Presentation on theme: "NC HPS Meeting 10/18-19/2001 Boone, NC Recent Advances in CT Technology and Issues of CT Dosimetry T. Yoshizumi 1,2, M. Sarder 1, R. Reiman 1,2, E. Paulson."— Presentation transcript:
NC HPS Meeting 10/18-19/2001 Boone, NC Recent Advances in CT Technology and Issues of CT Dosimetry T. Yoshizumi 1,2, M. Sarder 1, 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
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 NC HPS Meeting 10/18-19/2001 Boone, NC
1.Recent advances in CT technologies 2.Current radiation safety issues in multi- detector CT 3.Comparison between single-detector and multi- detector CT dosimetry 4.Results and discussions 5.Conclusions Recent Advances in CT Technology and Issues of CT Dosimetry
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.
Faster tube rotation times 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
GE QXi (multi-detector CT) acquires four interweaving helices simultaneously. e.g., 4 x 5 mm slice = 20 mm total scan width 4-slice in one rotation
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) Pitch = 15mm/20 mm =0.75 Pitch = 30mm/20 mm = mm 15 mm table travel 30 mm table travel 20 mm
(high quality: 15 mm table; 5 mm slice; 4x5 mm=20 mm) (high speed 30 mm table; 5 mm slice; 20 mm total width) Pitch (old) P’=0.75 P’=1.5 GE QXi Two scan modes: HQ and HS.
How fast is it?
2. Current radiation safety issues in multi- detector CT Insufficient organ dose data in multi- detector 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)
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 single- and multi-detector CT scanners under protocols that are currently used at our institution. 3. Comparison between single-detector and multi-detector CT dosimetry
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 )Phantoms 32 cm Cylindrical CT Phantom Anthropomorphic RANDO Phantom MATERIALS AND METHODS
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]. 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 QXi KEY DESIGN DIFFERENCES BETWEEN CTI AND QXI TUBE TO CENTER OF ROTATION (COR) DISTANCE
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 EXPERIMENT NO. 1- DOSE PROFILE STUDY
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 4 th slice in Qxi under HQ mode Figures taken from McCollough and Zink: Med Phys 26, TECHNICALLY EQUIVALENT SCAN PARAMETERS
4. RESULTS AND DISCUSSIONS EXPERIMENT NO. 1- DOSE PROFILE STUDY
Design: measure organ doses between single- and multi-detector CT scanners under protocols that are currently used at Duke University EXPERIMENT NO. 2- ORGAN DOSE COMPARISONS WITH PHANTOMS (A) 32 CM CT PHANTOM (B) RANDO PHANTOM
32 CM DIAM. PHANTOM ADULT BODY PROTOCOL 7 TLDs (21 mm) 7 TLD chips 6 inches (15 cm thick) A B Z-axis Center hole CTi Chest(140 kVp, 170 mA, 0.8 sec 7 mm thick, 10 mm spacing, pitch 1.5) 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) QXi Chest/Abdomen/Pelvis – same technique :(140 kVp, 170 mA, 0.8 sec, 5 mm thick, HQ 15mm/rot) EXPERIMENT NO. 2- ORGAN DOSE COMPARISONS WITH 32 CM PHANTOM
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 radiation- related risks. CONCLUSIONS