Role of Adaptive Statistical Iterative Reconstruction (ASIR) in lowering radiation dose for pediatric head CT Electronic Poster-eP 140 Azadeh Tabari, MD Dhruv Singh, Sarabjeet Singh, MD, MMST Michael S. Gee, MD, Sandra Rincon, MD Paul Caruso, MD

Ethical Committee Approval Financial Disclosure None of the authors had any financial disclosures pertinent to this study Ethical Committee Approval The study was approved by the IRB and was compliant with HIPPA guidelines

Background Iterative reconstruction has recently shown promising results for substantial CT dose reduction. However, there are concerns about image quality at very low dose levels and careful selection of settings of the Adaptive Statistical Iterative Reconstruction (ASIR) is recommend to achieve optimal image quality.

Background Filtered Back Projection {FBP} used since 1970’s with advantages Speed Closed loop solution Ignores that fact that projection data is corrupted by quantum and electronic noise Prorogates this noise and sometimes even amplify ASIR Takes into account these inconsistencies Lower image noise and improves quality

Basics of Filtered “Back” Projection Tube Detectors Patient X-rays from one angle are recorded at detectors as measured projection data As the tube rotates, x-rays from different source locations or “angles” results in a “set” of projection-data Set of projection-data is called the sinogram

“filtered” back projection Acquired sinogram data are back projected to obtain 2D image Results in dense center and blurred periphery Hence the need to filter/preprocess the images Projection 3

Image reconstruction Step 1: Data Acquisition Step 2: Reconstruction One angle –one projection Step 1: Data Acquisition Different angles-set of projections (Sinogram) Step 2: Reconstruction FBP Iterative Reconstruction Better defined mathematical estimates Filtration Back projection Multiple iteration Final image Final image

FBP ignores actual dimensions ASIR accounts for actual dimensions FBP ignores actual dimensions X-ray source/focal spot: assumed to be infinitely small Actual x-ray source/focal spot: 1 mm * 1 mm Image voxel: Depends on FOV and slice thickness E.g. for 50 cm FOV and 1 mm slice thickness, Reconstructed voxel size is 0.98*0.98*1 mm Image voxel size ignored Photon interaction assumed To be at exact geometric center of detector For 1mm detector cell spacing: Active detector area is 80% of spacing 8

Purpose The purpose of this study was to assess the potential benefit of ASIR in low dose pediatric head CT by comparing image quality with standard dose filtered back projection (FBP).

Materials & Methods Institutional review board approval was obtained for this study and HIPPA guidelines were followed. Study cohort was selected as all consecutive pediatric head CT performed on Discovery 750 HD (GE Healthcare) with ASIR 90%.

FBP and ASIR “Blending” interface ASIR and FBP can be selected based on desired reconstruction ASIR can be used as increments of 10% ASIR review tool is available to select the desired levels of blending Presently ASIR 30-50% is recommended by the vendor

Materials & Methods For comparison, pediatric head CT examinations performed on scanners with standard Filtered Back Projection (FBP) was included. Patient demographics, including maximum skin-to-skin transverse head diameter scanner information {mA, kVp} as well as radiation dose information were recorded.

Materials & Methods Effective dose was calculated as per the ICRP103 guidelines. Image quality was assessed my measuring image noise as standard deviation of HU values as well as Contrast to Noise Ratio {CNR} for grey white matter. Statistical analysis was performed with student t-test.

Results Fifty-six children {average age 12.0 ± 4.0, M: F 32:24} underwent head CT examinations with ASIR 90 enabled protocol as compared to 82 head CT {average age 12.7 ± 4.7, M: F 46:36} with standard FBP reconstruction. There was no significant difference in head diameter between ASIR90% {166.6 ± 10.5 mm} and FBP {170.5 ± 22.6 mm} (p = 0.07).

Results However, there was significant reduction of radiation dose of 52% with ASIR90% {CTDIvol 14.8 ± 10.4, DLP 272.6 ± 217.9 mGy.cm, 0.5 ± 0.4 mSv}, as opposed to FBP {CTDIvol 31.1 ± 17.4, DLP 593.6 ± 376.9 mGy.cm, 1.2 ± 0.7 mSv} (p <0.001). Furthermore, objective image noise measure in white and grey matter with ASIR90% was found to be similar as FBP (p = 0.5-0.6). In addition, contrast to noise ratio {CNR} was not affected with ASIR90%, when compared to FBP (p = 0.67).

14 yr girl with ischemic stroke and known infarct; hypodensity seen left caudate lobe, putamen, globus pallidus Prior scan: (FBP) Follow up: (ASIR) Scan parameters 120 kVp 140 mA Pitch: 0.531 ASIR:90 Estimated dose: CTDIvol: 26.3 mGy DLP: 476.8 mGy.cm 0.9 mSv Scan parameters 120 kVp 290 mA

18yr old boy, s/p hemicraniotomy and right MCA infarct Prior scan: (FBP) Follow up: (ASIR) Scan parameters 120 kVp 308 mA NI: 35 Estimated dose: CTDIvol: 52.1 mGy DLP: 1114.6 mGy.cm 2.2 mSv Scan parameters 80 kVp 100 mA Pitch: 0.531 ASIR:100 Estimated dose: CTDIvol: 5.4 mGy DLP: 103.5 mGy.cm 0.2 mSv

18yr old boy, with subarachnoid blood along the b/l frontal convexities and small SDH along the left frontotemporal convexity Prior scan: (FBP) Follow up: (ASIR) Scan parameters 120 kVp 159 mA, NI: 42 Pitch: 0.531 ASIR:90 Estimated dose: CTDIvol: 25.4 mGy DLP: 459.8 mGy.cm 0.9 mSv Scan parameters 120 kVp 231 mA Estimated dose: CTDIvol: 65.6 mGy DLP: 1223.6 mGy.cm 2.4 mSv

Conclusion Pediatric head CT could be acquired at 52% lower radiation dose with Adaptive Statistical Iterative Reconstruction {ASIR90%} without affecting image noise and contrast to noise ratio.

Thank you ssingh6@mgh.harvard.edu