Outline. Lifetime Attributable Risk 10 mgy in 100,000 exposed persons (BEIR VII 2006) SPECT/CT and PET/CT Dosimetry

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1 SPECT/CT and PET/CT Dosimetry Lifetime Attributable Risk 10 mgy in 100,000 exposed persons (BEIR VII 2006) All Solid Tumors Leukemia Frederic H. Fahey DSc Male Female Male Female Children s Hospital Boston Harvard Medical School Excess Cases frederic.fahey@childrens.harvard.edu Excess Deaths Note: About 45% will contract cancer and 22% will die. Excess Attributable Risk (Deaths) from All Solid Tumors per 10,000 Person-Year-Sv by 60Y (BEIR VII 2006) Age at Exposure (Y) >30 EAR (Mortality) Relative to >30Y Thus, if 1,000, YOs receive 10 msv, 25 will die from solid tumors by age 60 due to this exposure. Outline SPECT Cardiac PET FDG Application of CT Dosimetry Attenuation Correction Anatomical Correlation Diagnosis 1

2 D = à φ/m MIRD Equation Where D is radiation dose in Gy à is the cumulated activity in MBq-h is mean energy per disintegration in g-gy/mbq-h m is mass of the target organ in g Factors Affecting Dose in NM Administered activity Biology Uptake and clearance rates of activity in various source organs (biology) Physics Patient geometry (organ size and orientation) Radioactive decay characteristics Half-life Number, type and energy of emissions Factors Affecting Dose in SPECT Injected activity Total counts and imaging time Choice of camera Crystal thickness (1/4, 3/8, 1/2, 1 ) Number of heads Choice of collimator Hi Sens, Gen Purpose, Hi Res, Ultra-High Res Image processing? Patient Effective Dose (msv) Summary Newborn 1 Year 5 Year 10 Year 15 Year Mass (kg) Tc-MDP (20 mci*) Tc-ECD (20 mci*) I-123 MIBG (10 mci*) Tc-MIBI (20 mci*) Ga-67 (6 mci*) Adult *max admin activ Courtesy of Rad Internal Dose Information Center 2

3 SPECT: Laboratory Throughput SPECT has Established Value for Risk Stratification Rates of Cardiac Death and Myocardial Infarction by Scan Result 99m Tc 8-11 mci 99m Tc mci 60 min Gated Rest 60 min 35 min Ado/Dipy Total admin act = mci 4-6 min 3-44 hr 99m Tc SPECT Gated Stress 30 min Courtesy of Sharmila Dorbala,, MD Brigham and Women s s Hospital * p<0.001 **p<0.01 Event Rate per Year (%) CD MI ** * Scan Result * Normal Mildly Moderately Severely Abnormal Abnormal Abnormal Hachamovitch et al, Circulation 1998 Factors Affecting Dose in PET Injected activity Total counts and imaging time Choice of scanner Crystal material and thickness 2D vs 3D Axial field of view Image processing? PET Imaging Agents 18 F Fluoro-deoxy-glucose (FDG) Practically all clinical PET imaging Oncology Brain and cardiac 82 Ru for cardiac imaging 18 F NaF for bone imaging Other agents (FLT, F-DOPA, F-Miso ) 3

4 Patient Dose from FDG (msv) Patient Dose from 18 F NaF and 82 Rb (msv) Summary Newborn 1 Year 5 Year 10 Year 15 Year Adult Summary 1 Year 5 Year 10 Year 15 Year Adult Mass (kg) Mass (kg) F NaF Act (mci) Act (mci) Bladder* Bladder* Eff Dose* Eff Dose* Rb-1.3m Act (mci) Act (mci) Bladder* Thyroid* Eff Dose* Eff Dose* Courtesy of Rad Internal Dose Information Center *ICRP 80 Pediatric Administered Dose Survey Surveyed 15 dedicated pediatric hospitals in North America Requested information on 16 studies commonly performed in pediatric NM Administered dose per kg Maximum administered dose Minimum administered dose Treves ST, Davis RT, Fahey FH. J Nucl Med, 2008;49: Pediatric Administered Dose Survey Radiopharmaceutical Parameter Number of Minimum Maximum Median Mean Respondents Tc-99m DMSA Activity/Mass MBq/kg (0.030) 3.70 (0.100) 2.22 (0.060) 2.35 (0.064) (0.150) (2.000) (0.500) (0.714) (2.00) (6.000) (5.000) (4.091) Tc-99m MAG3 Activity/Mass MBq/kg (0.050) (0.280) 5.55 (0.150) 5.69 (0.154) (0.500) (4.000) (1.000) (1.458) (3.000) (10.000) (10.000) (7.538) Tc-99m MDP Activity/Mass MBq/kg (0.200) (0.360) (0.300) (0.294) (0.600) (5.000) (2.500) (2.700) (18.000) (25.000) (20.000) (22.154) Tc-99m DISIDA Activity/Mass MBq/kg (0.050) 3.70 (0.100) 2.78 (0.075) 2.97 (0.080) (0.400) (2.000) (1.000) (0.978) (2.500) (10.000) (5.000) (5.423) 4

5 Pediatric Administered Dose Survey Pediatric Administered Dose Survey Radiopharamceutical Parameter Number of Respondents Minimum Maximum Median Mean Radiopharamceutical Parameter Number of Respondents Minimum Maximum Median Mean I-123 MIBG Activity/Mass MBq/kg (0.140) 7.40 (0.200) 5.55 (0.150) 5.45 (0.147) (1.000) (5.000) (1.000) (2.045) (8.000) (10.000) (10.000) (9.731) Tc-99m SestaMIBI Activity/Mass MBq/kg (0.154) (0.500) (0.350) (0.323) (1.000) (14.000) (4.000) (5.700) (10.000) (30.000) (21.000) (21.417) Tc-99m NaTcO4 (Meckels) Activity/Mass MBq/kg (0.044) 5.92 (0.160) 5.18 (0.140) 4.46 (0.121) (0.200) (4.000) (1.000) (1.332) (3.000) (15.000) (10.000) (9.417) Tc-99m MAA Activity/Mass MBq/kg (0.030) 4.88 (0.132) 1.85 (0.050) 2.31 (0.062) (0.200) (1.000) (0.500) (0.578) (3.000) (6.000) (3.750) (3.892) I-123 (NaI) (Thyroid) Activity/Mass MBq/kg (0.0015) 0.22 (0.0060) 0.10 (0.0028) 0.12 (0.0033) (0.0150) (0.3000) 3.70 (0.1000) 3.76 (0.1015) (0.1000) (0.5400) 8.14 (0.2200) 9.45 (0.2555) Tc-99m Ultratag (GI Bleed) Activity/Mass MBq/kg (0.100) (0.300) 8.33 (0.225) 7.92 (0.214) (1.000) (4.000) (2.000) (2.111) (5.000) (20.000) (20.000) (16.636) Tc-99m ECD Tc-99m HMPAO Activity/Mass MBq/kg (0.050) (0.425) (0.285) (0.292) (0.500) (10.000) (5.000) (4.727) (10.000) (30.000) (20.000) (21.917) Tc-99m Denatured RBC (Spleen) Activity/Mass MBq/kg (0.050) 2.59 (0.070) 2.22 (0.060) 2.22 (0.060) (0.500) (2.500) (1.000) (1.083) (2.000) (20.000) (3.000) (5.833) Pediatric Administered Dose Survey Radiopharamceutical Parameter Number of Minimum Maximum Median Mean Respondents Tc-99m Ultratag Activity/Mass MBq/kg (0.200) (0.400) 8.14 (0.220) 9.69 (0.262) (MUGA) (1.200) (10.000) (2.750) (3.570) (15.000) (25.000) (20.000) (19.818) Ga-67 Activity/Mass MBq/kg (0.040) 2.59 (0.070) 1.85 (0.050) 1.90 (0.051) (for Inflammatory Disease) (0.250) (2.000) (0.750) (0.938) (3.000) (5.000) (4.500) (4.167) Ga-67 Activity/Mass MBq/kg (0.080) 5.25 (0.142) 4.07 (0.110) 3.90 (0.105) (for Tumor Imaging) (0.250) (3.000) (1.000) (1.110) (6.000) (10.000) (9.000) (8.458) Variability in Administered Doses in Pediatrics Admin dose/kg and maximum dose varied, on average, by a range of a factor of 3, but by as much as a factor of 10 Minimum dose varied, on average, by a factor of 10 and as much as a factor of 20 F-18 FDG Activity/Mass MBq/kg (0.140) 7.40 (0.200) 5.37 (0.145) 5.67 (0.153) (0.500) (2.000) (1.000) (1.250) (10.000) (15.000) (11.000) (11.625) 5

6 Minimum Administered Dose in Pediatrics Based on clinical task, subjective appearance, tradition and instrumentation May not be based on current state-of-the-art Establishment of universally accepted values Consensus among experts Acquisition of clinical data at variety of dose levels Simulation of clinical studies with noise Use of Image Processing and Reconstruction Filtered backprojection routinely used Newer image processing and reconstruction algorithms may provide adequate diagnostic image quality with fewer counts Lower administered dose or faster imaging times Subjective grading of studies acquired with varying numbers of counts Use of OSEM-3D Reconstruction CT in PET/CT and SPECT/CT FBP Full Cts OSEM Half Cts FBP Full Cts OSEM Half Cts Risk Dosimetry Application CT-Based attenuation correction Anatomical correlation Diagnostic 6

7 CIRS Tissue Equivalent Phantoms Radiation Risk for CT Phantom Newborn AP x Lat (cm) 9 x 10.5 Circum (cm) 32 For 10 & 1 YO, ~ 4X & 7X higher lifetime risk per CT scan, respectively Brenner, Hall, NEJM, 2007;357: Dosimetric CT phantoms Simulated spine Five 1.3 cm holes Five different sizes 1 Year Old 5 Year Old 10 Year Old Med Adult 11.5 x x x x Fahey et al. Radiology 2007;243: Dosimetry of PET-CT and SPECT-CT PET/CT GE Discovery LS SPECT/CT Philips Precedent CTADIvol (mgy) Dose from CT of PET-CT GE Discovery LS (4-slice) CTDIvol (160 m A, 0.8 s, 1.5:1 pitch) Tube Voltage (kvp) New Born 1 Year Old 5 Year Old 10 Year Old Med Adult ED from 14 mci of FDG 11 msv 7

8 Dose from CT of PET-CT GE Discovery LS (4-slice) Dose from CT of SPECT-CT Philips Precedence 6-slice CTDIvol (10 year old, 0.8 s, 1.5:1 pitch) CTADIw for 10 YO, pitch =1: mgy CTADIvol (mgy) kvp 100 kvp 120 kvp 140 kvp CTADI (mgy) kvp 120 kvp 140 kvp mgy Tube Current (ma) mas Dose from CT of SPECT-CT Philips Precedence 6-slice Typical Effective Dose Values CTADIw (mgy) CTADIw for 140 kvp, Pitch = 1: mas Infant Body 1 YO Body 5 YO Body 10 YO Body Med Adult Body Low Dose Chest CT 0.7 msv (70 mrem) Head CT 1-2 msv ( mrem) Chest CT 5-7 msv ( mrem) Abdomen CT 5-7 msv ( mrem) Pelvis CT 3-4 msv ( mrem) Abd & pelvis CT 8-11 msv ( mrem) Average U.S. background radiation 3 msv (300 mrem) 8

9 Estimated Cumulative Radiation Dose from PET/CT in Pediatric Patients with Malignancies A 5 Year Retrospective Review, Soni C. Chawla, MD, UCLA 248 PET-CT studies on 78 patients 50 males, 28 females, YO CT effective dose estimated by ImPACT msv/scan PET effective dose estimated by OLINDA msv/scan PET-CT effective dose msv/scan Average number of scans/patient (range 1-14) PET-CT Attenuation Correction Cumulative effective dose per patient 78.9 msv (range ) 27% of patients received > 100 msv Poster PO71at SPR Meeting Pediat Radiol 2008;38(Suppl 2):S339 PET-CT Attenuation Correction Acquire CT Scan and reconstruct Apply energy transformation Reproject to generate correction matrix Smooth to resolution of PET Apply during reconstruction Applications for very low-dose CT for anatomical correlation Pediatrics MR is correlative imagine modality of choice (e.g. brain imaging) Multiple acquisitions in single imaging session (stress-rest 82 Rb cardiac imaging) 9

10 Quality of CTAC Quality of CTAC 80 kvp 10 ma 0.5 s/rot 1.5:1 140 kvp 160 ma 0.8 s/rot 1.5:1 80 kvp 10 ma 0.5 s/rot 1.5:1 140 kvp 160 ma 0.8 s/rot 1.5:1 Effect of Patient Size 80 kvp, 10 ma, 0.5 s/rotation Accuracy of Attenuation Correction with Patient Size SPECT-CT CTAC Hounsfield to Linaer Attenuation Coefficient Linear Attenuation Coefficient (cm-1) New born 1 YO 5 YO 10 YO 15 YO Small Adult Med Adult Large Adult Linear Attenuation Coefficient (cm -1 ) kvp 140 kvp Hounsfield Units /10/.5 10

11 SPECT-CT CTAC SPECT-CT CTAC CT image reoriented into SPECT coordinates CT image resampled to match SPECT pixel size CT image smoothed to SPECT resolution Conversion from HU to linear attenuation coefficient (piecewise linear) CT Scan (140 kvp, 225 mas) CTAC Map SPECT-CT CTAC SPECT-CT CTAC 90 kvp/22 mas 140 kvp/225 mas 90 kvp/22 mas 140 kvp/225 mas Jaszczak DS Torso Phantom 11

12 CT-Based Attenuation Correction For PET/CT, adequate attenuation correction can be obtain for pediatric patients with acquisition parameters as low as 80 kvp, 10 ma and 0.5 s per rotation. In adults, 120 kvp, 10 ma, 0.8 s. For pediatric patients, if one is only using the CT for attenuation correction, the CT radiation dose can be substantially reduced, by on the order of a factor of 50. For SPECT/CT, adequate attenuation correction can be obtained with parameters as low as 90 kvp, 15 mas for pediatric and adult patients. CT for Anatomical Correlation In some cases, CT only needed for anatomical correlation Potential for large reduction in dose in these cases How much less? Clinical cases acquired with less dose Noise simulation Low Dose CT for PET-CT Previous conventional imaging at standard pediatric CT technique 80 kvp mas No missed diagnoses if PET +ve or in follow-up PET abnormality more important finding than CT localization Courtesy of Helen Nadal, British Columbia Children s Conventional CT kvp 90 mas 175 PET CT kvp 80 mas 35 Dose reduction by factor of 6! 12

13 Non avid Pulmonary nodule PET/CT Dedicated CT CT for Diagnosis Use of contrast material Respiration Breath hold Gating Axial extent Axial Extent of CT Whole Body PET typically acquired Eyes to Thighs Potential for SPECT acquisitions to all be extended, particularly with more efficient reconstruction Thus CT component can be combination of head & neck, thoracic, abdominal and pelvic CT One size fits all Different Paradigm for PET/CT and SPECT/CT Suggested by George Segall (Stanford) Limit high-quality CT to area of interest, i.e. area traditionally imaged in CT Very low-dose CT for remainder of PET/SPECT axial field of view If necessary, acquire additional high-quality CT in areas indicated on PET/SPECT 13

14 Summary SPECT and PET Increasing use, particularly cardiac SPECT and FDG tumor imaging Evaluation of pediatric dosimetry Use of image processing CT Large reduction in dose if only used for attenaution correction Possible reduction in dose for anatomical correlation For diagnosis, consider new acquisition paradigms 14