Haptic Palpation for the Femoral Pulse in Virtual Interventional Radiology
T.Coles1,2, N.W.John2, D.A.Gould3 ,D.G.Caldwell1,
1
Institute of Italian Technology, 2Bangor University, 3Royal Liverpool University Hospital
[email protected]
Abstract
Interventional Radiology is a rapidly expanding
speciality using minimally invasive techniques to treat
a multitude of clinical problems. Current work in
progress aims to create an affordable virtual training
tool to reduce training times and patient risk during a
trainee practitioners learning cycle. The procedure of
arterial catheterisation has been broken down into a
number of subtasks, one of which requires an operator
to locate the femoral artery pulse by palpation. This is
performed in preparation for a needle insertion to
allow the entry of a guide wire and catheter into the
patient. This paper presents the current state of
research into a unique solution for affordable haptic
simulation of pulse palpation in a virtual environment.
1. Introduction
Modern methods of training a medical procedure
are little changed since the earliest beginnings of
medicine: the “apprenticeship” model uses the
principle of “see one, do one, teach one”, where a
trainee observes a procedure, practices it under
supervision and, when proficient, becomes a mentor
him/herself. Interventional Radiology (IR), as with
most other specialisations utilises this approach.
The IR procedure uses minimally invasive
techniques to perform tasks such as unblocking or
blocking vessels, biopsies, draining abscesses and,
infusing drugs to specific sites in the vascular system.
The procedure starts with the Seldinger technique [1],
the first and most essential step in performing an
endovascular procedure. In this procedure a needle is
inserted into an artery or vein guided commonly by
palpating for a pulse. Through this entry point a
guidewire is then navigated along the vessel. The
practitioner uses x-ray guidance to see the position of
objects within the patient. The wire acts as a guide for
a catheter which is fed over the wire so that
fluoroscopic dye can be injected for guidance
purposes; a catheter sheath also may act as a channel to
insert tools such as balloons and stents or, fluids to a
specific location. Palpation for a pulse and instrument
guidance is highly tactile as the visual feedback is
reduced. Whilst guiding a wire it is essential to
respond to the force feedback felt at the finger tips as
the visual feedback is limited.
The number of straightforward invasive diagnostic
procedures, which are the main source of IR training
opportunities, have greatly reduced as non-invasive
imaging methods have improved. At the same time,
cost minimisation is high on hospital agendas, yet
training under the apprenticeship model is expensive
as it increases procedure duration [2]. This is due to
the increased time an expert practitioner must dedicate
to simple cases when training. This increased time
applies to all staff involved, as well as the patient. In
addition, work time directives in the US and EU are
reducing trainee practitioners’ hours of work, further
limiting the available training time.
A further significant disadvantage to the
apprenticeship model is that learning, in part, involves
the experience of errors, albeit under the guidance of
an expert mentor. Yet performing an operation
incorrectly through inexperience can lead to patient
discomfort and complications. The latter can prolong a
patient’s hospital stay or, in the worst case scenario,
can cause permanent damage or death. It is not
acceptable to make mistakes on patients when
alternatives are available.
There are many ways of simulating medical
procedures for training purposes. Traditionally,
students train using anesthetised animals or cadavers,
or by practicing on mannequins or fellow students.
However, the deformations that occur in an animal’s or
cadaver’s tissues vary from those of living humans due
to varying anatomy or absence of blood pressure. Not
only are cadavers expensive, but procedures can only
be performed once and a mistake can render the body
useless to re-demonstrate a procedure. This type of
simulation also raises ethical issues. Sophisticated
mannequins are also becoming increasingly common
to simulate a patient e.g. [3], though they are limited in
their replication of physiology, and have at best a
limited range of anatomical variability.
One alternative that is making an impact on the
medical community is computer simulation [4-6],
which can train practitioners by critically analysing
skills and providing feedback on the performed
procedure. This feedback can then be used to refine the
required skills until the operator reaches an agreed
level of proficiency before commencing training with
patients. Simulations can also provide the user with an
opportunity to virtually practice difficult cases, or
those in which patient anatomy is unconventional
before performing the procedure in a patient. Such
‘mission rehearsals’ can highlight operational and
equipment difficulties that would otherwise be
overlooked until they are encountered during the real
procedure.
Simulation involving the manipulation of living
human structure can require a much greater complexity
than that already achieved in simulations for aviation
or military applications. Aviation and military
simulation follows a known set of rules. A medical
simulation on the other hand would ideally simulate
the mechanical properties of organs which are still
little known. Further to the obvious computational
difficulty, procedural variety between practitioners
causes a problem. The simulation should accommodate
all correct and incorrect approaches to performing a
procedure. Despite these complexities, both
computational and mechanical approximations can be
made that provide sufficient visual and haptic fidelity
to effectively simulate real world situations.
Palpation is a very common medical examination
that relies on haptic response which has been little
researched for simulation [7-8]. The clinician presses
lightly on the surface of the patient’s body to feel the
organs or tissues underneath. Palpation is used in the
IR procedure to guide a needle into an artery in the
initial phase of the Seldinger Technique. One available
entry site for the procedure is the femoral artery.
Although commercial endovascular simulations
such as [9] exist they include neither the palpation nor
needle insertion procedure. Including the palpation
stage within a virtual training simulation for IR would
give the simulation an essential component of validity,
allowing the trainee to practice palpating a pulse in a
variety of patients, and to use this to guide a virtual
arterial puncture procedure.
Depending on a patient’s body habitus, which is in
turn related to their quantity of muscle and fat, the
displacement of tissues necessary to feel a patient’s
pulse varies. The strength of the pulse also varies due
to tissue thickness, as well as a range of pathologies
that might be present. These varying conditions must
also be taken into consideration and simulated in the
virtual environment.
This paper outlines research into the affordable
simulation of palpating for a pulse in the femoral
artery. The developed software and hardware solutions
are explained and reviewed in the following chapters.
2. Pulse Palpation
From firsthand experience, observation in theatre
and video documentation, the task of palpation for the
femoral artery specifically during an IR procedure has
been analysed. In addition, research into the exact
forces applied during palpation is being performed.
Figure.1: Palpation for a pulse during needle insertion
In combination with visual feedback, the haptic
component of the simulation (the feeling of the pulse)
has been investigated using both force and tactile
feedback.
2.1. Virtual Environment
The goal of this simulation is to create a realistic,
high fidelity visual model that also provides a realistic
touch sensation. The simulation hardware and software
is made up of the following components:
• A flat screen LCD monitor mounted horizontally
such that the user looks down into the monitor to
give an impression of seeing the patient lying
below. It was initially, but incorrectly, thought that
a time sequenced stereo image could be projected
in this manner, but the refresh rate of the LCD
display proved to be too low to permit this
approach [10]. The visual display is now being
replaced with a Zalman ZM-M220W [11], an
LCD display capable of projecting a stereo image
using linear polarisation.
•
•
•
A haptic device is placed below the monitor and a
hand avatar is displayed in a position that matches
the users hand on the haptic device to help the
user visualise the situation [12].
An anatomical model of a virtual patient has been
created by segmenting the visible human dataset
using ITKSnap [13]. A three dimensional mesh of
a segment of the skin’s surface (seen in figure. 3)
is used in the visualisation. Future developments
will implement the underlying structures, using
soft tissue modelling techniques, allowing an
accurate model of the deformation.
The simulation has been developed using Chai3D
1.51 [14] for force feedback support, and
proprietary algorithms for tactile response.
Pressure
(a) Approximation
of pulse pressure
waveform.
(b) Simulated
square waveform.
Time
(c) Simulated
Triangular
waveform
(around 1400 euro). The relatively new Falcon haptic
device has undercut this with an extremely low price
(200 euro in the EU), offering some interesting options
particularly for teaching and training.
As standard, the Omni uses a pen like end effector
and the Falcon a ball shaped end effector. The Omni
offers six DOF positional / orientation sensing and
three DOF force feedback. The Falcon offers a more
limited three DOF of force feedback and positional
sensing. Nonetheless, after preliminary studies, the
lower cost, limited DOF Falcon device proved to be
better suited to this application due to the nature of a
palpation.
During an IR palpation the patient lies flat on their
back upon the radiological table and the practitioner is
tasked with locating the position of the femoral artery.
The approximate location of the femoral artery on a
patient’s right leg is indicated by the dark strip in
Figure 3. The pulse is palpated by the practitioner,
usually using 2-3 fingers (subject to practitioner
preference). An example of this can be seen in Figure
1. The Omni’s armature based mechanics did not offer
such an intuitive palpatable platform in comparison to
the Falcon device.
Figure.2: Pressure variations of a pulse and simulated pulse
profiles.
2.2. Force feedback
Hardware solutions for force feedback are readily
available and many medical simulators use an off-theshelf haptic device – such as the PHANToM range
from SensAble Technologies (Woburn, USA), with
some using a purpose built haptic device – such as the
Laparoscopic Impulse Engine from Immersion
Medical (Gaithersburg, USA). Ideally the haptic
device will provide a full six degrees of freedom
(DOF) feedback, with an active force response to both
position and orientation. However, devices with 6
DOF are expensive and most off-the-shelf haptics
devices only provide 3 DOF force feedback to the
stylus position, through which a force is applied to a
user. Commercially available haptic devices are
usually preferred over a custom build as they tend to
be lower cost, stemming from higher production
numbers. Further simulation development time can be
reduced as such devices are pre-tested and API’s have
been pre developed.
The two devices considered in this project are
SensAble’s PHANToM Omni [15] and, Novint’s
Falcon [16]. Both are at the low cost end of the market
with the Omni having achieved comparatively wide
spread acceptance as “the” low cost haptic device
Figure.3: Location of the femoral available in palpation.
Removing the Falcon’s ball shaped end effector
uncovers a vertical triangular plane. The device has
been modified such that this vertical plane is now
horizontal to the table top on which the device stands.
To do this, the device (Figure.4(a)) has been rotated
through 90 degrees and the commercial end effector
replaced with a thin flat end effector (Figure.4(b)).
This more closely mimics the orientation of the
patient’s skin in the area to be palpated. The end
effector has been fitted with two finger loops
(Figure.4(c)) which ensure the users fingertips are
always close to the end effector, making both the
positioning of the hand avatar correct, and allowing the
user to lift the end effector easily. This allows the user
to navigate in the three dimensional simulation
environment. Upon the end effector, in the position of
the three palpating finger tips, three tactile devices are
mounted (Figure.4(d)). These can be used to further
simulate the pulsing effect and are described in section
2.3.
The Falcon is used to convey force feedback when
the model is palpated. A small amount of friction is
applied in addition to the skins reactive force feedback
when the skins surface is palpated to increase fidelity.
When the area of the model corresponding to the site
of the femoral artery is palpated, the haptic device
conveys an additional force response. The force profile
of each pulse wave is triangular such as the wave in
figure 2(c). This profile is used as an approximation of
a human pulse. Although the double peak (figure 2(a))
of a real pulse is ignored, it is a barely detectable
feature in the femoral pulse of patients undergoing the
IR procedure. The triangular force profile produces a
sinusoidal motion of the end effector, due to device
inertia, combined with the inertia of the user’s fingers.
This inertia creates a slow in and out motion
characteristic of a bouncing ball and, of the
pressure/force profile of a pulse. Although the double
peak of a real pulse could be simulated it didn’t seem
necessary in the initial stages to do this due to the
compelling feeling of a single peaked pulse.
square pulse (figure 2(b)) producing a compelling
pulse feeling. Presently a triangular wave profile
(figure 2(c), the same as used in the force feedback
simulation) seems to give the highest fidelity response.
A more sinusoidal shaped wave profile can be used
with the pin array device as the device’s power and
displacement makes the pulsing detectable.
2.3.1. Piezoelectric Pads
Piezoelectric material expands and contracts when
a voltage is applied. This expansion and contraction
can be controlled by varying the applied voltage.
When incorporated into a fingerpad assembly the
voltage changes create displacement of the
piezoelectric material. This can create a pulsing effect
that when pinched between thumb and forefinger
creates a distinct feeling closely resembling that of a
pulse. When driven at low frequencies (1Hz -3Hz) [17,
18] not typically used in applications of piezoelectric
material, these sensations can be quite compelling. The
pads are thin (<1mm) and have been mounted onto the
custom modified end effector plane attached to the
Falcon force feedback device (figure 5). A latex glove
with the pads inserted at the fingertips has also been
tested. The pads are driven by an integrated circuit
board consisting of a microcontroller and OLED
graphics display made by Luminary Micro (Austin,
USA), and signal amplification circuitry. The pulse
length and pause time can be manipulated to simulate
zero to 300 pulses per minute. A major advantage of
using piezoelectric pads is their extremely low cost.
Figure.4: Modified falcon force feedback device.
2.3. Tactile devices
To further increase the perception of pulse
discovery when the correct location of the pulse is
palpated, the use of tactile devices has been
investigated. Three devices; piezoelectric pads, micro
speakers, and a pin array have been used, each with
their distinct advantages and disadvantages. In each
case, a waveform is generated to represent a pulse. A
variety of waveforms have been tested with the
piezoelectric and speaker devices. A sinusoidal pulse
profile was found to be tactically undetectable, with a
Figure.5: Piezoelectric pads mounded onto customised
Falcon end effector. Hand superimposed.
2.3.2. Micro Speakers
Micro speakers have also been tested as potential
pulse generation modules. These speakers are driven
by small electromagnetic coils and can be activated
using similar circuitry to that for the piezoelectric pads.
The devices can be used in the same way to produce a
displacement and simulate a pulsing sensation. The
speakers produce a larger displacement than the
piezoelectric pads, but produce a small reactive force
which can be easily counteracted when applying a
downward force through the device. To effectively feel
a pulsing sensation, an alternative device arrangement
to that of the piezoelectric pads has been sought.
Figure.6: Micro speaker tactile device.
Figure 6 shows the current implementation where
the membrane of the speaker is strapped to the user’s
finger. The mass of the speaker system is forced to
move as the speaker is driven. The inertia involved in
the movement produces a response at the finger tip
which has been likened to a pulse. The fingertip device
weighs approximately 7.5 grams and the base of the
speaker has a diameter of 19mm.
Incorporating force feedback is harder than with
the piezoelectric pads as they can not be mounted
directly onto a force feedback surface due to the low
force produced by the electromagnetic coil. Alternative
mounting techniques are being explored.
3. Evaluation
Initial qualitative tests of the simulations have been
carried out with a number of medically inexperienced
subjects and an experienced interventional radiologist.
Current verbal evaluations of the visual and force
feedback components found that the pulse was
relatively easy to find even after a short explanation of
the position of the femoral artery. The users
commented that the skin did not deform and that the
3D visualisation was of low quality. However, the
simulation is to include a deformable mesh in future
models and a higher quality 3D model is to be used. A
low quality model was used in the initial stages as the
development system has been a standard desktop
computer for proof of concept. The simulation will be
optimised for a more powerful graphics orientated
machine. This feedback is promising especially as
further relatively simple developments have still to be
carried out. More structured and quantifiable
evaluations of the simulations are to follow as
development progresses.
A table of the advantages and disadvantages of
each tactile technology considered is given below in
table 1.
Table 1. Comparison of Tactile Technologies
Technology
Piezoelectric
Pads
Micro Speakers
Pin Array
2.3.3 Pin Array
A pin array device developed by Salford University
[19] and further commercialised by Aesthesis [20]
calling it Aphee-4x, demonstrates an alternative
promising concept. The device has 16 individual
remotely actuated pins in a 4 by 4 configuration. The
array is small enough to be clipped onto the tip of a
user’s finger. The actuators are housed in a wireless
unit, which drives the pins using cables. Each pin can
exert a force up to 1.3N and has a maximum
displacement of 2mm. The actuators can be used to
precisely manipulate the displacement of the pins at a
high frequency and as such arbitrary pulse formations
can be felt. An expert interventional radiologist’s
reaction to this device was very positive but due to its
high cost, a proposal to reduce the number of pins to a
single large pin per fingertip is being considered.
Advantages
•
Extremely low
cost
•
Thin
•
Medium force
•
Large
displacement
•
Low cost
•
High force
•
Large
displacement
Disadvantages
•
Small
displacement
•
Limited life
•
Limited force
•
High Cost
Currently a triangular pulse profile appears to
produce the best perceptual waveform when using
piezoelectric pads and micro speakers. As the pressure
profile of a pulse is a complex waveform (resembling
that of figure 2(a)), an investigation into the forces felt
during palpation for a pulse is being undertaken. From
this information, the waveform generated by the tactile
and force feedback devices will be modified in the
hope to further increase the realism of the tactile
response.
4. Conclusion
Simulating medical procedures at sufficient fidelity
is inherently difficult. This is due to computation and
hardware difficulties and also due to the
aforementioned
procedural
variety
between
practitioners. This difficulty, coupled with the
requirement for simulators to be available at an
affordable cost further increases the difficulty of
producing an effective simulator.
Although cost is high on the list of requirements,
this does not mean that money need not be spent in
development, and that a more expensive solution
should not be provided if its fidelity is much higher.
Simulation development should first look to achieve
fidelity, correct content and face validity. Meeting low
cost requirements can be sought from the initial high
quality target model produced.
During the visual and force feedback simulations,
experiments showed that although it was possible to
see a user was palpating in the vicinity of the pulse, it
was impossible to know if the user was feeling the
pulse or not, or to evaluate the force they applied to
palpate for the pulse. To combat this, it is proposed
that a separate visual interface for the trainer is
provided which will summarise the forces felt during
the palpation process. This can be used to guide the
user to feel the correct response, using the ideal force.
The virtual nature of the simulation theoretically
allows the simulation to be applied for various
palpation procedures. However, task analysis must be
performed to check the task similarity to the analysed
IR palpation procedure. The similarity of the IR
palpation to other procedures is currently unknown.
Future work will be carried out in the development
of the palpation simulation using both force and tactile
feedback. The tactile devices currently showing most
promise at this time are the piezoelectric pads and the
proposed single pin modification to the pin array
device. Measurements to record the exact forces
involved during palpation and throughout the
interventional procedure are being conducted and from
these measurements, the accuracy of the simulation can
be improved. This in turn increases the simulation’s
validity. Development of the palpation simulation is
not tied to any one device, either force feedback or
tactile, so that new approaches can be easily
implemented. Further professional validation of the
approaches will be sought but preliminary testing has
shown promise that a realistic palpation can be
developed.
5. References
[1] S.I. Seldinger, “Catheter replacement of the needle in
percutaneous arteriography; a new technique”. Acta
radiologica 39 (5) , 1953 pp 368–376
[2] D.A. Gould, “Interventional Radiology Simulation:
Prepare for a Virtual Revolution in Training” Journal of
Vascular Interventional Radiology, Vol. 18 Iss. 4, 2007, pp
483 - 490.
[3] Y. Zhu et al., “A Virtual Ultrasound Imaging System
for the Simulation of Ultrasound-Guided Needle Insertion
Procedures” Proc. Medical Image Understanding and
Analysis (MIUA) 2006, pp61-65, vol 1.
[4] A.G. Gallagher, E.M. Ritter, and H. Champion,
“Virtual reality simulation for the operating room:
proficiency-based training as a paradigm shift in surgical
skills training” Annals of Surgery. Vol. 241 No. 2, 2005, pp.
364–372.
[5] Y. Wang et al., “Real-time interactive simulator for
percutaneous coronary revascularization procedures”
Computer Aided Surgery, 1998, 3:211–227.
[6] F.P. Vidal et al., "Simulation of Ultrasound Guided
Needle Puncture using Patient Specific Data with 3D
Textures and Volume Haptics", Computer Animation and
Virtual Worlds. Vol 19, Issue 2, 2008, pp 111–127.
[7] A Liu et al., “A Survey of surgical simulation:
applications, technology, and education” Presence:
Teleoper.Virtual Environ., 2003, Vol. 12, No. 6. pp. 599-614.
[8] F.P. Vidal et al., “Principles and applications of
computer graphics in medicine”, Computer Graphics Forum,
2006 , Vol. 25 pp. 113-137.
[9] Mentice Procedicus VIST-Radiology. Carotid and
renal stenting. http://www.mentice.com/, Last visited August
2008.
[10] A.J. Woods, T. Rourke, and K.L. Yuen, “The
Compatibility of Consumer Displays with Time-Sequential
Stereoscopic 3D Visualisation” K-IDS Three-Dimensional
Display Workshop 2006, pp 7-10.
[11] Zalman. http://www.zalman.co.kr/, Last visited
August 2008
[12] N.G. Tsagarakis et al., “Haptic-Enabled Multimodal
Interface for the Planning of Hip Arthroplasty”, IEEE
MultiMedia Vol. 13, No. 3, 2006, pp. 40–48.
[13] P.A. Yushkevich et al., “User-Guided Level Set
Segmentation of Anatomical Structures with ITK-SNAP”
Insight Journal, Vol. 1. 2005.
[14] F. Conti et al., “CHAI: An Open-Source Library for
the Rapid Development of Haptic Scenes” IEEE World
Haptics, 2005.
[15] SensAble Ommi. http://www.sensable.com/hapticphantom-omni.htm, Last visited August 2008
[16] Novint Falcon.
http://home.novint.com/products/novint_falcon.php,
Last
visited August 2008
[17] D.G. Caldwell, S. Lawther, A. Wardle, “Multimodal cutaneous tactile feedback”, Intelligent Robots and
Systems, Vol 2, 1996, pp 465 – 472.
[18] D.G. Caldwell, S. Lawther, A. Wardle, “Tactile
perception and its application to the design of multi-modal
cutaneous feedback systems”, Robotics and Automation, Vol
4, 1996, pp 3215 – 322.
[19] I. Sarakoglou et al., “Free to Touch: A Portable
Tactile Display For 3D Surface Texture Exploration”,
Intelligent Robots and Systems, 2006, pp 3587 – 3592.
[20] Aesthesis. http://www.aesthesis.net, Last visited
August 2008.