Dose Survey in Computed Tomography DS /CM Kampala 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 Kampala

Dose Survey – What to think about What should be Surveyed –Examinations –Equipment –Patients Data collection Data analysis Reporting Investigating Kampala

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 Kampala

Planning a dose Survey in CT Examinations What examinations are you going to survey? Kampala

Criteria for inclusion Examinations must be performed reasonably frequently in your hospital. Data collection must be feasible Kampala

Your Presentations Brain Lung Lumbar Spine Abdomen Kampala

Relative Contributions – 5 Scanners HeadBodyOtherN NWIP52%40%8%1201 NWOP24%69%7%757 WGH38%58%4%1110 STJ40%54%6%866 DCN85%6%9%968 Kampala

Dundee Exams Kampala

Edinburgh Exams Kampala

An issue for PACS / RIS based surveys What are all these codes? Kampala

There are a large number of CT exams Anatomical Groups / Protocol Types –Head –Abdomen –Chest –Chest & Pelvis –Angiography –Choice of correct exam important Kampala

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) Kampala

Planning a dose Survey in CT Patient Selection Kampala

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 Kampala

Planning a dose Survey in CT How are you going to collect the dose data ? What Physical Quantities are you going to record? Kampala

Dose Length Product & CTDI vol 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? Kampala

Data Collection – What quantity CTDI vol – In My Opinion CTDI vol 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. Kampala

Dose Summary Sheet - Commercial CT Scanner Huda W, Mettler F A Radiology 2011;258: Kampala

Patient dose Survey in CT common scans patient weight, sex collect –CTDI vol total DLP, (calculate exam CTDI vol –Σ CTDI vol,i for overlapping ranges –do not mix head and body CTDI vol ) Kampala

Planning a dose Survey in CT How are you going to collect the dose data ? Kampala

Data Collection – What’s the protocol? Kampala

How to get the data ? Kampala

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? Kampala

Very Simple Paper Entry Form Kampala

Paper Entry – Get the design right! Kampala 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 Kampala

Clean up data Kampala

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! Kampala

“Cleaned” – exclude ridiculous numbers and zeros Kampala

Dose Survey Body Scans Ninewells Hospital, Dundee Kampala

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. Kampala

Patient Size CTDI vol 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. Kampala

How is CTDI vol related to patient dose? CTDI vol 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 CTDI vol and patient sizeAAPM Report 204 For the same CTDI vol, a smaller patient will tend to have a higher patient dose than a larger patient

How is CTDI vol related to patient dose? Both patients scanned with the same CTDI vol Patient dose will be higher for the smaller patient CTDI vol = 20 mGy 120 kVp at 200 mAs 32 cm Phantom

How is CTDI vol related to patient dose? Smaller patient scanned with a lower CTDI vol Patient doses will be approximately equal CTDI vol = 10 mGyCTDI vol = 20 mGy 120 kVp at 100 mAs 120 kVp at 200 mAs 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 Kampala

Effective Diameter – 16 cm reference Kampala

Free from ww.aapm.org and on your DVD Kampala

32 cm Phantom How is CTDI vol related to patient dose? Patients have equivalent SSDE CTDI vol = 10 mGy SSDE = 13.2 mGy CTDI vol = 20 mGy SSDE = 13.2 mGy 120 kVp at 100 mAs 27 cm 9 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 Kampala

Dose Survey – What to think about What should be Surveyed –Examinations –Equipment –Patients Data collection Data analysis Reporting ✔ Investigating Kampala

When doses are too high or too low: check… scan range –DLP too high, exam CTDI vol OK diagnostic task –e.g. brain CT Kampala

When doses are too high or too low: check… scanner performance –tube current modulation Kampala

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 Kampala

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 CTDI vol and DLP may need phantom –select reconstruction kernel to match resolution and image noise Kampala

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 Kampala

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)