A simple anthropomorphic phantom used to demonstrate the effectiveness of CT dose modulation functions.

Transcription

1 A simple anthropomorphic phantom used to demonstrate the effectiveness of CT dose modulation functions. Lynn Bateman & Peter Hiles North Wales Medical Physics

2 How can we test CT dose modulation? Lynn Bateman & Peter Hiles North Wales Medical Physics

3 Dose Modulation in CT All manufacturers offer some sort of dose modulation (AEC) As we have heard (I hope!) all have differing approaches Traditional CT Phantoms have circular cross sections New phantom required to ensure systems are working as specified and to compare different scanners and/or protocols

4 BOD Aunt Flo Here comes BOD. PC Copper Frank the Postman Farmer Barleymow Alberto the Frog and his Amazing Animal Band

5 An introduction to BOD - North Wales

6 An introduction to BOD - North Wales

7 Which parameters are adjusted on your system? Whilst all systems are aiming to obtain images at a defined noise level this is achieved by adjusting the ma per patient per projection per rotation

8 Know your target SD level Reference Image Reference mas

9 Does it work?

10 Does it work? Variation of predicted ma with quality setting High Quality Typical Quality Low dose Tube Current (ma) Head Abdomen Position (mm)

11 45 4 Does it work? Variation of predicted ma with quality setting High Quality Typical Quality Low dose 45 4 Tube Current (ma) Tube Current (ma) LOW DOSE 5 Head Abdomen Position (mm)

12 Does it work? Solid lines = ma Dashed lines = SD Variation of predicted ma with Quality setting Tu be C u rren t (m A ) SD 1 5 High Quality Typical Clinical HQ target SD = 5 Clinical, target SD = 2-25 Head Abdomen Position (mm) 5

13 Phantom Results Manufacturer Solid lines = Dose Dashed lines = SD Dose (mgy) Dose (mgy) SD SD Distance Head Abdomen

14 Changing clinical practice Local user was unsure how their AEC software worked. Result of a dose audit Request came for us to carry out measurements to show the effects.

15 Changing clinical practice Target SD Measured Dose (mgy) DLP (mgycm) CT# water Measured SD SD Variation from Target.6 cc ion chamber % % % Measurements taken in upper abdomen

16 Changing clinical practice Dose and Noise variation with Selected Noise Level 35 3 Organ Dose Image Noise Selected Noise Level

17 Changing clinical practice Exam Local DLP (mgy cm) DLP compared to pre change %NDRL NDRL 1st Audit 2nd Audit (post change) 3rd Audit (change + 1yr) 2nd Audit 3rd Audit 1st Audit 2nd Audit 3rd Audit DLP (mgy cm) Head % 81% 18% 9% 88% 93 Chest % 48% 14% 72% 67% 58 Chest/Abdo % 5% 29% 12% 15% 58 CAP % 56% 146% 76% 82% 94 Abdo/Pelvis % 48% 155% 79% 74% 56 Physics evaluation of AEC system

18 Changing clinical practice Request from another local user department to assess the impact of proposed protocol changes. Current Proposed Eff mas Chest Eff mas abdo SW 15 mas 2 mas.6 1 mas 15 mas 1.2

19 Changing clinical practice Effect on Image Noise Change Change Ref mas Ref mas Change Change SW Ref mas Change Change SW SW and Ref mas Change from "Standard" DLP change Shoulder Chest Chest Abdo 15mAs.6 mm -> 1.2 1mAs mm 2mAs -> 15mAs 15 mas -> 1 mas 2 mas -> 15 mas.6 mm -> 1.2 mm

20 The Future Split phantom into two to improve manageability Refine shoulders Different sized phantoms Work outside of CT!