HISTORY. CT Physics with an Emphasis on Application in Thoracic and Cardiac Imaging SUNDAY. Shawn D. Teague, MD

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1 CT Physics with an Emphasis on Application in Thoracic and Cardiac Imaging Shawn D. Teague, MD DISCLOSURES 3DR- advisory committee CT PHYSICS WITH AN EMPHASIS ON APPLICATION IN THORACIC AND CARDIAC IMAGING Shawn D. Teague, MD Associate Professor of Clinical Radiology and Imaging Sciences Chief of Cardiothoracic Radiology Indiana University School of Medicine MAINTENANCE OF CERTIFICATION LEARNING OBJECTIVES Compare and contrast the 4 generations of CT scanners Part 1: Evidence of Professional Standing Part 2: Lifelong Learning and Self-assessment Part 3: Cognitive Expertise 20% nonintrepretative- physics 80% clinical content Part 4: Practice Quality Improvement (PQI) Explain stochastic effects List the relationship between kvp, mas, and slice thickness on image quality. HISTORY CT SCANNERS mathematical principals of image reconstruction related to Radon st CT scanner Sir Godfrey N Hounsfield and Allan M Cormack awarded Nobel Prize- development of computer assisted tomography Helical single slice Dual-slice Quad-slice slice slice slice /320-slice slice 156

2 FIRST GENERATION EMI Mark I- first commercial scanner slice scanner pencil beam x-ray to single detector Straight-line motion of both x-ray source and detector (translate) then rotate 1 o then translate back 180 projections 4.5 min for a single scan (1 slice) Resolution 3 mm in plane and 13 mm slice thickness FIRST GENERATION EMI Mark I- first commercial scanner slice scanner pencil beam x-ray to single detector Straight-line motion of both x-ray source and detector (translate) then rotate 1 o then translate back 180 projections 4.5 min for a single scan (1 slice) Resolution 3 mm in plane and 13 mm slice thickness FIRST GENERATION EMI Mark I- first commercial scanner slice scanner pencil beam x-ray to single detector Straight-line motion of both x-ray source and detector (translate) then rotate 1 o then translate back 180 projections 4.5 min for a single scan (1 slice) Resolution 3 mm in plane and 13 mm slice thickness FIRST GENERATION EMI Mark I- first commercial scanner slice scanner pencil beam x-ray to single detector Straight-line motion of both x-ray source and detector (translate) then rotate 1 o then translate back 180 projections 4.5 min for a single scan (1 slice) Resolution 3 mm in plane and 13 mm slice thickness SECOND GENERATION Added multiple detectors- up to 53- each a different projection Rotate more degrees between each translation Allowed imaging of the trunk because speed fast enough to allow breath hold Issues with rotating anode x-ray tubes due to translate-rotatestationary anode x-ray tube Issue with thicker body parts Resulted in higher radiation to patient because of focal asymmetric focal spot SECOND GENERATION Added multiple detectors- up to 53- each a different projection Rotate more degrees between each translation Allowed imaging of the trunk because speed fast enough to allow breath hold Issues with rotating anode x-ray tubes due to translate-rotatestationary anode x-ray tube Issue with thicker body parts Resulted in higher radiation to patient because of focal asymmetric focal spot 157

3 SECOND GENERATION Added multiple detectors- up to 53- each a different projection Rotate more degrees between each translation Allowed imaging of the trunk because speed fast enough to allow breath hold Issues with rotating anode x-ray tubes due to translate-rotatestationary anode x-ray tube Issue with thicker body parts Resulted in higher radiation to patient because of focal asymmetric focal spot SECOND GENERATION Added multiple detectors- up to 53- each a different projection Rotate more degrees between each translation Allowed imaging of the trunk because speed fast enough to allow breath hold Issues with rotating anode x-ray tubes due to translate-rotatestationary anode x-ray tube Issue with thicker body parts Resulted in higher radiation to patient because of focal asymmetric focal spot THIRD GENERATION Fan beam x-ray with arc of detectors Tube and detectors rotate No translate only rotate so can use higher-output rotating anodes- decreased scan time Beam divergence- scan an additional arc of one fan angle beyond 180 o All modern scanners THIRD GENERATION Fan beam x-ray with arc of detectors Tube and detectors rotate No translate only rotate so can use higher-output rotating anodes- decreased scan time Beam divergence- scan an additional arc of one fan angle beyond 180 o All modern scanners HELICAL ~1990 Allowed acquisition in single breath hold Technical developments Slip-ring gantry- electrical conductive rings and brushes that conduct electric across moving surface Very high power x-ray tubes Interpolation algorithms to handle non-coplanar projection data (helix) Willi Kalender, PhD FOURTH GENERATION Only the source rotates inside stationary ring of detectors X-ray source rotates between patient and detector ring One design- tube outside the detector ring which wobbled as the tube moved Smaller detector ring with fewer detectors Limitations Less efficient use of detectors Scatter artifact since can not use antiscatter collimators 158

4 FOURTH GENERATION Only the source rotates inside stationary ring of detectors X-ray source rotates between patient and detector ring One design- tube outside the detector ring which wobbled as the tube moved Smaller detector ring with fewer detectors Limitations Less efficient use of detectors Scatter artifact since can not use antiscatter collimators RADIATION RISK POPULATION RADIATION RISK Est 6800 future cancers from Chest CT scans performed in % of all future cancer in US result of radiation from CT scans million by 2007 > 70 million CT scan use grew at 10 times US population growth % of CT scans included the chest Thyroid, breast, and lungs highly cancer-susceptible organs RADIATION EFFECT ON HUMANS Deterministic- threshold below which effect will not occur Acute high dose- >100 msv- radiation oncology Widespread cell death Dermatitis, pneumonitis, pulmonary fibrosis, GI illness Not occur with medical imaging Stochastic- linear no threshold Damage from low doses Unpredictable and random Long latency- 5 to 20 yrs Most significant is cancer Younger females at greatest risk RISK OF CANCER Linear, no-threshold model Dose and Cancer risk relationship is linear Some risk even at minimal doses Incremental increased risk with each exposure Atomic bomb survivors and nuclear power plant workers Organ doses msv = increased cancer risk Mean for atomic bomb survivors was 40 msv LUNG CANCER RISK Atomic bomb survivors Older age (50-60) at time of exposure- increased respiratory tract cancer Contrast to other cancers Synergistic carcinogenic effect of radiation and tobacco smoke? lung cancer screening? 159

5 SUNDAY INDIVIDUAL DOSE Dose Quality IMAGE QUALITY IMAGE QUALITY Quality ~ Signal/Noise Noise- manufacturer dependent Hardware Efficient detector Electronic shielding Smaller electronic devices Software Filters applied when making the source axial images Signal Hardware- efficient detectors Technique kvp, mas, pitch kvp Decreasing 120 kvp to 80 kvp Mean photon energy 66 kev to 52 kev Improves contrast resolution For calcium (Z=20) and iodine (Z=53) Increases HU due to higher absorption near the k-edge kvp EFFECT mas Linear relationship between mas, signal, and patient dose Noise increase 1/ (mas) 50% reduction in mas = 41% increase in noise 80 kvp HU = kvp HU = kvp HU =

6 mas Linear relationship between mas, signal, and patient dose DETECTOR SIZE- EFFECT SMALLER DETECTOR Noise increase 1/ (mas) 50% reduction in mas = 41% increase in noise DETECTOR SIZE- EFFECT SMALLER DETECTOR DETECTOR SIZE- EFFECT SMALLER DETECTOR DETECTOR SIZE- EFFECT INCREASE mas DETECTOR SIZE- EFFECT INCREASE mas 161

7 DETECTOR SIZE- EFFECT INCREASE mas DETECTOR SIZE- EFFECT THICKER SLICES DETECTOR SIZE- EFFECT THICKER SLICES DETECTOR SIZE- EFFECT THICKER SLICES Single Cycle Full Rotation Single Cycle Half Rotation Single Cycle Full Rotation Single Cycle Half Rotation 360 o From single detector 180 o 360 o From single detector 180 o TR=330 ms TR=165 ms TR=330 ms TR=165 ms 162

8 Single Cycle Full Rotation Single Cycle Half Rotation Multi-Cycle Reconstruction From two detectors 360 o From single detector 180 o 90 o 90 o TR=83 ms TR=330 ms TR=165 ms Multi-Cycle Reconstruction Multi-Cycle Reconstruction From two detectors From two detectors 90 o 90 o TR=83 ms 90 o 90 o TR=83 ms Multi-Cycle Reconstruction Multi-Cycle Reconstruction From two detectors From two detectors 60 o 120 o TR=110 ms 60 o 120 o TR=110 ms 163

9 Multi-Cycle Reconstruction MULTI-CYCLE RECONSTRUCTION Single Cycle Half Rotation From two detectors 60 o 120 o TR=110 ms Multi-Cycle MULTI-CYCLE RECONSTRUCTION MULTI-CYCLE RECONSTRUCTION Single Cycle Half Rotation Single Cycle Half Rotation Multi-Cycle Multi-Cycle 2 X-ray tubes 2 X-ray tubes From two detector array From two detector array 90 o 90 o TR=83 ms 90 o 90 o TR=83 ms 164

10 PITCH- EFFECT ON PENUMBRA PITCH- EFFECT ON PENUMBRA 20 cm scan, Pitch=1, 64 Channel (4 cm detector width) = 5 rotations 20 cm scan, Pitch=1, 64 Channel (4 cm detector width) = 5 rotations 20 cm scan, Pitch=0.2, 64 Channel (4 cm detector width) = 25 rotations 20 cm scan, Pitch=0.2, 64 Channel (4 cm detector width) = 25 rotations PITCH- EFFECT ON PENUMBRA PITCH & TIME OF EXPOSURE EFFECT ON SIGNAL Pitch = 1 Pitch = cm scan, Pitch=1, 64 Channel (4 cm detector width) = 5 rotations Pitch = cm scan, Pitch=0.2, 64 Channel (4 cm detector width) = 25 rotations PITCH & TIME OF EXPOSURE EFFECT ON SIGNAL Pitch = 1 Pitch = 0.5 PITCH & TIME OF EXPOSURE EFFECT ON SIGNAL Pitch = 1 Pitch = 0.5 Pitch = 0.2 Pitch =

11 PITCH & TIME OF EXPOSURE EFFECT ON SIGNAL Pitch = 1 PITCH & TIME OF EXPOSURE EFFECT ON SIGNAL Pitch = 1 Pitch = 0.5 Pitch = 0.5 Pitch = 0.2 Pitch = 0.2 MDCT DOSE EFFICIENCY 20 cm scan, Pitch=1, 4 Channel x 5mm MDCT DOSE EFFICIENCY 20 cm scan, Pitch=1, 64 Channel x 0.625mm 4 channel (20 mm) 20 cm scan, Pitch=1, 64 Channel x 0.625mm 64 channel (40 mm) 20 cm scan, Pitch=1, 128 Channel x 0.625mm 64 channel (40 mm) 128 channel (80 mm) RECONSTRUCTION Filtered back projection Rays are collected into sets called projections Back projection results in blurry image Filtering is applied first to remove the blurring Result in noisy images ITERATIVE RECONSTRUCTION Initial RAW DATA Compare New Final Voxel attenuation values are scaled to integers and normalized to water Water and air HU values are constant Other materials HU value will vary based on x-ray tube potential and from manufacturer to manufacturer RAW DATA Current Correction New Final Compare Compare Correction Correction 166

12 FACTORS AFFECTING DOSE DOSE Beam energy Higher the beam energy the higher the dose Beam filtration Different materials Different shapes- variable thickness Collimation Pre-patient- confine beam to section thickness Post-patient- reduce scatter and improve contrast resolution Number and spacing of adjacent sections Better resolution more dose Pitch with helical CT has little impact on dose quality and noise- noise and dose are inversely related POPULATION DOSE PATIENT DOSE CTDI vol 16 and 32 cm diameter phantoms for head and body Effective Dose DLP = CTDI vol x scan length msv= DLP x k-factor k-factor (0.014 or chest) k-factor (~0.03 cardiac) CTDI vol 16 and 32 cm diameter phantoms for head and body Size Specific Dose Estimate Lat + AP Effective Factor Lat Effective Factor AP Effective Factor Effective Factor AAPM Report 204 PATIENT DOSE CTDI vol 16 and 32 cm diameter phantoms for head and body Size Specific Dose Estimate Lat + AP Effective Factor Lat Should Effective not be Factor combined AP with Effective Factor k-factor to calculate msv in an individual patient Effective Factor THANK YOU! sdteague@iupui.edu AAPM Report