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Description : Radiological procedure and preparations

Tags : Radiology,medical imaging,ct scan

Published on : Mar 09, 2022 | Views : 3097 | Downloads : 3

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Slide 1-

James M. Kofler, Ph.D. Mayo Clinic Rochester, Minnesota Optimizing CT Image Protocols With Respect To Image Quality and Radiation Dose AAPM Annual Meeting, July 2008 Dianna D. Cody, Ph.D. U.T.M.D. Anderson Cancer Center Houston, Texas Scan Protocols Influence the image quality and radiation dose of EVERY CT scan Provide consistency within and among scanners Especially important in longitudinal exams And in clinics with many technologists Improves throughput and tech efficiency Should include all instructions to complete exam Where to Begin? New Protocol Use manufacturer’s suggested protocol Model after existing similar protocol Literature review for guidelines Ask your colleagues to share theirs Existing Protocol Determine SPECIFIC weakness of protocol Poor contrast, too noisy, dose seems high, etc. Consult with radiologist All protocol decisions must consider clinical task Single breath-hold (<15 seconds) Less patient motion Especially peds, ER patients Scan time also affects contrast timing Breathing motion in upper portion of image Major Clinical Considerations Need short scan time
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Major Clinical Considerations Need high spatial resolution Major Clinical Considerations Need good low contrast resolution Major Clinical Considerations Radiation Dose - Should be as low as possible without sacrificing diagnostic content. - Dose “ceilings” are now used as pass/fail criteria for ACR CT accreditation. - CTDIvol and DLP displayed on scanner console. Major Clinical Scan Parameters Tube rotation time mA Pitch kVp Image thickness Detector configuration Reconstruction kernel/algorithm Patient size-dependent techniques
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Tube Rotation Time Affects Total scan time (proportional) Noise / Low contrast resolution Dose (proportional) Generally want to minimize rotation time What to look out for… IV contrast timing may need adjustment mA needed may exceed tube/generator limits Tube Rotation Time Example: Limits are reduced by tube housing heating mA Affects Noise / Low contrast resolution Dose (proportional) What to look out for - mA near tube/generator limits can be problematic (especially when dose modulation is used) Pitch Affects Total scan time Noise / Low contrast resolution Dose What to look out for Pitches >1 may increase image thickness (vendor-specific) Pitches >1 may require mA to be increased near limits
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Pitch: 0.562 CTDIvol: 162 mGy Variable pitch. All other parameters constant. Pitch CTDIvol 0.562 162 Pitch Pitch: 0.938 CTDIvol: 97 mGy Variable pitch. All other parameters constant. Pitch CTDIvol 0.562 162 0.938 97 Pitch Pitch: 1.375 CTDIvol: 66 mGy Variable pitch. All other parameters constant. Pitch CTDIvol 0.562 162 0.938 97 1.375 66 Pitch Pitch: 1.75 CTDIvol: 52 mGy Variable pitch. All other parameters constant. Pitch CTDIvol 0.562 162 0.938 97 1.375 66 1.75 52 Pitch
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mA·s pitch Effective mAs = Terminology: Effective mAs Same Eff. mAs => comparable image quality VERY helpful to achieve uniform IQ across different scanners/platforms Typical targets (average size patients): Chest “ 180 eff. mAs Abd “ 200 mAs Pitch, Rotation Time, mAs Eff mAs = 280 Rotn time: 0.5s, Pitch: 0.8 Total scan time: 20s Want scan time to be 15s Maybe use p=1.0 (scan time=16s)? How about rotn time=0.33, p=0.6? Gives scan time=17.6s Change pitch to 1.1 (scan time=14.5s) But max eff. mAs=264 (need 280) kVp Affects Noise / Low contrast resolution Dose What to look out for… Low kVp may require mA values to exceed limits Confirm scanner is calibrated for proposed kVp Set mA by matching noise using a phantom Increasing kVp may be helpful for abdominal studies in large patients 140 kV 80 kV Mean: 112 o: 20 Mean: 243 o: 21 Mean: -98 o: 18 Mean: 273 o: 38 Mean: 483 o: 48 Mean: -108 o: 40 mAs=82 mAs=240 All other parameters are identical kVp
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Image Thickness Affects Noise / Low contrast resolution Dose (?) What to look out for… - Potential to dramatically increase mA (and dose) to compensate for increased noise with thinner images Noise  # Photons Better z-resolution (less partial vol. averaging) Increased image noise Potential for increased radiation dose 1 Image Thickness Image (mm): 10 5 2.5 1.25 Thinner slices => less partial volume effect Noise (HU): 2.93 3.84 5.89 7.82 All other parameters are identical Image Thickness 10mm image thickness All other parameters are identical Image Thickness
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5mm image thickness All other parameters are identical Image Thickness 2mm image thickness All other parameters are identical Image Thickness 1mm image thickness All other parameters are identical Image Thickness 0.6mm image thickness All other parameters are identical Image Thickness
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Detector Configuration Affects Total scan time Noise / Low contrast resolution Thinnest available recons Dose What to look out for… Recommend using thinnest channel widths possible for best IQ Some configurations (esp. narrow collimations) are less dose efficient (vendor-specific) Compare relative dose using CTDIvol on console Multi-slice Detectors * Doesn’t consider recons, not all available in helical 8 x 2.5 16 x 1.25 4 x 5 2 x 7.5 8 x 1.25 2 x 5 1 x 1.25 2 x 0.63 Number of slices x Image thickness 4 x 3.75 16 x 0.63 4 x 2.5 1 x 2.5 Detector Configurations - Many Options* Prospective images at 5mm Scanner: 16-channel Detector: 8 x 2.5 Pitch = 0.875 Retrospective images at 2.5mm Detector Configuration
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Same as patient study Pitch: 0.875, Detector: 8×2.5mm, Beam: 20mm SE 2, IM 2, 5mm SE 3, IM 3, 2.5mm Detector Configuration Change detector (incr. Z sampling), retain beam width Pitch: 1.375, Detector: 16×1.25mm, Beam: 20mm Effective mAs = 109 (decreased from 171) SE 10, IM 2, 5mm SE 11, IM 3, 2.5mm Detector Configuration Detector (output channel) size should be less than thinnest retro desired. Beam width may change with detector configuration. Changes in beam width and/or pitch will affect total scan acquisition time. Narrow collimations => less scatter, but less dose efficient. Compare relative dose using CTDIvol on console. Z-axis Sampling Summary “Wasted” radiation—contributes to dose only Multi-Slice and Dose MS dose is dependent on detector configuration Larger proportion of small beam is wasted!
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Affects Noise / Low contrast resolution Spatial resolution What to look out for… - Kernels/algorithms can have obvious-to-subtle differences—get consensus from radiologists. Reprocessing using different kernel is FREE (no dose cost) Kernel/Algorithm Both noise and frequency content affect “image quality” Kernel/Algorithm Both noise and frequency content affect “image quality” Kernel/Algorithm Technique Charts Pre-determined techniques based on patient size. Physicist needs to construct chart. Tech must measure patient size and manually enter technique. Not as eloquent or efficient as modulation but can save significant dose. Dose Modulation Technique determined automatically based on reference level of noise or image quality. Can reduce dose in x,y and z-directions (small pts). Patient Size-Dependent Techniques
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Technique Charts Mayo “imaging” HVLs Abd/Pelvis: 10 cm Chest: 13 cm Use known relationships to predict the mAs required to keep image noise/quality constant as thickness changed CT Technique Chart Example “AEC” approach for CT Final “scout” view typically used to determine mA modulation For SMALL patients can result in dose DECREASE (peds) For LARGE patients can result in dose INCREASE Tube Current Modulation Tube Current Modulation What to look out for… - Final scout acquired is typically used to assess size and should included entire scan area P-A instead of A-P for ALL patients If scout not adequate, repeat (scout dose “ 1-5 chest x-rays << spiral acquisition) - Patient centering is CRITICAL
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- Set Quality Reference mAs (vendor-specific) Build in scanner using appropriate base protocol - Calculating delivered dose challenging Changes per image and during tube rotation Can used exam-averaged mAs for dose estimate What to look out for… - Set mA FLOOR and mA CEILING (vendor-specific) Min. mA too low can cause high noise Max. mA too high can cause scary dose Tube Current Modulation Who should be involved? Medical Physicist: Technical issues Radiologist: Clinical issues Technologist: Implementation issues Others to consult… - Nurses, Schedulers, Billing, Vendor Apps, etc. Protocol Development The Physicist Assess which parameter(s) address the weakness of the protocol Provide options for optimizing the protocol (including minimizing dose and compromises to other parameters) The Technologist Provides their perspective on impact of implementation (workflow, patient issues, staff issues, etc.) Verifies settings in scanner The Radiologist - Provides their perspective on impact of implementation (workflow, patient issues, staff issues, etc.) Planning Case-by-case, with radiologist review after each case. - Ideally, get consensus of radiologists. If changes unacceptable, repeat planning phase. If changes acceptable… Change scanner program. Change written protocol. Notify all techs & radiologists of major changes. Document changes, justifications, and people involved. Clinical Evaluation and Implementation
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Example: One chest acquisition on 64-channel scanner 5mm transverse images 2.5mm transverse images 0.625mm images used for coronal & sagittal reformats 0.625mm images spaced at 10mm for high res General Tips: Watch for ‘Two-fers’ Become more savvy about using a dense helical data set for more than one purpose. Does this seem reasonable to you? Example Acquisition 120 kVp 64 x 0.625mm, pitch 0.938 0.4 sec per rotation 500 mA Construct 0.625mm images every 20mm General Tips: Watch for Oversights 213 eff. mAs (reasonable) For 40cm scan, 97% dose WASTED General Tips: Watch for Oversights Example Acquisition 120 kVp 64 x 0.625mm, pitch 0.938 0.4 sec per rotation 500 mA Construct 0.625mm images every 20mm - Details of interest are smaller in children, so greater CNR required - Radiologists are accustomed to “reading through the noise” on large patients - Radiologists require higher image quality in children to ensure high diagnostic confidence Equal noise is not the clinical ideal, because … - Children don’t have the fat planes between tissues and organs that adults do Special Cases: Pediatric
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Approach Scale down from a standard adult technique Adjust by ratio of image thickness for adult vs peds Tweak as necessary after review What to look out for… Want shortest possible scan time (kids squirm) Build scanner protocol using pediatric base (if available) Special Cases: Pediatric Pediatric Example Case: 6 year old, reduced-technique adult protocol Prescribed Quality Reference mAs : 165 Average mAs used: 38 Pediatric Example Case: 6 year old, reduced-technique adult protocol Prescribed Quality Reference mAs : 165 Average mAs used: 38 65mAs 28mAs 39mAs Great Resource www.ImageGently.com
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Growing issue across USA At MDA: ~ 50% ‘large,’ ~ 30% ‘average,’ ~ 20% ‘small’ Challenge to cross-sectional imaging Obligated to deliver diagnostic images First Considerations Is table safe for heavy patient (load limit)? Can patient fit into gantry? Can staff get patient on table? Special Cases: Heavy Patients Approach (prioritize according to clinical task) (1) Increase ceiling level on current modulation protocols. (2) Quality Reference mAs remains unchanged. Increase tube rotation time. Decrease pitch. Use larger collimation (e.g., 32x0.6 => 24x1.2) then decrease pitch. Increase kVp. May only need some options—listen to your scanner! Special Cases: Heavy Patients Hoping for adequate, not exquisite, images. Post-acquisition options Recon using a smoother kernel (or special kernels, if available). Recon to thicker images. Reprocessing is FREE (no dose cost) Special Cases: Heavy Patients 600 eff mAs GE VCT DFOV 48cm 120 kV 740 mA 0.8 sec/rotn Pitch .984 Special Cases: Heavy Patients
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Siemens Sens-64 DFOV 50cm 140 kV 334 mA 1.0 sec/rotn Pitch 0.5 10mm images 668 eff mAs Special Cases: Heavy Patients At MDA (and Mayo), we set up ‘average’ patient protocols At MDA, a ‘large patient’ duplicate set Patients who require S 42cm DFOV Increase eff. mAs by 30% At Mayo, a “bariatric” version and steps for heavy (but non-bariatric) patients Combo Protocol & Technique Chart Thinner images prospectively Thicker images built from reformats Reformat into sagittal and/or coronal planes Scan with higher kVp More photons produced at same mAs Photons are more penetrating Special Cases: Metal Implants Prospective axial series: 140kV 265 mA, 0.5 sec, pitch 1.5 Effective mAs = 88 mAs 2.5 mm image thickness 4 x 2.5 mm detector config. Metal Implants Example: Hip
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Special Cases: Metal Implants Cannot completely eliminate artifact Increasing mAs (dose) has diminishing returns Dose modulation should behave properly (i.e., not automatically max-out in metal)