Transcript AAPM Annual Meeting 7/26/01 Salt Lake City, Utah

NC HPS Meeting
10/18-19/2001
Boone, NC
Recent Advances in CT Technology and
Issues of CT Dosimetry
T. Yoshizumi 1,2, M. Sarder1, R. Reiman 1,2,
E. Paulson 2, D. Frush 2, F. Thornton 2
1
Radiation Safety Office, 2 Department of Radiology
Duke University Medical Center
Durham, NC
NC HPS Meeting
10/18-19/2001
Boone, NC
ACKNOWLEDGEMENTS
Sha Chang, Ph.D.
Department of Radiation Oncology
University of North Carolina at Chapel Hill
For Use of RANDO phantom
Thad Samulski, Ph.D., Kelly Ryan, M.S. and Mike
Scribner
Department of Radiation Oncology, Duke University
Medical Center
For Use of RANDO Phantom and TLD reader
Recent Advances in CT Technology and
Issues of CT Dosimetry
1. Recent advances in CT technologies
2. Current radiation safety issues in multidetector CT
3. Comparison between single-detector and multidetector CT dosimetry
4. Results and discussions
5. Conclusions
Recent advances in CT technologies
Original "Siretom"
dedicated head CT
scanner
CT was invented in 1972
by British engineer
Godfrey Hounsfield of
EMI Laboratories,
England.
Advances in CT Technology
Faster tube rotation times

0.5-0.8 s for a full rotation
shorter exposure time
tube current must
increase to get same photon statistics
higher Heat Unit
Greater anode heat capacity (HU=mA * kVp
* seconds)
 GE QXi 6.3 MHU
 Single-detector CT 2-3 MHU
 Axial scanner (old days) 1 MHU
Advances in CT Technology
Single-detector vs Multi-detector
Multi-detector planes
4-slice in one rotation
GE QXi (multi-detector CT) acquires four
interweaving helices simultaneously.
e.g., 4 x 5 mm slice = 20 mm total scan
width
Definitions of Pitch
Old definition:
Table travel per rotation
P=
slice thickness
New definition:
Table travel per rotation
P’=
Total nominal scan width
GE QXi High Quality (HQ) vs High Speed (HS)
20 mm
Pitch = 15mm/20 mm =0.75
15 mm table travel
20 mm
Pitch = 30mm/20 mm =1.5
30 mm table travel
GE QXi Two scan modes: HQ and HS.
(high quality:
15 mm table; 5 mm slice;
4x5 mm=20 mm)
P’=0.75
P’=1.5
(high speed
30 mm table;
5 mm slice;
20 mm total
width)
Pitch (old)
How fast is it?
2. Current radiation safety issues in multidetector CT
Insufficient organ dose data in multidetector CT
FDA worried about radiation risk from
WB CT for routine screening (5/18/01
Reuters Health)


“Boutique clinics” where patients pay $300$500 to get CT scans- a new nationwide
industry.
“We don’t have dose limits on CT. How the
operator uses it is totally out of control.” (Tom
Shope, FDA)
3. Comparison between single-detector and
multi-detector CT dosimetry
1) To compare the organ dose between
single- and multi-detector CT scanners
under technically equivalent exposure
parameters, thus quantifying the
magnitude of dose contributions due to
scanner design only;
2) To compare organ doses between singleand multi-detector CT scanners under
protocols that are currently used at our
institution.
MATERIALS AND METHODS
1) CT scanners
 GE CT/i Scanner (single-detector)
 GE QX/i Scanner (multi-detector)
2) Dose measurements
 Harshaw TLD-100 (LiF)
 Harshaw Auto TLD Reader Model QS
5500
3) Phantoms
 32 cm Cylindrical CT Phantom
 Anthropomorphic RANDO Phantom
KEY DESIGN DIFFERENCES
BETWEEN CTI AND QXI
 TUBE TO CENTER OF ROTATION (COR)
DISTANCE
Focal spot to ISO center is 541 mm
for QX/i and 630 mm for CT/i [tube is
closer to the patient in QX/i].
QXi
i.e., QX/i tube has more x-ray flux by
(630/540)2 = 1.36 or 36% or approx. 40%
more dose for the same mAs and kVp
[Inverse square law]
GE states that QX/i can generate
images with the same noise using 64%
(100-36= 64%) of CT/i mAs.
CTi
EXPERIMENT NO. 1- DOSE PROFILE
STUDY
Design: scan under the technically equivalent
parameters and see what happens to the
dose profile
 Only parameter adjusted: mA is
decreased by ~36% for QXi [focal
spot to ISO center adjustment
(630/540)2 = 1.36 (36 %)]
 Test to see if dose is comparable
between QXi and CTi
 If different, estimate the magnitude of
penumbra and scatter contributions in
QXi
TECHNICALLY EQUIVALENT SCAN
PARAMETERS
QX/i
140 kVp; 5 mm thick/15 mm
table travel(HQ); 130
mA;0.8 sec gantry rotation;
table =15 mm per rotation
CT/i
140 kVp;pitch=1; 5 mm
thick; 210 mA;0.8 sec
gantry rotation; pitch =1
Pitch = 1
Overlap @ 4th slice in Qxi under HQ mode
Figures taken from McCollough and Zink: Med Phys 26,2224 1999
4. RESULTS AND DISCUSSIONS
EXPERIMENT NO. 1- DOSE PROFILE STUDY
EXPERIMENT NO. 2- ORGAN DOSE
COMPARISONS WITH PHANTOMS
Design: measure organ doses between singleand multi-detector CT scanners under
protocols that are currently used at Duke
University
(A) 32 CM CT PHANTOM
(B) RANDO PHANTOM
EXPERIMENT NO. 2- ORGAN DOSE
COMPARISONS WITH 32 CM PHANTOM
32 CM DIAM. PHANTOM
ADULT BODY PROTOCOL
CTi
6 inches (15 cm thick)
A
Chest(140 kVp, 170 mA, 0.8 sec 7
mm thick, 10 mm spacing, pitch 1.5)
B
Z-axis
7 TLDs (21 mm)
Abdomen(140 kVp, 210 mA, 0.8sec,
5 mm thick, 7 mm spacing, pitch 1.5)
Pelvis (140 kVp, 210 mA, 0.8sec, 7
mm thick, 10 mm spacing, pitch 1.5)
7 TLD chips
Center hole
QXi
Chest/Abdomen/Pelvis – same
technique :(140 kVp, 170 mA, 0.8 sec,
5 mm thick, HQ 15mm/rot)
RESULTS AND DISCUSSIONS - (A) 32
CM PHANTOM
Relative Dose Comparison
between CTi and QXi
20
17
CTi
QXi
mGy
15
8.5
10
8.3
6.3
5
0
Chest
Abdomen
Pelv is
Duke Body CT Protocol
(32cm Acrylic Phantom with TLD
chips inside the center hole)
file: 32cm dose.pzm 2/23/01
EXPERIMENT NO. 2- (B) DOSE
COMPARISON WITH RANDO PHANTOM
DUKE PELVIS
PROTOCOL
QXi:
Scout view (140 kVp, 40 mA)
Pelvis (140 kVp, 170 mA,
HQ, pitch 3, 5 mm slice thick
)
CTi:
Scout view (140 kVp, 40 mA)
Pelvis (140 kVp, 210 mA,
pitch 1.5, 7 mm slice thick)
4. RESULTS AND DISCUSSIONS (B) RANDO PHANTOM
CONCLUSIONS
1) Using technically equivalent scan parameters,
QXi showed approximately 40 % higher dose
than CTi in the scanned region and 90% higher
dose in the beam penumbra region;
2) Even when the tube current settings of clinical
protocols were adjusted for the tube-to-COR
distance in QXi, body protocols still showed a
100% higher organ dose for the multislice
scanner. These data indicate that simply
translating clinical protocols between single
and multislice helical CT scanners still results
in excess organ dose and increased radiationrelated risks.