Dose Survey in Computed Tomography

DS /CM

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IAEA/RCA Kampala

Dose Surveys (Have you seen this slide?)

• Measure doses to patients.

• Compare measured doses to standards

• Decide if action is required

• Begin again

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Dose Survey – What to think about

• What should be Surveyed– Examinations– Equipment– Patients

• Data collection• Data analysis• Reporting• Investigating

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Surveys: Planning

• Identify scanners to be surveyed

• Identify procedures carried out in those rooms that you wish to survey

• Decide how to obtain Dose information

• Submit data for reference values

• OR compare against reference values

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Planning a dose Survey in CT

• Examinations• What examinations are you going to

survey?

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Criteria for inclusion

• Examinations must be performed reasonably frequently in your hospital.

• Data collection must be feasible

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Your Presentations

• Brain• Lung• Lumbar Spine• Abdomen

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Relative Contributions – 5 Scanners

Head Body Other N

NWIP 52% 40% 8% 1201

NWOP 24% 69% 7% 757

WGH 38% 58% 4% 1110

STJ 40% 54% 6% 866

DCN 85% 6% 9% 968

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Dundee Exams

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Edinburgh Exams

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An issue for PACS / RIS based surveys

• What are all these codes?

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There are a large number of CT exams

• Anatomical Groups / Protocol Types– Head– Abdomen– Chest– Chest & Pelvis– Angiography– Choice of correct

exam important

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Choice of exams - IAEA

• “In order to allow comparison of truly similar examinations conducted for similar purpose and requiring similar scan technique, you should specify detailed descriptions of CT procedures, including a clinical indication (such as CT abdomen in relation to liver metastases), rather than simply broad categories of examination (such as CT abdomen).”

• You must compare like with like, or as near as possible !! (This can be difficult / even impossible)

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Planning a dose Survey in CT

• Patient Selection

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Patient selection

• Standard sized patients

• Define weight range, or take everyone (cf Australia), but exclude very large and very small

• Make measurements / collect data for at least 10 patients – minimum of 20 preferable – more if no weight constraint.

• RIS enables thousands to be selected.Kampala

Dose Survey – What to think about

• What should be Surveyed– Examinations– Equipment– Patients

• Data collection• Data analysis• Reporting• Investigating

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Planning a dose Survey in CT

• How are you going to collect the dose data ?

• What Physical Quantities are you going to record?

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Dose Length Product & CTDIvol

• Are the dose related quantity measured and displayed on all modern CT scanners.

• Stored on DICOM Header • Can be transcribed to RIS system• Can be looked up on PACS using DICOM

query retrieve interrogation.• Can be calculated, not going to show you

how – see TRS 457.

Data Collection – What quantity

• DLP• CTDIvol• What are the relative merits of each?

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Data Collection – What quantity• CTDIvol – In My Opinion CTDIvol is best for

optimising your protocol• DLP – In My Opinion, DLP will tell you about

variations introduced by different patients and is therefore best for dose audit.

• DLP is analagous to KAP in conventional radiology.

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Dose Summary Sheet - Commercial CT Scanner

Huda W , Mettler F A Radiology 2011;258:236-242

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Patient dose Survey in CT

• common scans• patient weight, sex• collect

– CTDIvol total DLP,

• (calculate exam CTDIvol

– Σ CTDIvol,i for overlapping ranges

– do not mix head and body CTDIvol)

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Planning a dose Survey in CT

• How are you going to collect the dose data ?

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Data Collection – What’s the protocol?

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How to get the data ?

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Data Collection Paper or Electronic

– Paper• Data tailored to needs• Prospective• ? Accuracy of data entry

– RIS• Retrospective• Patient numbers• ?? Patient weight, kV/mAs• ? Accuracy and consistency of data entry (units)

– PACS• Accurate and consistent data• Patient size?

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Very Simple Paper Entry Form

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Paper Entry – Get the design right!

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Form 2: Patient data acquisition form One entry of this form should be filled in for each patient that receives a ‘standard’ CT CAP exam. For each patient entry, the date, CHI and DLP should be recorded. Only patients receiving the ‘standard’ CAP exam should be included in the Survey. If the patient’s CT exam deviates from the standard protocol, e.g. receives additional scans, the patient should not be included in the Survey. Patient size: we ask that only patients of average size are included in the Survey. Very large and very small patients should be excluded from the Survey. We are not asking you to weigh the patients, simply exercise your professional judgement on what an ‘average’ sized patient is. The position of the patient’s arms should be consistent for all patients in the Survey. If the protocol states that the patient’s arms should be above their head for the exam, only patients who meet this criterion should be included.  Data from 30 patients per site are required for the Survey.

Large scale data collection

• It will very likely that you will have transcription errors

• Incorrect data entry• Zeros and blanks• Multiple exposure asigned to single

exam• Lack of patient information

– Abnormal patients – eg very large

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Clean up data

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Filter the data

• The results of the Survey will only be as good as the data that goes into it!!

• It can be quite time consuming to extract the good data!

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“Cleaned” – exclude ridiculous numbers and zeros

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Dose Survey Body Scans Ninewells Hospital, Dundee

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Patient Size.

• CTDI is a measure of scanner output.• CTDI does take patient size into account• CTDI is NOT patient dose• Important for paediatrics and possibly bariatrics.• You can correct to SSDE

– Only going to mention, not discuss.

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Patient Size

• CTDIvol tells you about scanner output.

• It does not address patient size, so no dose information.

• For smaller paediatrics, interpreting DLP as patient dose could lead to underestimate of patient dose levels by a factor of 2-3 if the 32cm phantom is used for reference.

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How is CTDIvol related to patient dose?

• CTDIvol is not patient dose

• The relationship between the two depends on many factors, including patient size and composition

• AAPM Report 204 introduces a parameter known as the Size Specific Dose Estimate (SSDE) to allow estimation of patient dose based on CTDIvol and patient size

• For the same CTDIvol, a smaller patient will tend to have a higher patient dose than a larger patient

How is CTDIvol related to patient dose?

Both patients scanned with the same CTDIvol Patient dose will be higher for the smaller patient

CTDIvol = 20 mGy CTDIvol = 20 mGy

120 kVp at 200 mAs

120 kVp at 200 mAs

32 cm Phantom

32 cm Phanto

m

How is CTDIvol related to patient dose?

Smaller patient scanned with a lower CTDIvol Patient doses will be approximately equal

CTDIvol = 10 mGy CTDIvol = 20 mGy

120 kVp at 100 mAs

120 kVp at 200 mAs

32 cm Phantom

32 cm Phantom

Equivalent cylinder diameter

• Patient diameter can be equated to a standard cylinder of the same length and weight, if other data not available

• Equivalent diameter = 2√[Weight/( Height)]

• OR 2√[APxLat] – easy in CT

h

Effective Diameter – 32cm reference

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Effective Diameter – 16 cm reference

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Free from ww.aapm.org and on your DVD

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32 cm Phantom

32 cm Phanto

m

How is CTDIvol related to patient dose?

Patients have equivalent SSDE

CTDIvol = 10 mGySSDE = 13.2 mGy

CTDIvol = 20 mGySSDE = 13.2 mGy

120 kVp at 100 mAs

27 cm9 cm

120 kVp at 200 mAs

Patient size specific protocols

• different protocols for adults and children– different image quality requirements

• differences in pathology (contrast, size)• poorer organ delineation• use of contrast agents

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Dose Survey – What to think about

• What should be Surveyed– Examinations– Equipment– Patients

• Data collection• Data analysis• Reporting ✔• Investigating

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When doses are too high or too low: check…

• scan range – DLP too high, exam

CTDIvol OK

• diagnostic task– e.g. brain CT

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When doses are too high or too low: check…

• scanner performance– tube current modulation

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When dose does not change with patient size: check…

• If mA modulation is enabled– patient size, z and xy modulation– appropriate to anatomy

• head• Pelvis

• If no mA modulation– protocol, protocol, protocol

• If mA modulation– Protocol, protocol, protocol

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When doses are too high or too low: reference protocols

• DO NOT transfer protocols between scanners– unless same model

• SET UP a new scan protocol– match

• kVp, slice thickness, beam collimation, beam shaping filter, pitch

– adjust mAs / noise indicator• to match CTDIvol and DLP

• may need phantom

– select reconstruction kernel to match resolution and image noise

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Summary

• Dose Survey required:– To set Local DRLs and test compliance– Optimise new equipment/ protocols

• Choice of examinations and equipment• Patient selection and sample size• Data collection: paper or RIS• Need for special attention for

paediatrics

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WHATEVER YOU DO

• Know your scanner…!

• Collect scanner displayed values of DLP and CTDIvol

• Adjust data collection forms and information for scanner type (so radiographers understand clearly which parameters to record)