Compton Scattering as a Tool for Medical Imaging
Harris Kagan
Ohio State University
UCLA
April 14, 2005
Outline of Talk
• Introduction
• Compton Imaging
• PET
• Prostate Probe
• Summary
UCLA
April 14, 2005
The CIMA Collaboration
Compton Imaging for Medical Applications
- Collaborators • The Ohio State University Columbus, Ohio USA
• The University of Michigan Ann Arbor, Michigan USA
• CERN Geneva, Switzerland
• Instituto de Física Corpuscular Valencia, Spain
• Institut Jozef Stefan Ljubljana, Slovenia
UCLA
April 14, 2005
Introduction
Early Detection of disease is essential in:
– prostate cancer
• Most common diagnosed non-skin cancer in US
• Second leading cause of cancer-related deaths in Western
male population
• One in 6 men will develop prostate cancer; 1 in 26 will die
• American man is 33% more likely to develop prostate cancer
than an American woman to get breast cancer
– breast cancer
•
•
•
•
Most common cancer among women
One in 9 women will develop breast cancer; 1 in 27 will die
Number of new cases is growing at ~2% rate
Smallest detectable cancer 2-3 mm
Early detection  photon imaging
UCLA
April 14, 2005
Introduction
Many Imaging Tasks:
Detection
Classification
Choose
between two:
Choose
among many
Estimation
Choose a value from
a continuum:
•Tumor
present
•Radiotracer uptake
in a region
•Tumor
absent
•Exact location of
tumor
UCLA
April 14, 2005
Introduction
Three main interaction of photons with matter
•Photoelectric effect
•Compton Effect
•Pair Production
UCLA
April 14, 2005
Source
Energy [keV]
Half-life
99mTc
140.5
6 hr
111In
171, 245
2.8 days
131I
364
8 days
22Na, 18F, 11C, 15O
511
950 days, 1.8 hr
Introduction
Early Detection with Imaging
SPECT - Single Photon Emission Computed Tomography
UCLA
April 14, 2005
Introduction
For mechanical collimators the tradeoff between
detection efficiency and spatial resolution is
unfavorable.
object
S : collimator efficiency
Rc : collimator
resolution
collimator
S  Rc2
detector
UCLA
April 14, 2005
~10-15 mm FWHM spatial resolution
~detection efficiency of 10-4 for body imaging (99mTc)
Introduction
About Absorbing Collimation
• In the best case efficiency is only a few photons detected for
10,000 emitted
• Performance of absorbing collimation decreases rapidly with
increasing energy
- Photoelectric absorption coefficient changes as ~ Z4 / E3
• Typical collimator for 140 keV (Tc99m)– 9mm FHWM @ 10cm, 2.3e-4
• Typical for 364 keV (I-131) – 12mm FWHM @ 10cm, 1e-4
• Radionuclides with even a small fraction of higher energy
radiation will result in severely compromised imaging
performance
UCLA
April 14, 2005
Introduction
Existing Approaches for Improvement
• Larger detector area (multiple head, ring geometry)
sensitivity but  price & limited improvement
Non-uniform sampling (fan beam, cone beam collimators)
sensitivity & resolution but  FOV & distorted images
Multiplexing (multiple pinhole arrays, coded aperture)
high sensitivity but computationally costly & extra noise
propagation from one part of image to another part
UCLA
April 14, 2005
Compton Imaging
• Electronic collimation – uncouple resolution and efficiency
• Two detectors in coincidence
- Near field operation
- Photon scatters in first detector – tag, efficiency
- Photon absorbed in second – no collimation, efficiency
• Reconstruction of scattering angle based on kinematics
-Measure interaction points and energy in both detectors
• Good performance from 140-511 keV
• Should enable excellent images where conventional
techniques only partially successful
UCLA
April 14, 2005
Compton Imaging
Electronic Collimation
1st Det
Compton Camera
2nd Det
1
0
ee-

1 - 1
1
(1-cos)
E1 E0 = 511keV
E0: incident photon energy
E1: scattered photon energy
 : scattering angle
1st det: solid state detector
2nd det: scintillation detector
UCLA
April 14, 2005
Source
Image Plane
1st Detector
2nd Detector
Scattered
-Rays
Compton Imaging - Goals
• Investigate the performance of PET
–Goal
Detection efficiency
– Spatial resolution
• Investigate the performance of Compton probe
– Influence of non-prostate background activity
– Prostate tumor detection
UCLA
April 14, 2005
Compton Imaging - History
1922
1968
1974
1981
1983
1987
1993
1996
Compton scattering (Compton)
Compton telescope (White)
nuclear medicine application (Todd et. al.)
Compton image reconstruction (Singh et. al.)
first prototype Compton camera (Singh)
multiple Compton method (Kamae et. al.)
ring Compton camera (Martin et. al.)
Compton-SPRINT (Rogers et. al.)
Also Astrophysics: COMPTEL, TIGRE, MEGA, …
UCLA
April 14, 2005
Applications of Small High Resolution PET
 Metabolic studies with small animals
 Studies with genetically modified mice
 Radiotracer development
Sub-millimeter resolution is required for
small organs (thyroid 1~6mg, adrenal 3~40mg)
and functional regions of brain, kidney, etc.
UCLA
April 14, 2005
General Principles of PET
 Patient injected with drug having +
isotope
•Scintillation Detector Ring
•(continuous or segmented)
 Drug localized in patient
 Radioisotope decays and emits +
 + annihilates with e- from tissue and
forms back-to-back 511 keV photon pairs
•Patient
•e-
 511 keV photon pair detected with timing
coincidence window
 Annihilation position lies on line of
response (LOR) defined by detector pair
•Scintillation site
UCLA
April 14, 2005
•+
Role of Image Reconstruction
Scanner with some set of Line Of Responses
Projection data collected along a
“tube” or line of response (LOR)
Scanner
LOR data
Image
Reconstruction
Algorithm
Patient
Display
Effective field of view (FOV)
Assumption
 (events in LOR)  constant   (activity concentration in patient)
LOR
UCLA
April 14, 2005
Compton PET Concept
•BGO
•detector
•Si
•detector
•Three Major Coincidence Events
•Si-Si :
• Very High Resolution
•
•Si-BGO :
High Resolution
•Si-Si
•Si-BGO
•BGO-BGO
UCLA
April 14, 2005
•BGO-BGO :
• Conventional PET Resolution
Sensitivity and Spatial Resolution
•
Si-SI
Si-BGO
•Si-Si
• Sensitivity: 1.0 %
• *FWHM = 230 m
•Si-BGO
• Sensitivity : 9.0%
• *FWHM = 790 m
•BGO-BGO
1D Profiles
•BGO-BGO
• Sensitivity: 21.0 %
• *FWHM = 1.45 mm
•* Not including
•effects of annihilation photon
acolinearity and positron range
UCLA
April 14, 2005
•Image reconstruction: FBP
Overall Spatial Resolution
Si-BGO
Si-Si
Event
Geometric
BGO-BGO
Spatial Resolution (mm FWHM)
Geometric
Overall
+ Acolinearity
F-18
C-11
N-13
O-15
Si-Si
0.234
0.241
0.393
0.443
0.492
0.553
Si-BGO
0.788
0.816
1.062
1.261
1.419
1.742
BGO-BGO
1.452
1.458
1.977
2.270
2.490
3.069
UCLA
April 14, 2005
Spatial Resolution
UCLA
April 14, 2005
Compton Scatters in Scintillator
Conventional PET
• Scintillator
•~ 1 cm
Multiple interactions
Energy deposited over a volume
~ 1cm mean path
Penetration into crystal ring blurs measured
position due to depth of interaction (DOI)
uncertainty
Resolution ~ 1 – 2 mm
UCLA
April 14, 2005
Scatter and DOI Uncertainty in BGO PET
•- Using Monte Carlo simulations
•- BGO PET ( 17.6 cm I.D. 16 cm length segmented with 3 mm x 3mm x 20 mm crystals)
•- Point sources at 0, 3, 6, 9, 12, 15, and 18 mm from center of FOV
•- Filtered back projection (FBP) method
•True first interaction position
UCLA
April 14, 2005
•Centroid of scattered E distribution
•DOI uncertainty included in both images
Annihilation Photon Acolinearity
Deflection for Acolinearity
Distribution of Deflection Angles
511 keV photon
180
e


from 10 million photon pairs
Angular Uncertainty :
Gaussian distribution
Mean = 0 degree
FWHM = 0.5 degree
UCLA
April 14, 2005
[by Evans and Harrison]
Compton Kinematics
Compton Kinematics
Reduction of scatter,
random,
BGO
ESi
Si
and misclassified
events
Si
BGO
Angular Uncertainty Factors
Doppler Broadening
Detector Element Size
Si pad:
0.3 mm x .3 mm x 1 mm
BGO crystal:
UCLA
April 14, 2005
3 mm x 3 mm x 20 mm
EBGO
Energy Resolution
Photon Acolinearity
Blurring due to Positron Range
Intrinsic
F-18
C-11
N-13
O-15
Si-Si
Si-BGO
BGO-BGO
UCLA
April 14, 2005 All images include photon acolinearity.
Each image size is 10 mm x 10 mm
First Detector
UCLA
April 14, 2005
2D Images of Simulated Multiple Cylinder Sources
Sources
Si-Si (160k)
- F-18 Cylinder Sources :
1,2,4,6,8, and 10 mm dia,
10 mm long
-Centers of Each Source :
1cm from center of FOV
4 cm x 4 cm
-Filtered back projection (FBP)
-Images reconstructed
with different resolution and
sensitivity data
UCLA
Si-BGO
April 14, 2005
(1.4M)
BGO-BGO (3.1M)
Multi-resolution Image Reconstruction
- Imaging system generates data having wide different spatial resolution
and sensitivity.
- FBP does not account for the different information in the three datasets.
- How to combine these image ?
Si-Si (160k)
FWHM: 0.34 mm
UCLA
April 14, 2005
Si-BGO (1.4M)
1.07 mm
BGO-BGO(3.1M)
1.95 mm
All
from FBP
Image Combined with ML-EM
- Maximum likelihood Expectation Maximization (ML-EM)
- Iteration number = 200
4 cm x 4 cm
Si-Si (160k) + Si-BGO (1.4M) + BGO-BGO (3.1M)
UCLA
April 14, 2005
A Prototype Experimental Setup
BGO
detector
(non-position
sensitive)
Lead shielding
and slice
collimation
Source turntable
Silicon detector
in plastic box
Readout Board
for BGO
Intermediate Board
for Si readout
UCLA
April 14, 2005
Silicon and BGO Detector
Silicon detector
BGO detector
VATAGP3
HAMAMATSU PMT R2497
4.5 cm  2.2 cm and 1 mm thick
3216 (512) pads, 1.4 mm  1.4 mm pixel size
VATAGP3 : 128 slow shapers (VA part, preamp)
128 fast shapers (TA part, trigger)
UCLA
April 14, 2005
5.3 cm  5 cm and 3 cm thick
84 array, 12.5 mm  5.25 mm crystal size
Performance of Silicon Detector
UCLA
April 14, 2005
Baseline offset and Noise
Gain and FWHM
Energy Resolution of Silicon Detector
Am-241 (59.5 keV)
FWHM = 1.49 keV (2.5 %)
UCLA
April 14, 2005
Tc-99m (140.5 keV)
FWHM = 1.39 keV (0.99%)
Pb K1 = 74.969 keV, K2 = 72.804 keV,
K1 = 84.936 keV, and Compton edge = 49.8 keV
Energy Resolution of BGO Detector
Tc-99m (140.5 keV)
FWHM = 65.5 keV (46.6 %)
UCLA
April 14, 2005
Na-22 (511 keV)
FWHM = 116.6 keV (22.8%)
Compton edge : 340 keV for 511 keV
Compton edge : 1061.7 keV for 1274.5 keV
Detector Alignment
Silicon detector
Lead shielding
Silicon detector
Tungsten Slit
Source turntable
Laser beam
UCLA
April 14, 2005
Compton PET System Block Diagram
BGO
BGO
Si
Si
BGO
BGO
CFD
CFD
Coincidence
Unit
VME DAQ
Trigger
line
VME DAQ
Computer
Listmode data (E, position)
UCLA
April 14, 2005
Lines of Response (Alignment)
UCLA
April 14, 2005
Hit Map
UCLA
April 14, 2005
Images of Two Point Sources
Compton PET
MicroPET R4
Source
F-18
in glass capillary tubes
5
F-18
5
4
4
1.1~1.2 mm
3
0.4 mm
3
2
2
1
1
Glass wall
0.2 mm
0
0
cm
1
UCLA
April 14, 2005
2
3
4
5
0
0
cm
1
2
3
ML-EM Image reconstruction with Si-Si coincidence events
4
5
Images of Two Point Sources
5
5
Compton PET
MicroPET
4
1.1~1.2
mm
4
3
2.0 mm
3
2
2
F-18
1
0
0
cm
1
2
3
4
5
Glass wall
0.2 mm
1
0
0
cm
1
2
3
4
Source
FWHM = 1.14, 1.11 mm
UCLA
April 14, 2005
FWHM = 1.72, 1.61 mm
5
Intrinsic PET Resolution
Image Resolution
1.11~1.14mm
FWHM
Intrinsic
Resolution
~ 700 m FWHM
Intrinsic Resolution  Source Size = Image Resolution
UCLA
April 14, 2005
Intrinsic PET Resolution
F-18
0.254 mm
0.127 mm
Needle 25G (ID = 0.254 mm, OD = 0.5mm, SS_steel wall =
0.127 mm)
SS_steel wall
5
4
Image Resolution
= 700 m FWHM
3
2
1
0
0
cm
1
UCLA
April 14, 2005
2
3
4
5
Resolution Uniformity
5
5
4
4
3
3
2
2
1
1
0
0
cm
1
2
3
4
F-18 in capillary tubes
(I.D. 1.1 ~ 1.2 mm)
Resolutions:
~ 700 µm FWHM
5
0
0
5cm
1
2
3
4
5
1
2
3
4
5
4
3
2
1
UCLA
April 14, 2005
0
0
cm
Compton PET Summary

In order to achieve sub-millimeter spatial, a small animal PET based on
Compton camera concept was proposed.

Simulation results demonstrated sub-millimeter spatial resolution of the
Compton PET (0.4 mm FWHM from Si-Si and 1.0 mm FWHM from
Si-BGO).

Image reconstruction method using MLEM was developed to properly
combine images having different resolutions and sensitivities.

Experimental results with a prototype setup using 1.4 mm x 1.4 mm
x 1 mm silicon pads verified very high resolution (700 m FWHM).
UCLA
April 14, 2005
Compton Prostate Probe
The Big Question:
How well can a compton camera work
relative to a conventional collimated single
photon system?
UCLA
April 14, 2005
Compton Probe Monte Carlo Studies
Prob
e
Prostate
Geometry set-up for “voxel-man”
simulation
UCLA
April 14, 2005
• Human phantom
– “Zubal” male torso
phantom with 4mm3 voxel
size mated with EGS4
• Internal probe and 2nd
detectors
• Radioisotopes
– Tc-99m, In-111, I-131,
and positron emitters
Compton Probe Monte Carlo Studies
Counting Efficiency
Probe Efficiency
10% FWHM Energy Window
0.20%
0.18%
0.16%
0.14%
0.12%
0.10%
100
200
Tc99
m
300
In111
400
I131
(ProstaScint)
Gamma Photon Energy
UCLA
April 14, 2005
500
e+
600
Compton Probe Reconstruction
List-mode Likelihood Reconstruction Images of
“M” at 100 Iterations
141kev
171kev
245kev
364kev
511kev
4mm Point-to-Point, 1cm from probe
UCLA
April 14, 2005
Compton Probe
UCLA
April 14, 2005
Compton Probe
UCLA
April 14, 2005
Compton Probe
B cts/ Prostate cts
Background from Other Organs
0.25
0.2
0.15
0.1
0.05
0
0
100
200
300
400
Gamma Photon Energy
Target
Activity
Bkgd.
Activity
UCLA
April 14, 2005
liver (0.009)
kidney (0.06)
spleen (0.1)
bladder (1.0)
500
600
Compton Probe
Planar Images of Prostate with Tumors
Prostate with 4mm tumors
171keV (245k counts)
Tumor 2
15:1 conc.
Tumor 3
30:1
conc.
Tumor 1
10:1 conc.
80 iterations
UCLA
April 14, 2005
100 iterations
#1: 0.8cm away, 10:1 T/P concentration
ratio #2: 1.2cm away, 15:1 T/P
concentration ratio #3: 1.6cm away,
30:1 T/P concentration ratio
Compton Probe
Relative Uptake of In-111 Prostascint
Organ
Relative Uptakes
Prostate
1.0
Liver
2.0
Blood
1.5
Bone
0.7
Kidney
1.0
Spleen
1.0
Bladder
0.6
Rectum
0.4
Testes
0.6
Averaged from three In-111 Prostascint SPECT scans
UCLA
April 14, 2005
Compton Prostate Probe
Prostate Probe Reconstruction
171 keV
Prostate
w/tumor
Prostate
only
UCLA
April 14, 2005
245 keV
Compton Prostate Probe
Prostate Probe Reconstructions
245 keV only, 1.2 million events, 8mm lesion
5:1
w / tumor
bkgd
UCLA
April 14, 2005
10:1
15:1
20:1
SPECT
SPECT Reconstructions
5:1
w / tumor
bkgd
UCLA
April 14, 2005
10:1
15:1
20:1
Compton, SPECT Performance
Performance 8mm Tumor
Area Under ROC Curve: 8 mm Tumor
1
Area Under Curve (AUC)
0.95
Compton Probe
0.9
0.85
0.8
0.75
0.7
Conventional SPECT
0.65
0.6
0.55
0.5
1
UCLA
April 14, 2005
1.5
2
2.5
3
3.5
4
Tumor-to-background Ratio
4.5
5
Compton, SPECT Performance
Performance 5mm Tumor
Area Under ROC Curve: 5 mm Tumor
1
Area Under Curve (AUC)
0.95
0.9
0.85
Compton Probe
0.8
0.75
0.7
0.65
Conventional SPECT
0.6
0.55
0.5
1
UCLA
April 14, 2005
2
3
4
5
6
7
8
Tumor-to-background Ratio
9
10
Conclusions – Prostate Probe
• Compton probe has overall 0.16% counting efficiency
for Gamma energies ranging from 141kev to 511kev
• Compton probe has FWHM resolution <2.9mm; best
(2.1 mm) at 364 keV
• Simulations of probe exhibited 3.5x increase in SNR over
conventional Prostascint SPECT
• Compton performance improves dramatically with
energy
• Tumor much easier to visually locate than with
conventional SPECT
• Useful over a wide range of energies (140  >511 keV)
• Small device in construction
UCLA
April 14, 2005
Summary
• Compton Imaging has been demonstrated to
have applicability in many areas
• High Resolution PET with < 1mm FWHM has
been achieved
– Presently constructing full barrel
– Clinical tests?
• Prostate Probe works well in simulations
– Presently constructing new prototype
• Compton Scintimammography is still an open
question – simulations in progress
• Many open questions need attention
UCLA
April 14, 2005
Extra Slides
UCLA
April 14, 2005
Compton Scintimammography
Requirements:
• Reliable detection of small (<1cm diameter) lesions
• Ability to operate at many energies (inc. 511 keV) for
tracer flexibility
UCLA
April 14, 2005
The Compton Imaging Probe
“Near-field” operation provides
the following advantages
• High spatial resolution
• High counting
efficiency
• Useful for 140 keV and
higher energies
• Relatively low-cost
UCLA
April 14, 2005
2nd Detector
1st
Detector
Object
Object is placed
close to 1st
detector
Uncertainty
increases with
distance
The Contenders: SPEM
•
•
•
•
•
•
UCLA
April 14, 2005
Simple
Collimators close to breast have
high resolution
Can trade some resolution for
efficiency
Can see lesions in chest wall
using angled collimators
Flexible configurations
Efficiency very low – long
imaging times
The Contenders: PEM
• High efficiency and
resolution
• Tracers use “elements
of life”
• Sensitivity drops off
toward edges of FOV
• Difficult to see lesions
close to the chest wall
UCLA
April 14, 2005
Compton Emission Mammography
High efficiency and resolution
• “Sees” lesions throughout breast with
high efficiency
• Capable of operating over wide energy
range with good performance
•
Coded data – performance advantage is
not raw efficiency gain
• Influenced by background in this
configuration
• Computationally intense reconstruction
•
UCLA
April 14, 2005
CEM in more detail
• Resolution and efficiency vs. incident
energy
• Influence from background activity
• Preliminary lesion detection
performance.
UCLA
April 14, 2005
Configuration and Experiments
10cm x 10cm silicon detectors of
various thicknesses
• 2 40cm x 40cm scintillation
cameras
•
Data generated by attaching 2
10cm x 10cm x 5cm breast
phantoms to “Voxelman”/EGS4
Monte Carlo code
• ~1.3 M events / realization
•
UCLA
April 14, 2005
Spatial Resolution
4
Spatial Resolution (mm FWHM)
3.5 2.5 cm
3
2.0 cm
2.5
1.5 cm
2
1.0 cm
1.5
1
100
0.5 cm
150
200
250
300
350
Energy (keV)
UCLA
April 14, 2005
400
450
500
550
Efficiency
0.045
10mm
0.04
0.035
Absolute Efficiency
0.03
5mm
0.025
0.02
0.015
2mm
0.01
0.005
1mm
0
100
150
200
250
300
350
Energy (keV)
UCLA
April 14, 2005
400
450
500
550
Efficiency -5 mm thickness
0.05
0.5 cm
0.045
0.04
Absolute Efficiency
1.0 cm
0.035
1.5 cm
2.0 cm
0.03
2.5 cm
0.025
0.02
0.015
100
150
200
250
300
350
Energy (keV)
UCLA
April 14, 2005
400
450
500
550
Lesion Reconstruction Experiments
•5 cm
•8mm / 10:1
•4-plane
•8mm / 8:1
•3-plane
•5mm / 15:1
•5 mm
•8 mm
•Extra plane for 4 plane
reconstruction
•Note improvement in lesion contrast with better modeling
UCLA
April 14, 2005
Reconstruction – no background
•140 keV
•171 keV
•245 keV
•8mm
•5mm
•1.2 M events
UCLA
April 14, 2005
•364 keV
•511 keV
Reconstruction with background
•140 keV
•171 keV
•245 keV
•364 keV
•8mm
•5mm
•1.2 M events
•Liver:Heart:Normal Breast  4:2:1
UCLA
April 14, 2005
•511 keV
Background: Principle of Compton Camera
Source
Image Plane
• Detection coincident events
between two detectors
• Compton scatter equation
relates scatter angle  and Eo
and Ere
1st Detector
cos   1 
2nd Detector
Scattered
 - Rays
UCLA
April 14, 2005
511
511

E 0 E 0  E re
• Photon direction is determined
within conical ambiguity
“slab-man” Monte Carlo Simulation
Planar CZT
came ra
• Human phantom
– 40x40x20cm3 slab
tissue material
with
soft
• Internal probe
20cm 30cm
Silicon
detector
Human
phantom
40cm
40cm
UCLA
April 14, 2005
Planar CZT
came ra
– 1x4x1cm3 silicon with 1mm3
detector elements
• External 2nd detectors
– 40x40x2cm3 Cadmium
Telluride (CZT)
Zinc
• Radioisotopes
– Point sources of Tc-99m, In111, I-131, and positron
emitters
– Located 1cm away from the
front face of silicon detector
Applications
Use Silicon detectors for spatial resolution
BGO
detect
or
BGOBGO
5
Compton
scattering in silicon5
Si
detect
Si-or
Si
SiBGO
4
Compton PET
3
3
2
2
1
0
5 cm
0
Silicon detector
Lead shielding
4
1
2
3
4
1
MicroPET R4
0
5 cm
1
2
3
4
0
Silicon detector
Tungsten Slit
Source turntable
UCLA
April 14, 2005
Laser beam
Compton Probe
Probe-Gamma Camera Comparison
Efficiency and resolution for In111
Imaging Distance
Compton Probe
High-Sensitivity Collimator
High-Resolution Collimator
UCLA
April 14, 2005
10 cm
Efficiency
1.8e-3
1.11e-4
4.00e-5
Resolution
2.47mm
15.9mm
10.5mm
Monte Carlo Simulations
• Used EGS4/Voxelman simulation for prostate probe
data. 10 x 1.3 million event realizations generated.
• Realizations were pooled and another 9 generated by
resampling with replacement (“bootstrapping”)
• SIMND with Voxelman and model for hi res / medium
energy collimator used to generate SPECT data
• 25 realizations generated having counts same as
observed in Prostascint SPECT scans at UM
• NPW matched filter applied using templates from tumoronly images, SNR of filter output estimated
UCLA
April 14, 2005
SPECT Reconstruction
171 + 245 keV (20% window), Hi Res / Med Energy collimator
Without
Tumor
Prostate
8 mm dia
Tumor
Only
UCLA
April 14, 2005