnification, res = resolution,
SID = source-to-image distance, SOD = source-to-object
distance, TV = television camera.
(or, more accurately, the remaining time that
would have been spent scanning these 35 lines)
are spent in vertical retrace. With 490 lines, we
can represent 490/2 = 245 line pairs. We can
therefore say that our vertical resolution is 245
line pairs per scan height.
Actually, we should say that the maximum
achievable vertical resolution is 245 line pairs per
scan height. Several factors conspire to reduce
the typical vertical resolution from that which we
just calculated. For one, the electron-beam spot in
the pickup tube is not pillbox-shaped (with sharp
edges) but tends to be more Gaussian, resulting
in overlap between the lines (especially during interlaced scanning). More significantly, how the
line-pair gauge lines up with the raster scan (ie,
its phase) can significantly affect the outcome of
our observation (Fig 16). The calculation performed earlier assumes that the spaces and bars of
the line-pair gauge line up exactly with the scan
lines of the video system. If the line-pair gauge is
shifted by half the width of a line (90° phase shift),
contrast (modulation) in the resulting image disappears, since the signal read by each video line
represents half a bar and half a space. In the interest of quoting a single specification for limiting
resolution, it is common to reduce the maximum
achievable vertical resolution by an amount referred to as the Kell factor, after R. D. Kell, who
researched this issue in the 1930s. A typically
quoted value for the Kell factor is 0.7, resulting in
a practical vertical resolution for our 525-line system of 245(0.7) . 170 line pairs per scan height.
Line pairs per scan height is a bit removed
from what we really want to know: What is our
resolving power (eg, in line pairs per millimeter)
in the object plane (ie, the patient)? This conversion is performed by referencing resolution to the
object plane (Fig 17). Line pairs per scan height
specifies our resolution for objects located on the
RG ■ Volume 20 • Number 6
1782 November-December 2000
Figure 18. Determination of bandwidth
by proper selection of the video system
components. The horizontal resolution of
a properly operating video system is set by
the system bandwidth. The frequency-response characteristics of the video electronics are represented here by a low-pass
filter. The consequences of bandwidth that
is too low or too high are shown as well.
CCU = camera control unit, TV = television camera, VOUT(t) = video signal output.
video camera target. We can translate this value
into line pairs per millimeter at the face of the II
by simply dividing by the diameter of the relevant
II mode. We then translate that value into line
pairs per millimeter at the object plane by multiplying by the relevant geometric magnification.
The geometric magnification, which is a consequence of the fact that the x-ray field is diverging,
is calculated as follows:
mag =
SID
,
SOD
(4)
where SID is the distance from the source to the
II face (source-to-image distance) and SOD is
the distance from the source to the object plane
(source-to-object distance).
As an example, calculate the object-plane resolution of a 525-line video system when the 23-cm
II mode is used. Assume that the geometric magnification is 1.2 and the Kell factor is 0.7:
res =
490 æç line pairs ö÷
÷ ´ 0.7 ´ 1 ´1.2
ç
2 çè scan height ø÷
230 mm
= 0.9 ççèæç lp ÷÷÷öø
mm
.
(5)
The Table presents video resolution as a function of II mode. Resolution is seen to decrease
with increasing II size, since a fixed number of
video lines are stretched across an increasing field
of view. What is striking about this result is how
poor the video resolution is. Compare a video
resolution of approximately 1 lp/mm with the ap-
Vertical Object-Plane Limiting Resolution of
Video Systems as a Function of II Mode
Limiting Resolution (lp/mm)
II Diameter
(cm)
525-Line*
System
1,049-Line†
System
36
23
17
13
0.6
0.9
1.2
1.6
1.1
1.8
2.4
3.2
Note.—Assumptions include a geometric magnification of 1.2 and a Kell factor of 0.7.
*490 active lines.
†
980 active lines.
proximately 5 lp/mm limiting resolution of the II.
Film resolution (either cine or photospot film) is
so high that the imaging chain resolution for these
modalities is limited to that provided by the II.
As the role of the video system expanded in
the 1980s and 1990s, it became clear that video
resolution had to be improved. The result was
the adoption of high-line video for angiography
and interventional systems. Like the current
plans for high-definition television for broadcast
video, high-line video systems typically double
the number of lines (eg, from 525 to 1,049 lines),
along with a concurrent increase in video bandwidth (as discussed later in this article). Doubling the number of video lines doubles the vertical resolution of the video system. As seen in the
Table, the combination of a high-line system and
a small II mode produces a respectable resolution
(eg, 3 lp/mm). However, the application in which
high-line systems are crucially important is large
II formats.
RG ■ Volume 20 • Number 6
Horizontal Resolution.—Resolution in the horizontal direction is limited by the bandwidth of the
video system. The concept of video bandwidth
can be explained by drawing an analogy with the
stereo systems that many people have in their
homes. When designing the electronics for an audio system, the designer makes use of the fact
that the human ear is capable of detecting audio
frequencies of up to 20,000 cycles per second (20
kHz). The designer therefore makes sure that the
electronics are capable of transmitting voltage
waveforms of up to 20 kHz (ie, that the bandwidth is 20 kHz). If the designer does not do so,
the listener will not hear the high-frequency tones
in the music because they were not passed from
the source to the speakers by the electronic components (such as the amplifier). Conversely, any
money or effort spent to pass frequencies above
20 kHz is wasted because the listener cannot hear
them.
This analogy can be extended to video systems
by imagining a low-pass filter at the end of the
amplifier chain that transmits the signal from the
camera to the monitor (Fig 18). A low-pass filter
passes all frequencies up to a cutoff frequency,
attenuating frequencies above the cutoff frequency. We can equate the video system bandwidth with this cutoff frequency. It is the task of
the system designer to make sure that this filter
has the appropriate frequency-transfer characteristics. The consequences of improper bandwidth
selection are significant (Fig 18). Say we desire to
transmit the image of a line-pair gauge. If the
bandwidth is set too low, a rounded-off, blurry
representation of the gauge will be passed to the
video monitor. We will have degraded the horizontal resolution of the video system. However,
we also want to make sure that we do not set the
bandwidth too high because there is a great deal
of electronic noise, generated by the video system, at high frequencies. Passing these frequencies through to the monitor will result in an excessively noisy image.
The appropriate bandwidth is the one that
yields a horizontal resolution equal to the vertical
resolution. Since the vertical resolution is dictated by the number of video lines, it is straightforward to calculate this bandwidth. Step 1 is to
determine how many cycles we require along a
horizontal line. What we mean by “cycles” are sinusoidal variations from bright to dark (or from
Van Lysel 1783
high voltage to low voltage). For our purposes,
we can equate cycles with line pairs, even though
the equivalence is not exact. Say we wish to calculate a bandwidth for the standard 525-line video
system. We know from the earlier discussion that
the maximum vertical resolution is 245 line pairs
per scan height. Therefore, to achieve equal horizontal and vertical resolution, we require that the
video system pass up to 245 cycles per scan line.
Step 2 is to determine how much time is spent
scanning one line. In our example, we have a 525
lines per frame system that scans 30 frames per
second. Therefore, the period of a single line is as
follows:
TH
=
1
æ lines ö÷ æç frames ö÷
çè frame ø÷´
÷ 30 ççè sec ÷÷ø
525 çç
= 63.5 m sec
.
(6)
However, we learned earlier that 15% of this period is spent in horizontal retrace, during which
no information is transmitted by the system.
Therefore, we have only 63.5(1 - 0.15) = 54 msec
to transmit our 245 cycles.
Step 3 in our bandwidth calculation is to divide the required number of cycles per scan line
(245 cycles) by the time we have to transmit them
(54 msec):
245 cycles
= 4.5 ´106 cycles = 4.5 MHz . (7)
sec
54 ´ 10-6 sec
The only other video scanning mode typically
encountered in a fluoroscopy system is the highline mode discussed earlier. The required bandwidth for a 1,049 lines per frame, 30 frames per
second scanning mode is calculated as follows:
BW =
æ lines ö÷
980 æç cycles ö÷
÷
1, 049 ççç
´
÷
ç
÷
ç
2 è line ø
è frame ø÷
´ 30 æçççè frames ÷÷÷öø ´
sec
» 18 MH z
.
1
(1
- 0.15)
(8)
RG ■ Volume 20 • Number 6
1784 November-December 2000
In this calculation, it is assumed that there are 980
active scan lines (69 are lost to vertical retrace)
and that the active horizontal period is reduced
by 15% due to horizontal retrace. The reader will
note that the high-line bandwidth is four times
that of the standard-line mode. This increase is a
result of two facts: (a) We require twice as many
cycles per scan line to increase the horizontal
resolution by the same amount as the improved
vertical resolution (the equal vertical and horizontal resolution criterion) and (b) the horizontal
period is half as long (31.75 msec vs 63.5 msec)
because we need to scan twice as many lines in
the same 1/30th of a second frame period.
What if the bandwidth of a particular system is
less than that required? We are now in a position
to calculate the resulting resolution. Let us assume
that the video system in question has a bandwidth
of only 3 MHz. If we are imaging in the 23-cm II
mode, the limiting resolution at the face of the II
is calculated as follows:
æ cycles ÷ö ´54 ´10èç sec ÷÷ø
= 0.7 æçççè lp ÷÷÷öø .
mm
res = 3 ´ 106 çç
6
çççæ sec ÷÷÷ö ´ 1 çççæ lines ÷÷÷ö
è line ø 230 è mm ø
(9)
If the geometric magnification was 1.2, then
the object plane resolution would be 0.7 ´ 1.2 =
0.85 lp/mm.
Frame Rate.—Inspection of the relationships
in the previous section indicates a direct proportionality between frame rate and bandwidth. Doubling the frame rate doubles the required bandwidth because it halves the horizontal line period.
However, a point that is sometimes misunderstood is that lower x-ray pulse rates do not typically reduce the required video bandwidth. It is
becoming increasingly common to perform both
fluoroscopy and fluorography at reduced pulse
rates (eg, 15 or 7.5 pulses per second) to reduce
x-ray dose (as discussed later in this article). It is
common to refer to these rates as 15 or 7.5
“frames” per second. However, the implication
that the video system is operating at a reduced
frame rate is usually incorrect. For example, both
coronary angiography and pulsed fluoroscopy are
now often performed at “15 frames per second.”
Figure 19. Relationship between x-ray pulses and
video camera scanning for reduced frame rate coronary
angiography (also applicable to reduced frame rate
pulsed fluoroscopy) and for slow frame rate (eg, 1
frame per second) DSA. The “high” state indicates “x
rays on,” and the “low” state indicates “video signal
blanked.” fps = frames per second.
The typical manner in which this rate is achieved
is to produce x-ray pulses at 15 pulses per second
but continue to scan the video camera at 30 frames
per second. The first camera scan after the x-ray
pulse contains the video information from that
pulse (Fig 19 [middle waveform]). This frame is
both sent to the video monitor for display and
stored in a digital frame buffer. The second time
the camera target is scanned, there is no new information because there was not an x-ray pulse.
Therefore, the first frame, stored in the frame
buffer, is displayed again. This process retains
the necessary 30 frames per second display rate
(to avoid display flicker). At the beginning of the
third frame, a new x-ray pulse provides new video
information.
When the x-ray pulse rate is less than 30 pulses
per second, a less often used alternative is to use
the additional time to reduce the frame rate at
which the camera is scanned. The advantages of
this slow-scan mode include a lower bandwidth
(and thus less video noise) and reduced demand
on the analog-to-digital converter of the digital
image processor.
When the x-ray pulse width becomes greater
than about 10 msec, other methods must be used
to scan the camera. This situation occurs in low
frame rate (eg, 1 frame per second) DSA and
digital photospot imaging. In this case, the detected x-ray exposure can run from 100 mR to
1,000 mR per frame (at the face of the II) to reduce the x-ray noise amplitude in the resulting
RG ■ Volume 20 • Number 6
Van Lysel 1785
Figure 20. Operation of a video monitor.
image. To achieve this exposure level, x-ray pulse
widths on the order of 0.1 second or greater must
be used. The x-ray pulse then extends over several video frame periods. Two different methods
can be employed to scan this signal. One method
is to operate the video camera continuously in
the normal 30 frames per second scanning mode.
Video frames that span the x-ray pulse period are
integrated (summed) by the digital image processor to produce one image. This image is stored in
the image processor and displayed repeatedly until
the next x-ray pulse provides a new image. The
other method is as follows (Fig 19): For a pickup
tube–based camera, the scanning electron beam
is blanked, just as it is during retrace, during the
x-ray pulse. After termination of the x-ray pulse,
the beam is turned back on (synchronous with V
sync) to read the image. CCD camera operation
is similar; the shift operations are suspended during the x-ray pulse.
As mentioned earlier, it is becoming increasingly common to encounter systems that offer
“frame rate reduction” as a means to decrease
patient and staff x-ray exposure. Such systems require that the x-ray source be pulsed. Cardiac angiography (ie, cine) has always been pulsed, but
in the past most fluoroscopy was performed with
“continuous” x rays. Frame rate reduction also
requires that scanning of the video camera be progressive rather than interlaced. However, it is not
true, as is often claimed, that pulsed-progressive
fluoroscopy, in and of itself, results in exposure
reduction. Rather, pulsed-progressive fluoroscopy enables frame rate reduction, which results
in exposure reduction.
Exposure reduction is not as straightforward
as it might at first appear. If the frame rate is
dropped from 30 to 15 frames per second, the
x-ray exposure drops by a factor of two only if the
milliampere-seconds per pulse remains constant.
However, the vendor might raise the milliampere-seconds per pulse to “compensate” for the
reduction in frame rate, reducing or negating the
exposure reduction. If it is assumed that the milliampere-seconds per pulse remains fixed, it is important to understand that, although the exposure does drop by two times, image quality also
drops. This effect occurs because, when viewing
a rapid succession of images, the eye integrates
the noise content of all images presented within
a period of approximately 0.2 seconds. If fewer
images are presented within this “eye integration
period,” the observer perceives an increase in
noise. Research shows that if image quality is
held constant by increasing the milliampere-seconds per pulse as the frame rate is reduced, frame
rate reduction results in only modest exposure reduction (4). However, as a practical matter, frame
rate reduction can be a powerful tool for exposure
reduction. In the past, changing the fluoroscopic
exposure rate was an involved process requiring a
service engineer. However, with pulsed-progressive fluoroscopy, the physician can be given tableside control of the fluoroscopic frame rate. If it is
assumed that the vendor maintains a constant milliampere-seconds per pulse, the physician now has
the ability to decrease or increase the exposure
level in response to the demands of the procedure.
Image Display
The image that is read from the video camera
target is displayed on a video monitor (Fig 20).
The similarities in operation between the monitor and the camera pickup tube are conspicuous.
RG ■ Volume 20 • Number 6
1786 November-December 2000
In a monitor, an electron beam is produced by
thermionic emission from a hot cathode. This
beam is accelerated toward the face of the monitor by a high-potential anode. When the electron
beam strikes the fluorescent screen on the monitor face, light is emitted. Deflection coils cause
the electron beam to be scanned across the monitor screen in the required manner (usually in a
2:1 interlaced fashion). The magnitude of the
electron beam current is determined by the voltage applied to the control grid, which in turn is
determined by the video signal originating from
the camera. If the point at which the electron
beam is currently striking the monitor face is to
be dark, the control grid reduces the electron
beam current. If the image is to be bright, more
electrons are allowed to pass through the control
grid to reach the phosphor. During horizontal
and vertical retrace, the control grid blanks the
electron beam.
In the absence of a digital image processor, the
scanning electron beams in the pickup tube and
the monitor are precisely synchronized (by V sync
and H sync). The presence of an image processor
allows decoupling of camera and monitor scanning. Earlier in this article, scan conversion to allow progressive acquisition and interlaced display
was discussed. A second common scan conversion operation is upscanning. The most typical
upscanning method is to convert a 525-line
frame acquired by the camera into a 1,049-line
frame for display. The digital processor interpolates the acquired data to double the number of
lines. Although this procedure does not increase
the spatial resolution of the video system (that
would require 1,049-line acquisition), it does
provide an improved display (5). Close inspection of a 525-line display shows dark gaps between adjacent lines. Upscanning fills in the gaps
between the lines, producing a more continuous
and higher-contrast display. An alternative method to achieve the same result, defocusing of the
scanning electron beam of the monitor, results in
an unacceptable loss of resolution in the horizontal direction.
Conclusions
Although it is unnecessary for the operators of
fluoroscopy equipment to be familiar with all of
the technical details pertaining to the imaging system, there are some principles that should be understood. For example, the x-ray exposure level
between particular systems can vary substantially,
primarily due to choices made by the installer or
vendor but also due to maintenance issues. Routine quality assurance testing addresses the latter
issue. The former issue is best addressed by active participation by the user in the installation
process. Responsible use of x-ray equipment requires the physician to be aware of the x-ray exposure level employed on his or her machine. If
this level is higher than average, questions should
be asked. To ensure that the answers received are
valid, it is important to understand how the optical distributor works.
Proper performance of the video system is
critically important to good image quality. Ensuring optimum performance starts with the equipment purchase; here again, a sound understanding of system operation will allow intelligent
choices. Once the system is in operation, parameters such as edge enhancement, lag, and gamma,
which can often be adjusted through the digital
system, can have significant effects on the perception of image detail. Finally, modern systems
often afford the operator the option of reducing
“frame rate” to reduce radiation exposure. It is
beneficial to understand how such reduction is
accomplished.
References
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2. Sandrik JM. The video camera in medical imaging.
In: Fullerton GD, Hendee WR, Lasher JC, Properzio WS, Riederer SJ, eds. Electronic imaging in
medicine. Medical Physics Monograph no. 11. College Park, Md: American Institute of Physics, 1984;
145–183.
3. Seibert RA, Barr DH, Borger DJ, Hines HH, Bogren HG. Interlaced versus progressive readout of
television cameras for digital radiographic acquisitions. Med Phys 1984; 11:703–707.
4. Xue P, Wilson DL. Pulsed fluoroscopy detectability
from interspersed adaptive forced-choice measurements. Med Phys 1996; 23:1833–1843.
5. Holmes DR Jr, Wondrow MA, Reeder GS, et al.
Optimal display of the coronary arterial tree with an
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