dose reduction in mdct . daniel j.p , khorfakhan hospital . uae

DOSE REDUCTION IN MDCT

DANIEL JAYAPRAKASH TECHNOLOGISTRADIOLOGYKHORFAKKAN HOSPITAL


• Radiation safety…we’ve come a long way


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DOSE REDUCTION IN MDCTProgress of technologyGeneration X-ray beam Detector Motion1 generation Pencil beam Single

detectorLinear and rotary (translate rotate

2 generation Fan beam Multiple detector

As above


Rotary motion only


Only tube moves in a circle

5 generation Electron beam CT

6 generation Spiral CT7 generation Multi

detectorArray of detectors


Outline Reasons for increased dose in MDCT.Dose optimization techniques

Dose Reference LevelsCT dose unitsEffective dose unitsBismuth shieldingSummary


Algorithm for image reconstruction

1. Back projection

2. Iterative method

3. Analytical method (a)2D fourier analysis (b) Filtered back projection

4. ASIR – adaptive statistical iterative reconstruction

DOSE REDUCTION IN MDCTCT image acquisition

Intensity profile- measurement of attenuation values at detectorEach projection is an intensity profile 1000 projection for 1 sliceEach projection has 704 variables sample70,000 samples for each slice

DOSE REDUCTION IN MDCT(contd)

Pre-processing – intensity profile are converted into an electronic formatAny errors due to physical or technical reasons are corrected

Back projection- the intensity profile are stretched corresponding to the direction from which the profile was measured at the detector array

Convolution – the back projection images are blurred and these are corrected by mathematical procedure

Filtered Back Projection– data further filtered and any profile responsible for blurring is filtered

algorithms – smooth , standard , sharp

ASIR-Adaptive statistical iterative reconstruction

DOSE REDUCTION IN MDCTPitch in SSCT = table travel/slice thickness

Pitch in MSCT= table travel/sum of active detectors

Contrast resolution – ability of the system to detect a single structure that varies slightly from the surrounding.

Spatial resolution – ability of system to distinguish two small adjacent objects.

Noise-portion of signal which contains no information.

Linearity –ability of the system to display accurate attenuation values.


Are doses in MDCT different and why ?

•Scan Volume•Shorter source to pt distance•Increase in MAS to over come graininess in thin slices•larger pt volume•Multiple contrast phases •Over lapping scans•Over beaming and over ranging.•Pitch


Dose optimisation techniques

• Tube current modulation(X,Y,Z)• AEC • Table speed(cm/s)• Gantry rotation time• KVp• Minimizing contrast phases


Tube current modulation(angular X & Y , longitudinal Z)Adjust the tube current according to pt size and pt anatomyReduce d dose with consistent image quality Dose reduction by 10 to 5o%

AECScanogram suggests required MAS according to pt size Dose reduction from 20 to 44%


Z – dom D- dom

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DOSE REDUCTION IN MDCT(cotd)Table speed(cm/s)- inversely propotional to dose (halving the table speed doubles the exposure).

Gantry rotation time – directly propotional to exposure(doubling the gantry time doubles the exposure)

KVp – reducing tube potential reduces dose but increases grains(90 to 120KVp)

Contrast studies should be phase specific


Paediatric imaging - diagnostic quality not optimum quality

Reduce MAS , FOV , KVp(80 to 100KVp)

Shorter tube rotation time

DOSE REDUCTION IN MDCTDose reference levelsReference level appropriate for average sized patient,age ,clinical data and region of imaging.

Advantage

•Compare CT dose with other modality

•Compare practise with other centers

•Realize if we have certain margin of optimization

•Detect abnormal situation where dose limits are increased

DOSE REDUCTION IN MDCTDRL

Why do we need DRLs?

•Optimization

•Protocols tailored

•Promote and develop current protocol

•NDRLs and LDRLs - awareness , audit and comparing


1. CT dose index(CTDI)- measures absorbed dose(mGy)

2. Dose Length Product(DLP) – CTDIxL (mGy/cm)

3. Effective dose-estimate of stochastic radiation risk (effective dose (mSv)=DLPxCF

4. Conversion factor(CF)from ICRP

DOSE REDUCTION IN MDCT ICRP 2007 Twf

Tissue TwfGonads 0.08Bone marrow , Colon , Lung , Stomach , Breast

0.12

Bladder , Liver , Oesophagus , Thyroid

0.04

Skin , Brain , Bone surface 0.01


Effective dose conversion tableBody region Conversion factor(mGy/cm)Head 0.0023

Neck 0.0054

Chest 0.017

Abdomen 0.015

Pelvis 0.019


Establishing DRL”s•Audit dose report for different body size•Record DLP and CTDIvol •Develop DRL”s•Published DRL”s from UK

Exam DRL(CTDI mGy) DLP(mGy /cm)

Head 60 1060

Sinus 35 360

Vertebra 70 460

Chest 30 650

HRCT 35 280

Abdomen 35 780

Pelvis 35 570


DRL in Pediatrics

AGE BRAIN/CHEST

CTDI DLP

Under 1yr 20/25 180/150

5yrs 25/25 200/200

10yrs 50/30 750/600

UPP/LOW ABD

Under 1yr 20/20 330/170

5yrs 25/25 360/250

10yrs 30/30 800/500

DOSE REDUCTION IN MDCT Effective Dose=DLPxCF (mSv)

Examination Average Effective Dose

HEAD 2mSv

NECK 3mSv

CHEST 7mSv

CHEST ANGIO 15mSv

ABDOMEN 8mSv

PELVIS 6mSv

TRI PHASE LIVER 15mSv

SPINE 6mSv

CORONARY ANGIO 16mSv

CALCIUM SCORING 3mSv

VIRTUAL COLONOSCOPY 10mSv

DOSE REDUCTION IN MDCTB S C A P NCTDI DLP ED

2mSVCTDI DLP ED

2mSv

CTDI DLP ED5-7mSv

CTDI DLP ED8-11mSv

CTDI

DLP ED3-4mSv

CTDI DLP ED3mSv

45.7 798.47

1.83 8.5 112.2 .25 6.6 210.75 3.58 10.9 334.98

5.02 9.8 253.49 4.8 3.1 36.24

45.7 721 1.65 10.6 104.3 .23 5.9 361.70 6.14 24.6 991.03

14.865

4.6 108.53 2.06 12.8 284.26

1.5

45.7 700 1.61 10.6 106.4 .24 7.0 201.10 3.41 8.4 335.25

5.028 11.0 159.26 3.02 23.3 328.9 1.7

45.7 764.29

1.75 10.6 102.7 .23 11.8 425.38 7.2 26.6 925.16

13.877

7.8 129.10 2.45 19.4 307.9 1.6

45.7 636.50

1.46 10.6 106.4 .24 9.2 273.70 4.65 11.4 375.11

5.62 12.7 225.12 4.27 19.4 242 1.32

30.4 459.28

1.05 10.6 139.8 .32 7.0 156.04 2.65 8.3 253.10

3.79 8.4 113.23 2.15 19.3 415.4 2.2

33.5 487.20

1.12 10.6 101.1 .23 3.4 66.7 1.13 23.7 448.22 8.5 19.3 215.79

1.16

33.4 569.53

1.30 2.7 46.8 .795 16.9 404.7 7.6

33.4 546.18

1.25 2.7 40.10 .681

38 577.85

1.32


Bismuth shielding1. Reduces dose to skin and superficial organs 2. Reduces primary beam attenuation3. In thoracic scans reduces breast dose by 43 – 70

% (In practise female <50 years)4. Attenurad bismuth shield 0.06mm Pb covered with

plastic5. Shield placement - tip of the shield on sternal

notch and cover around axilla (after scout)6. Other application if compatible use for thyroid

and eye

DOSE REDUCTION IN MDCT1. Gonadal shield in abdomen reduces dose

from 2.4 – 0.32 mSv2. In CTA prospectively 35 – 40%3. In CT urograph reduction in kV reduces dose

by 7 – 2.9 mSv4. X, Y &Z axis current modulation reduces dose

in colonography5. Paediatric image of diagnostic quality not

optimum quality (reduced mAs ,FOV ,kV between 80 - 100 ,shorter rotation time )

6. Low contrast volume and low KV in renal dysfunction


Images in a 70-year-old woman show normal brain. Standard- and low-dose nonenhanced head CT scans at identical levels are compared. A and B, Posterior fossa (medulla and cerebellar hemispheres) at 170 (A) and 90 (B) mAs. C and D, Thalamus (deep GM) and forceps major (WM) at 170 (C) and 90 (D) mAs. E and F, Centrum semiovale (WM) at 170 (A) and 90 (B) mAs

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58-year-old man with body mass index (kg/m2) of 26.5. Images in both panels matched same level ofliver, which includes main portal vein and medial edge of posterior liver.A, Representative low-dose CT image with adaptive statistical iterative reconstruction (volume CT dose index[CTDIvol], 9 mGy; 120 kV; 3.75 mm slice thickness) is shown. Note decreased sharpness of aortic wall.

B, Representative routine-dose CT image with filtered back projection (CTDIvol, 18 mGy; 120 kVp; 3.75 mm slice


Transverse CT scans obtained at different milliampere-second settings show mass on lower lobe of right lung on 74-year-old man. Both scans were rated as normal-quality images. Transverse CT scans were obtained at 115 mAs (150 mA) (A) and 25 mAs (30 mA) (B).

Transverse CT scans obtained at different milliampere-second settings show mass on lower lobe of right lung on 74-year-old man. Both scans were rated as normal-quality images. Transverse CT scans were obtained at 115 mAs (150 mA) (A) and 25 mAs (30 mA) (B).


75-year-old man with body mass index of 22. Low-dose CT scans without (A) and with (B) adaptive statistical iterative reconstruction (120 kVp; 3.75-mm slice thickness; CT dose index [CTDI], 11) and routine-dose CT scan (C) (140 kVp; 3-mm slice thickness; CTDI, 20) all show hepatic cysts, but sharpness of cyst edges is best in C. B has least image noise.

DOSE REDUCTION IN MDCTTake home points:1. Know your CT dose units 2. Audit CT doses 3. Archive dose reports4. Think about possible site related DRL”s5. Review dose technique6. Use dose modulation whenever possible7. Use shielding if available8. Ask Radiologist to accept images with noise.9. Empower Technologist to adjust protocols 10. ASIR .11. Decision support software12. Think of alternate modalities.13. Provision of patient information material.14. Always keep ALARA in mind!

PROGRESS…CT doses 1/3 lower than a decade agoExpect 10-fold+ or more reduction in next few years

ACR National Dose Registry-collect data on all CT exams


DANIEL

dose reduction in mdct . daniel j.p , khorfakhan hospital . uae

Education

increased dose

dose limits

pt size dose reduction

mdct pitch

mdct different

mdct drl

mdct icrp

ct dose indexctdi measures