ELEC ENG 3BA3:
Structure of
Biological Materials
Notes for Lecture #16
Wednesday, November 7, 2012
6. FOUNDATIONS FOR MEDICAL IMAGING
We will look at the:
– basic concepts of medical imaging, and
– common modalities of imaging:
 X-ray imaging (radiography) (e.g.,
screen-film & digital radiography, CT)
 nuclear medicine imaging (e.g., PET,
SPECT)
 magnetic resonance imaging (MRI)
 ultrasound
2
The first radiographic image by Roentgen:
3
(Bushberg et al., 2001)
6.1 Introduction to medical imaging
Medical imaging involves the measurement
of energy emanating from the body. The
two main forms of energy utilized in medical
imaging are:
 photonic energy (electromagnetic waves)
— x-ray, nuclear medicine imaging, MRI
 mechanical energy (sound pressure waves)
— ultrasound
4
The main modern imaging modalities:
5
Images are formed based on either:
 transmission properties of different
tissues in the body ⇒
transmission imaging
or
 emission properties of different tissues
in the body or of different physiological
activity in the body ⇒
emission imaging
6
Transmission vs emission imaging:
7
Planar images:
 projections onto a 2D plane
Tomographic images:
 3D projections in a slice
(tomos is Greek for “slice”)
8
Tomographic imaging:
(Bushberg et al., 2001)
9
Planar transmission imaging—
Screen-film radiography:
10
(Bushberg et al., 2001)
Planar emission imaging—
Nuclear medicine planar imaging:
11
(Bushberg et al., 2001)
Tomographic transmission imaging—
X-ray computed tomography (CT), a.k.a.
computed axial tomography (CAT):
12
(Bushberg et al., 2001)
Tomographic emission imaging—
Positron emission tomography (PET):
13
(Bushberg et al., 2001)
Tomographic
transmission/
emission
imaging—
Magnetic
resonance
imaging (MRI):
14
(Bushberg et al., 2001)
Tomographic transmission imaging—
Ultrasound:
15
(Bushberg et al., 2001)
Spatial resolution of the different modalities:
16
(Bushberg et al., 2001)
6.2 Attenuation of photonic energy
In x-ray, nuclear medicine and magnetic
resonance imaging, photonic energy is
detected as at is transmitted through or
emitted from the body.
In transmission imaging, the attenuation of
photonic energy as it passes through the
body is the basis of image formation.
In emission imaging, attenuation of photonic
energy by the body is a confounding factor.
17
Photon attenuation:
The intensity (or number) of photons
detected is given by:
where I0 (i0) is the intensity from the source,
¹ (¹x) is the attenuation coefficient (in units
of reciprocal length), and z (x) is the path
length.
18
Attenuation coefficients for different
materials:
19
(Bushberg et al., 2001)
The electromagnetic spectrum:
20
(Bushberg et al., 2001)
Rayleigh scattering:
21
(Bushberg et al., 2001)
Compton scattering:
22
(Bushberg et al., 2001)
Compton scattering angle vs photonic
energy:
23
(Bushberg et al., 2001)
Photoelectric absorption:
24
(Bushberg et al., 2001)
Sources of diagnostic photon attenuation:
25
(Bushberg et al., 2001)
Photon attenuation in soft tissue:
26
(Bushberg et al., 2001)
Effects of scatter in narrow-beam vs broadbeam geometry:
27
(Bushberg et al., 2001)