What do we mean by workload? Number of scans Type of scans/mix of scans? Total mas Total dip

Transcription

1 CT & Workload (BIR) David Sutton

2 What do we mean by workload? Number of scans Type of scans/mix of scans? Total mas Total dip

3 What do we do with it? Need to relate workload to total scatter dose in the room. Manufacturer s data generally an isodose plot showing scatter distribution, typically in ngy per mas for a given scan protocol. Manufacturer s data typically for a head phantom and a body phantom and in two perpendicular planes.

4 Typical Isodose Plot

5 Critical Direction Isodose plot not symmetrical, ti and dh has critical directions. Shielding should be specified according to these worst case scenarios A H

6

7

8 /(distance)2 A B C D E F G H Dose per slice (microgray y)

9 120 kvp; 250 mas

10 CT Calculation (G) Distance: (4.0/cos 36) = 4.9 m Weekly workload: 3340 x 250 mas slices Distance to 2 µgy isodose: 1.7 m Transmission: 7.2 x 10-3 Concrete thickness: 100 mm Thickness at 36 : 100 x (1 + cos36)/2 = 90 mm Dose per week: 2 x 3340 x (1.7/4.9) 2 = 800 µgy

11 Scatter from walls in CT Protection against radiation scattered from adjacent walls or the ceiling is usually required It is likely to be necessary: 1) to take shielding to ceiling height 2) to have a fully enclosed operator s cubicle with door

12 Relating plot to dose Isodose plot not symmetrical, and has critical directions. Shielding should be specified according to these worst case scenarios Ideally then, one needs to determine the total mas for the two generic types of phantom, head and body.

13 Getting the data Information available from a scanner can include predicted mas predicted dlp, CTDI vol True dlp mas. It all depends on the scanner being used and its age.

14 Getting the data - What data? Which scans are being done? Which scans are going to be to be done? Crucial questions to answer How many of them there are going to be?

15 How to get the data The first thing to do is to find out what the scanner is going to be used for. This means speaking to a human being! Or even more than one!

16 Data The length of the working day The number of patients per day The scans to be performed The relative number of different types of scan. Crystal ball gazing g

17 Data For all of the major scan types you need some idea of the scan protocol e.g. 400 ma max on automa 120 kvp Pitch 1.375:1 Projected dip

18 Getting the data Information available from a scanner can include predicted mas predicted dlp, CTDI vol True dlp mas. It all depends on the scanner being g used and its age. Getting the data - What data? Which scans are being g done? Which scans are going to be to be done? Crucial questions to answer? How many of them there are going to be? Data The length of the working day The number of patients per day The scans to be performed The relative number of different types of scan. Crystal ball gazing Data For all of the major scan types you need some idea of the scan protocol e.g. 400 ma max on automa 120 kvp Pitch 1.375:1 Projected dip Not a trivial task!

19 Exisiting Data You may be able to base your decision on existing data, for example this may be a replacement room, or a room which will be used for a similar purpose as one elsewhere. Conceptually this should ease the process since it is very unusual not to have access to a RIS system. To prepare and to give some examples of the process we chose 5 scanners and looked at data at between 750 and 1000 sequential patients on each.

20 Clean up data

21 Cleaned

22 Dundee

23 Edinburgh

24 Relative Contributrions 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

25 ? Others Others WGH CT Angiogram Aorta CT Angiogram Renal/Abdominal CT Guided Ablation CT Guided Aspiration CT Guided Biopsy CT Guided Drainage CT Hip Rt CT Liver Triple Phase Others - NW CAICN CANKL CANKR CART CBIOPB CCALR CCHAP CCHES CCHESC CCSPN CDRAID CELBL CFACE CFACI

26 Options Your scanner gives you an mas reading / you have mas workload data. You want to use the isodose method You want to use the dlp method? Your scanner gives you a dlp reading / you have dlp workload data You want to use the dlp method You want to use the isodose method

27 You want to use the isodose CT Abdomen and Pelvis method CT Abdomen b With Anatomical Groups / Protocol Types Contrast CT Abdomen/Pelvis With Contrast CT Angiogram Aorta CT Angiogram Pulmonary CT Angiogram CT Chest CT Chest and Biopsy CT Chest High Resolution CT Chest/Abdo CT Chest/Abdo/Pelvis/ With Contrast CT Colonoscopy (Virtual) CT Head CT KUB CT Urogram Head Abdomen Chest Chest & Pelvis Angiography

28 Workload Determination Chest Abdo Pelvis An Example

29 1) Determine Scan Length

30 2) N Months Workload DLP (Smart ma) from RIS

31 3) Data nctdiw = 8.3 mgy/100 mas Pitch = nctdivol= 6 mgy/100 mas Scan Length = 75 cm (from phantom) ndlp= 450 mgycm/100mas

32 4) Workload Mean dlp = 930 mgycm Mean mas per CAP = 100*930/ 450 mas =200 mas (using ma modulation)

33 Options Your scanner gives you an mas reading / you have mas workload data. You want to use the isodose method You want to use the dlp method? Your scanner gives you a dlp reading / you have dlp workload data You want to use the dlp method You want to use the isodose method

34 Example from 2006 (i) Workload- Patients & Protocols Scatter diagrams state the dose per rotation at given exposure parameters. To calculate annual dose at each barrier, need to: (i) correct dose for exam exposure parameters (ii) multiply by total number of rotations per year Estimations are therefore required for typical exam parameters (ma, rotation time, kvp, exam time & beam width) and number of examinations per year.

35 Example from 2006 (ii) Number of Examinations Based on Hospital A workload (2002) and amended by CT radiographer to anticipated demand. Estimated daily workload: 7 head exams, 7 AP body exams, 8 CAP body exams. Cardiac session half day per week, 8 patients. On remaining half day assume 3 head exams, 3 AP body exams, 4 CAP body exams Assume scanner working 5 days a week, 52 weeks per year. Annual workload: 1612 head exams, 1612 AP body exams, 1872 CAP body exams, 416 cardiac exams

36 Example from 2006 (iii) Exam Parameters Four standard protocols are head abdo-pelvis chest-abdo-pelvis cardiac ac CTA. Protocols have been derived form number of sources. Parameters have been corrected for Toshiba Aquilion 64 MSCT scanner.

37 Example from 2006 (iv) Annual Dose Estimate Scatter diagrams are for Toshiba Aquilion 64 CT scanner. Dose is given per rotation at standard scanning parameters: EG Body: 120 kvp, 200 ma, 1 sec, 32mm beam Once the dose at a given position is calculated it must be scaled for the actual scan parameters estimated for Aquilion 64 head, body & cardiac protocols (see table below). Assume linear scaling factors for slice thickness, mas and beam width. Scaling factor for energy is taken as kvp 2.

38 Example from 2006 (v) Annual Dose is given by: number of exams x rotations/exam x scaling factors x dose/rotation Sc aling Factors Ex am Type & No of Rot s/ kvp mas Sl ice Para met ers Ex am Ex am [120] [100] [32 mm] s Head (PF) k Vp, 330 mas, 4mm Head (HS) k Vp, 330 mas, 8mm Bo dy (AP) k Vp, 200 mas, 32 mm Bo dy (CA P) k Vp, 200 mas, 32 mm Cardiac k Vp, 200 mas, 32 mm

39 Example from 2006 (vi) Annual Doses Corridor Reporting Outside Corner of Ctrl Ceiling above Ceiling above Corner of Wall Wall Room Corridor and Corridor Exam room Exam room and Ceiling Body Dose AB BB CB DB EB FB Per Rot ugy Body (AP) Body (CAP) Cardiac Corridor Reporting Outside Corner of Ctrl Ceiling above Ceiling above Corner of Wall Total Annual Wall Room Corridor and Corridor Exam room Exam room and Ceiling Dose mgy

40 Totally New Facility Identifying proposed workload may be tricky. Understand what the unit is intended to be used df for.

41 Any Facility Beware of using published reference levels as the basis of your calculation. There is a significant time lag between data collection and publication.

42 Reminder The use of CT increases inexorably The indications for CT increase inexorably 0.25 head of population Per Year England USA

43 Why The Increase? Not just CT replacing other techniques Multi Slice CT and slice wars Changes in practice Changes in protocol / acquisition modes Differing volumes Differing numbers of slices Hypertestosis Catch all protocols Standards, What Standards?.

44 Any Facility Beware of using published reference levels as the basis of your calculation. There is a significant time lag between data collection and publication. Reference levels l are not necessarily indicative of current practice Need to know protocols before designing shielding.

45 Thank You The Scary McLarey Family