CT Dose Reduction Techniques

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Presentation Description

CT Dose Reduction Techniques, A practical approach.


By: xh990 (78 month(s) ago)

Hi, i am a researcher of CT physics. Your presentation is very interesting and valuable to me. could you please email me a copy. I'd like to learn it in details. Thank you!

By: jakey39 (69 month(s) ago)

The presentation is now available for download. Pls remember to reference Lauren Smith as the author of this work.


By: kieran19 (81 month(s) ago)

Hi, I'm a medical imaging student doing a paper on radiation dose reduction in paediatric patients. Could you please email me a copy of this powerpoint, if possible. Thank you.

By: jakey39 (69 month(s) ago)

Hi Kieran, sorry for the delay. The presentation is now available for download. Pls remember to reference Lauren Smith as the author of this work.


By: caro15 (87 month(s) ago)

As Clinical specialist radiographer in ireland this presentation is great. I would appreciate a copy for staff training if possible. Many thanks

By: jakey39 (69 month(s) ago)

The presentation is now available for download. Pls remember to reference Lauren Smith as the author of this work.


By: kassyb (89 month(s) ago)

wow, this is excellent. I am a Radiology Educator in a hospital setting and would love a copy to use with staff in a power point format. I have read and read and this is so clear. Liked the pics on isocenter- one of the major errors I see in a high workload enviroment.

By: jakey39 (89 month(s) ago)

sorry for the delay, the author is happy for you to download, as long as you use her name. The file is available as a PowerPoint or Video.

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Presentation Transcript

CT Dose Reduction Techniques : 

CT Dose Reduction Techniques A Practical Approach Lauren Smith Western Health

Outline : 

Outline CT Dose Units Effective Dose Dose Reference Levels CT Dose Optimisation Techniques CT Dose Modulation Bismuth Shielding Breast Shields in Practice Summary

CT Dose Units : 

CT Dose Units CT Dose Index - measures Absorbed Dose in a CT phantom (mGy) CTDIw = CTDI . tissue weighted factors CTDIvol - weighted average of CTDI from within a phantom and corrected for pitch or table increment DLP = CTDIvol (mGy) . L (mGy.cm) Where L = Scan Length Allows us to calculate Dose Effective dose – Estimate of Stochastic Radiation Risk Effective Dose (mSv) = DLP . CF Where CF is the conversion factor from IRCP table Takes Organ Sensitivity weighting factors into account

103 ICRP Tissue Weighting Factors : 

103 ICRP Tissue Weighting Factors ICRRP 103, 2008

Effective Dose Conversion Table : 

Effective Dose Conversion Table Effective Dose = DLP . CF Ref. European Guidelines on Quality Criteria for Computed Tomography EUR 16262, May 1999

CT Radiation Sources : 

CT Radiation Sources CT is 13% of medical x-ray exams, but accounts for 70% of medical dose (Lee, 04) In Australia CT accounts for 50% of all medical radiation dose (06-07) ARPNSA looking at establishing national DRLs From NCRP Report No. 93 US Radiation sources to Population

DRL’s : 

DRL’s Dose Reference Level A reference level of dose likely to be appropriate for average sized patient undergoing medical diagnosis and treatment DRLs allow us to: Compare CT dose in mSv with other Modalities Compare our practice with other centers Realise if we have a certain margin for Optimisation Detect abnormal situations with high radiological risk to the patient

Establishing DRLs : 

Establishing DRLs How Audit dose reports for range of body sizes of each scan type Record DLP and CTDIvol Employ your in house Physicist or Radiation Safety Officer to develop DRLs - third quartile values of CTDIvol and DLP Published DRLs Reference NRPB data survey 1990 ACR Recommendations European Guidelines 16262 ICRP

UK DRL Guide : 

UK DRL Guide Ref. European Guidelines on Quality Criteria for Computed Tomography EUR 16262, May 1999

US Typical Effective Radiation Dose Values : 

US Typical Effective Radiation Dose Values Mayo Clinic, 06

What should we be Doing? : 

What should we be Doing? Archiving Dose Reports Employ Dose Reduction Techniques Ask your Radiologist’s to Accept more Noise in your Images Look at developing your own site related DRL’s

Dose Optimisation Techniques : 

Dose Optimisation Techniques Patient Positioning Scouts kV FOV and Filters Pitch Image Noise Rotation Time Dose Modulation

Patient Positioning : 

Patient Positioning Take the time to position the patient in isocentre Use different tilt Positions when scanning the head Reduces scan Volume Ensure the patient is flat in the Z plane This effects optimal dose modulation

Positioning and Dose Modulation : 

Positioning and Dose Modulation Correct Alignment can reduce dose up to 56% (Banghart, 06) Centered too high = Increased Dose Centered too low = Reduced Dose and Increased breast dose

Tube Position for Scouts : 

Tube Position for Scouts Make sure that tube position is PA when scanning scouts Reference vendor user guide to find out tube home position Kv must be the same for the scout and scan acquisitions for optimal dose modulation

kV : 

kV kV and Dose have an exponential relationship by a factor of 2 Lower kV = better image contrast resolution Generally standardised at 120 kV Try using 100kvp for smaller patients on chest scans Isolated Extremities can be scanned at 80-100kV Cardiac Scan performed at 100kV for patients <180pds Use 80- 100kV for Paediatrics When kV is increased from 120 to 140kV = 39% dose increase

FOV and Filters : 

FOV and Filters Always choose the smallest FOV possible for the body part being examined Use Appropriate Filters provided by vendor Bow tie Filters can reduce skin dose by 50% Reduce noise and Artifact Use Paediatric Filters if Available Post Processing Filters Neuro Cardiac Ref: www.gehealthcare.org

Pitch : 

Pitch Pitch = table increment per rotation /beam collimation Inversely Proportional to patient dose Larger Pitches Lower Radiation Dose Faster Scan times More image Noise Decreased Resolution Paediatric scans should have pitch of 0.9-1.5

Image Noise : 

Image Noise Noise is related to Dose Overcoming Noise Increase MPR Thickness Use Post Processing Filters Use Appropriate Algorithms Ask Radiologists to accept more image noise Phantom A (80 mAs) Phantom B (40 mAs) A B

Rotation Time : 

Rotation Time Rotation Time is related to dose in a linear fashion Trade off with image noise Shorter Rotation Time Advantages Linear decrease in dose Faster scan time Less motion/breathing artifact Use Short Rotation times for Paediatrics 0.4s RT (200mAs) 1s RT (200mAs)

Dose Modulation : 

Dose Modulation Scanner adjusts the Xray tube mA automatically with changes in patient anatomy during the scan and from patient to patient Produces reduced dose scans without image quality compromise Ref. Radiographic Journal ,2006

Advantages of Dose Modulation : 

Advantages of Dose Modulation More consistent signal to detectors Image quality is maintained at a constant level Tube Heat capacity conserved Reduction in (photon starvation) streak artefact Dose Optimisation Dose Reductions from 10-50% Able to set Reference or noise levels Some vendors allow you to cap a max and min mA

Bismuth Shielding : 

Bismuth Shielding Shielding that can be used on in plane MDCT scanning Has been shown to reduce radiation dose to skin and superficial organs without compromising image quality Reduces Primary beam Attenuation Ref. Medscape.com

Bismuth Breast Shielding : 

Bismuth Breast Shielding Used to Reduce unwanted radiation to the breast without degrading image quality Can reduce dose to breast from 43-73% for Thoracic scans Ref. Medscape.com

Breast Shielding In Practice : 

Breast Shielding In Practice Patient Selection All Females of child bearing age ( <50yrs ) Where the anatomical Thorax is being scanned Shield Parameters Attenurad Bismuth Shield 0.06mm Pb 0.675cm offset – applied to each side Covered in plastic for cleaning and reuse

Breast Shielding Protocol : 

Breast Shielding Protocol Shield Placement Top of shield is placed on sternal notch to cover breasts – curve round auxilla Shield is positioned after scouts have been performed

The Resultant Images : 

The Resultant Images

Other Bismuth Applications : 

Other Bismuth Applications Ask your Vendor if Bismuth Shielding is compatible with your scanner Paediatric Breast Shielding Thyroid and Eye Shield Ref. Impactscan.org

Summary : 

Summary Know your CT dose Units Audit CT Doses Archive Dose Reports Think about possible site related DRL’s Review Dose Optimisation Techniques Use Dose Modulation where possible Ask your Radiologists to accept more image noise Use Shielding if available

Any Questions? : 

Any Questions?

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