CT Dose Optimisation

RANZCR QUDI MMC July 2010

MultiMulti--detector CT (MDCT) detector CT (MDCT)

Technology & OptimizationTechnology & Optimization

John C.P. Heggie, Ph.D.

MultiMulti--detector CT (MDCT) detector CT (MDCT)

Technology & OptimizationTechnology & Optimization

John C.P. Heggie, Ph.D.

Consultant Medical PhysicistConsultant Medical Physicist

BreastScreenBreastScreen VICVI


The Evolution of CT from The Evolution of CT from

Head scan circa 1974 Head scan circa 1974 Matrix 80 x 80Matrix 80 x 80EMI Mark I s canner


To To

Siemens Somatom Sensation 64 and beyond


MDCT What about Dose?MDCT What about Dose?

The potential The potential downside of downside of MDCTMDCT


Topics for DiscussionTopics for Discussion

Dose terminology for risk assessmentDose terminology for risk assessment

Should we have any concerns about Should we have any concerns about MDCT patient doses?MDCT patient doses?

What can we do to keep doses under What can we do to keep doses under control in MDCT? control in MDCT? Understanding t he technology o f MDCTUnderstanding t he technology o f MDCT

OPTIMIZATIONOPTIMIZATION


Useful Dose IndicesUseful Dose Indices

CTDICTDIvolvol Dose length product (DLP)Dose length product (DLP)

Effective dose (E)Effective dose (E)

Absorbed dose (D)Absorbed dose (D)


Effective Dose (E)Effective Dose (E)

Measure of Measure of riskrisk of hereditary effects & of hereditary effects & cancer caused by ionizing radiation cancer caused by ionizing radiation

Accounts f or varying t issue Accounts f or varying t issue

radiosensitivitiesradiosensitivities

Accounts for partial body irradiation Accounts for partial body irradiation

Measured in Measured in SievertSievert ((SvSv), ), mSvmSv or or SvSv 1 1 SvSv = 1,000 = 1,000 mSvmSv = 1,000,000 = 1,000,000 SvSv


Conversion from DLP to E(from ICRP 102)

Conversion from DLP to E(from ICRP 102)

Example: Example: AbdoAbdo pelvis CT has DLP of 500 mGy.cm. pelvis CT has DLP of 500 mGy.cm. Effective dose is 500 x 0.015 = 7.5 Effective dose is 500 x 0.015 = 7.5 mSvmSv

Body part k (mSv/mGy.cm)

Head & neck 0.0031

Head 0.0021

Neck 0.0059

Chest 0.014

Abdomen & pelvis 0.015

Trunk 0.015


MDCT Doses SVHM Median ValuesMDCT Doses SVHM Median ValuesCT Examination (Adult) Effective Dose (mSv)

Brain 1.4

Chest 4.0

with contrast 7.2

Chest/abdo/pelvis with contrast 11

Abdo/pelvis 4.4

with contrast 6.0

triple phase 13


Comparison with other modalitiesComparison with other modalitiesProcedureProcedure SVHM Ef fective Dose (SVHM Ef fective Dose (mSvmSv))

Chest (P/A)Chest (P/A) 0.0230.023

Abdomen (A/P)Abdomen (A/P) 0.350.35

Lumbar spine seriesLumbar spine series 0.470.47

PTCAPTCA 3.13.1

IVP (8 images)IVP (8 images) 3.73.7

Bone scan (Bone scan (99m99mTc)Tc) 4.24.2

Brain SPECT (Brain SPECT (NeuroliteNeurolite)) 6.96.9

Barium enemaBarium enema 7.97.9

Natural Natural bgbg/annum/annum 2.02.0


CT Organ Doses CT Organ Doses Examination Organ Absorbed Dose (mGy)

Brain lens 45-50brain 38

Chest - with contrastlung 11breast 7stomach/liver 14

Abdo/pelvis - triple phasestomach/liver/kidneys 30-35gonads (f) 13SI 22


As far as diagnostic radiological procedures are concerned CT is a

high dose procedure.

But are these doses a cause for concern?

CT DosesCT Doses


Risks at CT DosesRisks at CT Doses Brenner et al AJR 2001: 176; 289Brenner et al AJR 2001: 176; 289

Sparked debate about number of possible Sparked debate about number of possible cancer deaths caused by paediatric CT cancer deaths caused by paediatric CT scanningscanning

Brenner et al NEJM 2007: 357; 2277Brenner et al NEJM 2007: 357; 2277 Discussed increasing CT usage patterns & Discussed increasing CT usage patterns &

riskrisk


Risks at CT DosesRisks at CT Doses

What do we really know about cancer risks What do we really know about cancer risks at low dose?at low dose? Most information about human exposure Most information about human exposure

comes from the 1945 Japanese experimentscomes from the 1945 Japanese experiments


Risks at CT DosesRisks at CT Doses

Japanese LSS low dose data (RERF Japanese LSS low dose data (RERF Study)Study)

440 deaths attributable to radiation out of ~9350440 deaths attributable to radiation out of ~9350 Risk decreases with increasing age at time of Risk decreases with increasing age at time of

exposureexposure

Other sourcesOther sources Canadian & Massachusetts TB fluoroscopy Canadian & Massachusetts TB fluoroscopy

patientspatients Swedish benign breast disease patientsSwedish benign breast disease patients

IN SHORT - NOT MUCH!


Cancer Mortality Excess Radiation Induced Risk Male 30y

Cancer Mortality Excess Radiation Induced Risk Male 30y

0

0.02

0.04

0.06

0.08

0.1

0 25 50 75 100 125 150

Effective dose (mSv)From uni form whole bo dy e xposure

E

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CT dose range

Pierce et al: Rad Res 146 (1996) 1Preston et al Rad Res 160 (2003) 381

Linear model based on risk o ver range 0-2 Sv


Cancer Incidence Excess Radiation Induced Risk Male 30y

Cancer Incidence Excess Radiation Induced Risk Male 30y

0

0.05

0.1

0.15

0.2

0.25

0.3

0 50 100 150 200

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Pierce & Preston: Rad Res 154 (2000) 178-186

Effective dose (mSv)From uni form whole bo dy e xposure

CT dose range

Linear model based on risk o ver range 0-2 Sv


Hypothetical CaseHypothetical Case

A 30 y old man undergoes two triple phase A 30 y old man undergoes two triple phase CT CT abdoabdo procedures with an effective dose of procedures with an effective dose of ~25 ~25 mSvmSv. What is the hypothetical risk of a . What is the hypothetical risk of a radiation induced cancer in this man?radiation induced cancer in this man?

Assuming cancer incidence of ~40% in male Assuming cancer incidence of ~40% in male population then:population then:

AAdditional risk = 0.025 x 40% = 1%dditional risk = 0.025 x 40% = 1% Young children are thought to be more Young children are thought to be more

radiosensitive radiosensitive risk will be higherrisk will be higher ffood for thought at the very least!


Helical CT - Key ParametersHelical CT - Key Parameters

kVpkVp determines subject contrast determines subject contrast Increasing t he Increasing t he kVpkVp will decrease the subject contrast will decrease the subject contrast Increasing t he Increasing t he kVpkVp will increase dose will increase dose -- all other all other

parameters being kept constantparameters being kept constant Consider using 80 Consider using 80 kVpkVp or 100 or 100 kVpkVp with children & with children &

for CT angiography where intrinsic contrast is highfor CT angiography where intrinsic contrast is high

mAmA & rotation time (& rotation time (mAsmAs)) determines noise & hence low contrast determines noise & hence low contrast detectabilitydetectability Increasing t he Increasing t he mAsmAs increases dose in direct increases dose in direct

proportionproportion



Reconstruction algorithm (kernel) Reconstruction algorithm (kernel) High resolution kernel will increase noise

SD=6.0

Smoothing filter

Wire

SD=32

Bone filter

0

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n

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BoneSmoothing



Reconstruction algorithm (kernel) Reconstruction algorithm (kernel) If high spatial resolution is not crucial then

reconstruct using a smoothing k ernel

And/or reconstruct using thicker slices

To decrease impact of noise in any CT image utilise a smoothing k ernel rather than increase the mAs



90 mAs Kernel = B80

Mean = 43 98

90 mAs Kernel = B41

Mean = 44 19

To achieve the same level of noise reduction would require an INCREASE in mAs (dose) by a factor of 25!



Image slice widthImage slice width impacts on spatial resolution in zimpacts on spatial resolution in z--direction direction

& image noise & image noise

Scanned volume Scanned volume Increase the volume and effective dose Increase the volume and effective dose

increases because more organs includedincreases because more organs included

PITCH Dictates the total scan time and MAY

impact on image quality & dose


Single slice Helical CT - PitchSingle slice Helical CT - Pitch

Patient motion

Distance advanced by table during one rotation of X-ray tube (TF)

Pitch = table feed per rotation collimated nominal slice width= TF/h

Pitch values typically 1Pitch values typically 1--22 A pitch = 1 at same mAs A pitch = 1 at same mAs

has same dose implications has same dose implications as contiguous step and as contiguous step and shoot sequential scanningshoot sequential scanning

A pitch of >1 at same mAs A pitch of >1 at same mAs implies a dose saving but implies a dose saving but tissues are not scanned tissues are not scanned from all anglesfrom all angles

A pitch


MDCT Helical & PitchMDCT Helical & Pitch A modified definition is required A modified definition is required

because image slice width & collimated because image slice width & collimated beam width are decoupled:beam width are decoupled:

Pitch = Table feed per rotationPitch = Table feed per rotationTotalTotal nominal beam widthnominal beam width

= = TF / (N . h)TF / (N . h)N= Number of detector rows usedN= Number of detector rows used

With this definition a pitch > 1 implies With this definition a pitch > 1 implies dose reduction as for single slice helical dose reduction as for single slice helical scanningscanning

Historically we have recommended USING A PITCH > 1

N . h

Detector elements

(4 slices N=4)


Effective mAsEffective mAs

Effective mAs = mA * s / pitchEffective mAs = mA * s / pitch

= mA * s * beam collimation / table feed= mA * s * beam collimation / table feed


Effective mAs & DoseEffective mAs & Dose

Dose index (Dose index (CTDICTDIvolvol) i s a measure) i s a measure

of the average absorbed dose to of the average absorbed dose to

one slice of the patient assuming one slice of the patient assuming

typical patient dimensionstypical patient dimensions

CTDICTDIvolvol effective effective mAsmAs= 8.4 = 8.4 mGymGy

Double the effective Double the effective mAsmAs & & CTDICTDIvolvol doublesdoubles


Causes of Dose Increases with MDCT

Causes of Dose Increases with MDCT

OverOver--beaming beaming (penumbral effects)(penumbral effects)

OverOver--scanningscanning (over(over--ranging)ranging)

MDCT MDCT detector inefficiencies detector inefficiencies

MDCTMDCT geometrygeometry

Improved Improved radiation utilisationradiation utilisation

Increasing use of Increasing use of thin slice reconstructionthin slice reconstruction


axis ofrotation

Single active element detects umbra & penumbra & uses most if

not all

Focal spot

1 x 4 mm single slice CT

Umbra

Penumbra4 mm slice includes

penumbra

Collimator

Over beaming (Penumbra)Over beaming (Penumbra)Focal spot

4 x 1 mm multislice CT

Each active element detects unequal amounts of the umbra

Each active element detects the umbra equally

Collimator opened further

Focal spot


Impact of Total Beam WidthImpact of Total Beam Width

Unused umbra represents a smaller proportion of beam

when total beam width (N . h )

increases

collimator

Small beam width N.h Large beam width N.h


Over-beamingOver-beaming

Depends on focal spot size Depends on focal spot size

OverOver--beaming is increased by having:beaming is increased by having: Large focal spot &Large focal spot &

Poor radiographic geometry (i.e. short SDD) Poor radiographic geometry (i.e. short SDD)

OverOver--beaming is common t o both beaming is common t o both sequential (axial) and helical MDCTsequential (axial) and helical MDCT


Head scan

Do the right thing and carefully avoid

orbits!


Demonstration of Over-beaming (head)

Demonstration of Over-beaming (head)

ParameterParameter 12 mm12 mm

SequentialSequential

24 mm24 mm

SequentialSequentialDLP DLP (mGy.cm)(mGy.cm)

421421 379379

E (mSv)E (mSv) 1.41.4 1.21.2

Lens dose Lens dose (mGy)(mGy)

5.15.1 4.64.6


Over-beaming MinimisedOver-beaming Minimised

Geleijns et al PMB 54 (2009) 3141

Aquillion One120 kVp

CTDI normalised to 1.0 for 160 mm collimation


Over-beaming MinimisedOver-beaming Minimised

Note that situation is alleviated somewhat Note that situation is alleviated somewhat with 32 or 64 slice scanners as the with 32 or 64 slice scanners as the unused penumbra b ecomes a smaller unused penumbra b ecomes a smaller proportion of total beam width provided proportion of total beam width provided you use a large you use a large N.hN.h product. product.

In general recommended that use scan protocols with collimation large (eg. N.h= 64 x 0.5 mm) for all studies except those that require high spatial resolution but there is the then the issue of over-scanning in helical mode!


Helical scanners must irradiate a larger Helical scanners must irradiate a larger volume of patient than is ultimately displayed volume of patient than is ultimately displayed in the image data set:in the image data set: Necessary evil to avoid having incomplete data Necessary evil to avoid having incomplete data

for reconstruction of first and last slices of interest for reconstruction of first and last slices of interest

Typically an extra rotation is required at Typically an extra rotation is required at beginning a nd end of imaged volumebeginning a nd end of imaged volume

Common to all scanners used in helical mode Common to all scanners used in helical mode except with a few members of the Siemens except with a few members of the Siemens Definition family of scannersDefinition family of scanners

Over-scanningOver-scanning


Over-scanningOver-scanning

More significant for MDCTMore significant for MDCT May irradiate several extra cmMay irradiate several extra cm

Increases patient dose in general but may Increases patient dose in general but may lead to unintended irradiation of key lead to unintended irradiation of key organsorgans Lens of eye in head scanLens of eye in head scan

Should routinely use sequential scanning unless Should routinely use sequential scanning unless clinically indicated or doing MPR clinically indicated or doing MPR

Male gonads in abdomen/pelvic scanMale gonads in abdomen/pelvic scan


Outcome of Over-beaming & Over-scanning (head)

Outcome of Over-beaming & Over-scanning (head)



24 mm24 mm


24 mm24 mm

HelicalHelical

P =1P =1DLP DLP (mGy.cm)(mGy.cm)

421421 379379 460460

E (mSv)E (mSv) 1.41.4 1.21.2 1.51.5

Lens dose Lens dose (mGy)(mGy)

5.15.1 4.64.6 35.735.7


MDCT Optimization MDCT Optimization

Perform head scans in sequential mode Perform head scans in sequential mode using broad collimation with angled using broad collimation with angled gantry to avoid orbits gantry to avoid orbits unless o therwise unless o therwise indicated by clinical c onsiderations ( e.g.)indicated by clinical c onsiderations ( e.g.) MPR requiredMPR required

3D reconstructions required3D reconstructions required


Do the right thing and carefully avoid

testicles!

Abdomen & pelvis

scan


Outcome of Over-beaming & Over-scanning (abdo/pelvis)Outcome of Over-beaming & Over-scanning (abdo/pelvis)


HelicalHelical

P = 1.0P = 1.0

24 mm24 mm

HelicalHelical

P = 1.0P = 1.0

24 mm24 mm

HelicalHelical

P =0.5P =0.5DLP DLP (mGy.cm)(mGy.cm)

478478 462462 446446

E (mSv)E (mSv) 8.48.4 9.29.2 8.28.2

Testicular Testicular dose (mGy)dose (mGy)

4.84.8 11.011.0 7.27.2



ConsiderConsider using s equential s canning w ith using s equential s canning w ith large collimation when short scan blocks large collimation when short scan blocks intended (e.g. small children)intended (e.g. small children)

Scan patient anatomy in one large block Scan patient anatomy in one large block rather than two or more smaller blocks:rather than two or more smaller blocks:

Use pitch < 1.0 !!!!! Use pitch < 1.0 !!!!! PROVIDEDPROVIDED reduce reduce mAsmAs to keep to keep effectiveeffective mAsmAs the samethe same

There are exceptions to this approach!There are exceptions to this approach!


Exceptions to General RuleExceptions to General Rule

A different level of noise may be tolerated in A different level of noise may be tolerated in one part of anatomy versus anotherone part of anatomy versus another Scan separately with different Scan separately with different mAsmAs

Combined head/neck scans with some Combined head/neck scans with some 16 slice 16 slice scannerscanner designs would result in a dose penalty designs would result in a dose penalty because head scans ar e usually p erformed with because head scans ar e usually p erformed with less filtration which cannot be increased in less filtration which cannot be increased in middle of scan blockmiddle of scan block dose penalty would more than offset the dose penalty would more than offset the

advantage of scanning as a single block.advantage of scanning as a single block.


MDCT Detector InefficienciesMDCT Detector Inefficiencies The limiting spatial The limiting spatial

resolution in the zresolution in the z--direction direction is dictated by the width of is dictated by the width of the discrete detector the discrete detector elementselements of necessity there are t iny of necessity there are t iny

gaps between detector gaps between detector elementselements

leads to wasted radiation and leads to wasted radiation and a loss of geometric a loss of geometric efficiencyefficiency

Siemens UFC detector

GE ceramic detector


Single Slice v MDCT GeometrySingle Slice v MDCT Geometry


Scan protocols should Scan protocols should NOTNOT be be transferred from one scanner (particularly transferred from one scanner (particularly a single slice CT) to another without due a single slice CT) to another without due consideration to differences in consideration to differences in scanner geometryscanner geometry

Detector efficiencyDetector efficiency

filtration.filtration.

MDCT Detector & GeometryMDCT Detector & Geometry


For the CT abdominal example discussed For the CT abdominal example discussed previously, us ing t he same scan protocol previously, us ing t he same scan protocol (effective (effective mAsmAs) would result in a DLP ) would result in a DLP increase of 90% in going to the GE increase of 90% in going to the GE LightspeedLightspeed 16 from the GE CT/16 from the GE CT/ii

MDCT Detector & GeometryMDCT Detector & Geometry


Improved Radiation Utilisation Improved Radiation Utilisation

A greater proportion of XA greater proportion of X--rays generated rays generated by Xby X--ray tube are utilised with cone ray tube are utilised with cone beam (MDCT) versus fan beam (single beam (MDCT) versus fan beam (single slice). This has advantages from Xslice). This has advantages from X--ray ray tube loading perspective but .. tube loading perspective but ..


The DownsideThe Downside

Volume s canned

Increase the Increase the scanned scanned volume orvolume or

Undertake Undertake more phasesmore phases DOSE

INCREASES Multiphase scan



Do Do NOTNOT increase scanned vo lume jus t increase scanned vo lume jus t because you can:because you can: Rather only if clinicallyRather only if clinically justifiedjustified

MinimiseMinimise the use of multithe use of multi--phase phase examinationsexaminations Do you always need the venous and delayed Do you always need the venous and delayed

phases?phases?


Thin Slice ReconstructionThin Slice Reconstruction

MDCT allows the same volume to be MDCT allows the same volume to be reconstructed with thin slices without reconstructed with thin slices without increasing scan times increasing scan times However i mage noise increasesHowever i mage noise increases

Some manufacturers ha ve changed geometry so Some manufacturers ha ve changed geometry so that patient & detector are closer to focus (see that patient & detector are closer to focus (see earlier GE design) and/orearlier GE design) and/or

Operator may increase tube loading (mAs) to Operator may increase tube loading (mAs) to compensate for increased image noise compensate for increased image noise

DOSE INCREASES


Time Out!Time Out!


MDCT OptimizationMDCT Optimization

Optimization means reducing dose whilst maintaining acceptable image quality

It is NOT about having either The lowest dose or

The best image quality



Tailor techniques to clinical Tailor techniques to clinical requirements. Choose:requirements. Choose: scanned vo lume scanned vo lume pitchpitch rotation speed rotation speed and collimation carefullyand collimation carefully

Adjust Adjust mAsmAs (in absence of AEC) (in absence of AEC) & & kVpkVp to be patient size specificto be patient size specific Paediatric protocolsPaediatric protocols


Dose & Technique FactorsDose & Technique Factors

Dose increases with kVp for same mAsDose increases with kVp for same mAs

Dose increases wi th volume scanned Dose increases wi th volume scanned

use small volumeuse small volume

Dose increases with mAs Dose increases with mAs

use low mAsuse low mAs

Dose decreases with pitchDose decreases with pitchDosemAs effective mAs

pitch



Check your doses against published Check your doses against published Diagnostic Reference Levels ( DRLs)Diagnostic Reference Levels ( DRLs) If others are using much lower doses you If others are using much lower doses you

probably need to modify your protocolsprobably need to modify your protocols

Employ Automatic Exposure Control Employ Automatic Exposure Control AEC (AEC (mAmA modulation) technology modulation) technology whenever possible:whenever possible: Smart Smart mAmA -- GEGE Care DoseCare Dose -- SiemensSiemens DoseRightDoseRight -- PhilipsPhilips SureExposureSureExposure -- ToshibaToshiba


Automatic Exposure ControlAutomatic Exposure Control

The scanner adjusts the XThe scanner adjusts the X--ray tube ray tube mAmAautomatically with changes in anatomy automatically with changes in anatomy both during the scan and from patient to both during the scan and from patient to patientpatient Rotational (xRotational (x--y) current modulationy) current modulation

Longitudinal (ZLongitudinal (Z--axis) current modulationaxis) current modulation

Combined rotational & zCombined rotational & z--axis modulationaxis modulation


Z-axis Current ModulationZ-axis Current Modulation

The The mAsmAs is altered from rotation to is altered from rotation to rotation as the patient moves through the rotation as the patient moves through the gantrygantry Uses attenuation data generated from a Uses attenuation data generated from a

SPR (Scout) to determine required SPR (Scout) to determine required mAsmAs as as a function of distance (z)a function of distance (z) To be fully successful need to ensure the SPR To be fully successful need to ensure the SPR

is acquired is acquired over the full length of the intended helical acquisitionover the full length of the intended helical acquisition at the same at the same kVpkVp as the helical acquisitionas the helical acquisition


Effective mAs versus distanceReference effective mAs = 300

0

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250

0 50 100 150 200 250Relative distance (mm)

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Lung apex 160 mAs

Mid cervical 58 mAs

Orbits 75 mAs

SPR Attenuation MeasurementsSPR Attenuation Measurements

Aortic arch 200 mAs


Rotational Current ModulationRotational Current Modulation

The The mAmA is altered as Xis altered as X--ray tube rotates ray tube rotates about patient about patient


Rotational Current ModulationRotational Current Modulation

The The mAmA is altered as Xis altered as X--ray tube rotates ray tube rotates about patient about patient It may use angleIt may use angle--dependent a ttenuation dependent a ttenuation

information on the fly information on the fly -- Siemens & PhilipsSiemens & Philips

OROR AP and Lateral SPRs AP and Lateral SPRs GE & ToshibaGE & Toshiba

Typically the Typically the mAmA used in the AP & PA used in the AP & PA projections will be significantly less than projections will be significantly less than when the lateral projections are being when the lateral projections are being acquiredacquired


Image noise is largely dictated by the projections in which the radiation is most strongly attenuated

Why Use Rotational Current Modulation?

Lateral projection

AP projection

Intensity IAP = IlatNoise sdAP = sdlat

AP projection

Intensity IAP > IlatNoise sdAP < sdlat


(b) Constant tube current

(c) mA modulation - for the same dose level the noise can be significantly reduced

(d) & (e) or for the same noise level the dose can be reduced, for example by 32%

Rotational Current Modulation

sd = 33 sd = 25

(from Kalendar 2005)


(a) Typical mA and noise distributions in scanning with constant mA. Noise (white dots) is highly variable

AEC Controlled mA Modulation


(a) Typical mA & noise distributions in scanning with constant mA. Noise (white dots) is highly variable

(b) AEC-controlledscanning with the mA adapted to the attenuation & modulated as a function of projection angle. A constant noise level is achieved at significantly reduced dose.

AEC Controlled mA Modulation

(from Kalendar 2005)


Automatic Exposure ControlWARNING!

Automatic Exposure ControlWARNING!

The use of AEC does NOT in itself The use of AEC does NOT in itself guarantee dose reductions but should guarantee dose reductions but should ensure ensure consitentconsitent image qualityimage quality

In film/screen radiography the film In film/screen radiography the film blackness (Optical density) controls doseblackness (Optical density) controls dose

No such physical limitation applies in No such physical limitation applies in CT or any other digital modalityCT or any other digital modality

REQUIRES HUMAN INTERVENTIONREQUIRES HUMAN INTERVENTION



Clinical d ecisions need to be made as Clinical d ecisions need to be made as to what constitutes acceptable image to what constitutes acceptable image quality or noise. Scanners require input quality or noise. Scanners require input of either:of either: a soa so--called predetermined called predetermined referencereference noise noise

index (NI) or standard deviation (SD) orindex (NI) or standard deviation (SD) or a a reference effective reference effective mAsmAs judged to be judged to be

suitable for imaging the suitable for imaging the average sizedaverage sized (80 (80 kg) patientkg) patient



Once decision is made the AEC should Once decision is made the AEC should ensure consistent image quality and ensure consistent image quality and inevitable r educe doses.inevitable r educe doses.


Suppose noise index of about 30 HU is desired for coronary Suppose noise index of about 30 HU is desired for coronary calcium measurements. The calcium measurements. The mAsmAs values can be reduced values can be reduced significantly for smaller patient crosssignificantly for smaller patient cross--sections, shown here for a sections, shown here for a Cardiac CT phantom with 40 and 35 cm lateral diameter, Cardiac CT phantom with 40 and 35 cm lateral diameter, respectively. Dose is reduced by ~ 65% when the respectively. Dose is reduced by ~ 65% when the mAsmAs is lowered is lowered from 200 to 69 under AEC. from 200 to 69 under AEC.

(From Kalendar 2005)(From Kalendar 2005)


Toshiba Aquillion 16Toshiba Aquillion 16


Toshiba Aquillion 16Toshiba Aquillion 16

Standard deviation is measure of noise


Toshiba Aquillion 16Toshiba Aquillion 16Scanner indicates the

variation of mA on the SPR


GE VCTGE VCT


GE VCT ScreenGE VCT Screen

These arrows adjust NI by 5% and hence dose by 10%



Decision about the reference noise index Decision about the reference noise index should be made on a protocol by protocol should be made on a protocol by protocol basis.basis. The nonThe non--contrast phase of contrast phase of abdoabdo/pelvis scan, for /pelvis scan, for

example, does not need the same low noise example, does not need the same low noise requirement as the contrast phasesrequirement as the contrast phases Set reference noise index higher (or reference Set reference noise index higher (or reference mAsmAs

level lower)level lower)

The The mAsmAs actually utilized for a particular actually utilized for a particular patient may be very patient may be very much l owermuch l ower for slightly for slightly built patients than the maximum value the built patients than the maximum value the AEC allowsAEC allows


AEC Controlled mA Modulation6 year old child scanned using adult protocol. Reference effective mAs = 165 and mean actual effective mAs = 38

From McCollough et al Radiographics 26 (2006) p 503 -513



The correct choice of noise index (NI) or The correct choice of noise index (NI) or standard deviation (SD) to be used is dictated standard deviation (SD) to be used is dictated by the image slice width of the by the image slice width of the first series in first series in the reconstruction.the reconstruction. A noise index of 12 may seem appropriate for A noise index of 12 may seem appropriate for

abdominal imaging with 5 mm image slicesabdominal imaging with 5 mm image slices This would be totally inappropriate for This would be totally inappropriate for

reconstruction with 0.625 mm slices!! If this was reconstruction with 0.625 mm slices!! If this was done the implication would be an increase in dose done the implication would be an increase in dose by a factor of 8!!by a factor of 8!!

Thin slices should be reconstructed with much Thin slices should be reconstructed with much higher values of the NI.higher values of the NI.



A modest change of 10% in NI results in a A modest change of 10% in NI results in a change in dose of change in dose of 20%.20%. For example, a change in the NI from 12 to 14 For example, a change in the NI from 12 to 14

implies a dose reduction of implies a dose reduction of 36%!!36%!! Such a change is barely perceptible in the Such a change is barely perceptible in the

imagesimages OPTIMIZATION is not about tinkering at

the edges large dose savings can be made with little implication for image quality



While the avoidance of both extremely low While the avoidance of both extremely low and extremely high tube currentand extremely high tube currenttime pr oduct time pr oduct values is necessary with regard to the xvalues is necessary with regard to the x--ray ray generator, we also found it most appropriate generator, we also found it most appropriate from a patient dose and image quality from a patient dose and image quality perspective. More aggressive dose reduction perspective. More aggressive dose reduction was not acceptable in children, and more was not acceptable in children, and more aggressive dose increase was not necessary in aggressive dose increase was not necessary in obese patients. obese patients. McCollough et alMcCollough et al



Translated:Translated: You require You require lessless noise with childrennoise with children

You can tolerate You can tolerate moremore noise with obese noise with obese patientspatients

The NOISE INDEX should be changed The NOISE INDEX should be changed when changes in patient size are extreme.when changes in patient size are extreme.


Optimization (early days before AEC)Optimization

(early days before AEC)

Effective mAs = 380Effective mAs = 380 Effective Effective mAsmAs = 300= 300Equivalent change in NI Equivalent change in NI 12%12%


Optimization(with CareDose)Optimization(with CareDose)

Reference effective mAs = 180Reference effective mAs = 180Effective mAs = 149Effective mAs = 149

Reference effective Reference effective mAsmAs = 120= 120Effective Effective mAsmAs = 105= 105Equivalent change in NI Equivalent change in NI of of 20%20%


OptimizationOptimization

Fixed effective Fixed effective mAsmAs = 360= 360

3mm sliceB60s

L5

3mm sliceB60s

L5

Effective Effective mAsmAs = 232= 232Equivalent change in NI Equivalent change in NI of of 25%25%


Optimization - SummaryOptimization - Summary Perform head scans in sequential mode using wide

collimation with angled gantry to avoid orbits

Consider using wide collimation to minimise over-beaming and scan patient anatomy in one large block to minimise over-scanning rather than two or more smaller blocks

Consider using sequential scanning with wide collimation when scanning short scan blocks to avoid over-scanning issues

Minimise use of multi-phasic examinations


Optimization - SummaryOptimization - Summary Do Do NOTNOT transfer scan protocols from one scanner to transfer scan protocols from one scanner to

another without due consideration to differences in another without due consideration to differences in scanner geometry and filtrationscanner geometry and filtration

In absence of AEC keep the In absence of AEC keep the effective mAseffective mAs as low as as low as clinically indicatedclinically indicated Effective mAs = mAs/pitch Effective mAs = mAs/pitch

Use tube current modulation technology (AEC) Carefully select reference effective mAs or noise index

Do NOT accept manufacturers default scan protocols as being optimized


RecommendationsRecommendations Optimization of scan protocols depends upon the Optimization of scan protocols depends upon the

technical expertise of clinical users (radiographers & technical expertise of clinical users (radiographers & radiologists) and scientific support staff. radiologists) and scientific support staff.

Thus, before attempting to optimize scan protocols, Thus, before attempting to optimize scan protocols, radiographers, radiologists and support staff need to radiographers, radiologists and support staff need to understand the scanner technology and to appreciate understand the scanner technology and to appreciate clinical imaging requirements.clinical imaging requirements.

Practices should work towards development of Practices should work towards development of optimized doseoptimized dose--image q uality relationship(s).image q uality relationship(s).

When choosing/optimizing protocols take care to When choosing/optimizing protocols take care to ensure that they are tailored to clinical need & patient ensure that they are tailored to clinical need & patient size size -- no universal right dose answersno universal right dose answers..


CT doses are of a magnitude that may represent a risk of cancer in the patient.

MDCT doses may be higher t han single slice CT doses

Many factors may contribute to increased patient dose when MDCT is utilised.

Care must be taken in choosing protocols so that they are tailored to clinical need & patient size.

SummarySummary


ReferencesReferences Heggie JCP et al Importance in optimization of multiHeggie JCP et al Importance in optimization of multi--slice slice

computed tomography scan protocols Austral Radiol (2006) computed tomography scan protocols Austral Radiol (2006) 50, 27850, 278--285.285.

Kalra M et al Strategies for CT radiation dose optimization Kalra M et al Strategies for CT radiation dose optimization Radiol (2004) 230, 619Radiol (2004) 230, 619--628.628.

McCollough CH et al CT Dose Reduction and Dose McCollough CH et al CT Dose Reduction and Dose Management Tools: Overview of Available Options Management Tools: Overview of Available Options RadioGraphics (2006) 26, 503RadioGraphics (2006) 26, 503512.512.

ICRP Publication 102 (2007). ICRP Publication 102 (2007).

Heggie JCP Patient doses in multiHeggie JCP Patient doses in multi--slice CT and the slice CT and the importance of optimization Austral Eng Phys Sci Med importance of optimization Austral Eng Phys Sci Med (2005) 28, 86(2005) 28, 86--96.96.


In the end there was a beginning!In the end there was a beginning!

Pregnancy diagnosed by CTPregnancy diagnosed by CT


The Public Perception of Radiation!

The Public Perception of Radiation!

He has grown a foot since I last saw him


Breast Dose & Lifetime Attributable Risk of Cancer

Induction per 100,000

Breast Dose & Lifetime Attributable Risk of Cancer

Induction per 100,000Age at exposure (yr)

Dose (mGy) 15 30 50

10 55 25 7

50 270 126 35

100 553 253 70

Derived from BEIR VII Report 2006


Achievable MDCT Doses Achievable MDCT Doses

Examination (Adult) DLP (mGy.cm) E(mSv)

Brain 610 1.4

CTPA 160 3.3

KUB 240 4.2

Lumbar spine 435 6.9


Additional ReadingAdditional Reading


The Physicists Compliant Friend:Mr/Mrs Walker

The Physicists Compliant Friend:Mr/Mrs Walker

ImPACT

CT Patient Dosimetry Calculator

version 0.99x, 20/01/2006

CT-Expo V 1.5.1 Nov 2005


Siemens Sensation 16Siemens Sensation 16ProcedureProcedure Reference Effective mAsReference Effective mAsHeadHead 260260Chest routineChest routine 9090Chest with portal venousChest with portal venous 90/15090/150Chest HRChest HR 100100Chest/Abdo/pelvisChest/Abdo/pelvis 90/15090/150Abdo/Pelvis with contrastAbdo/Pelvis with contrast 150150Abdo/Pelvis triple phaseAbdo/Pelvis triple phase 120/150/150120/150/150Renal (KUB)Renal (KUB) 120120Lumbar s pineLumbar spine 300300


Focal Spot WanderingFocal Spot Wandering Thermal/mechanical Thermal/mechanical

effects cause focal effects cause focal spot position to move spot position to move during scanningduring scanning Some manufacturers Some manufacturers

increase umbra & increase umbra & signals from two signals from two detector arrays detector arrays outsideoutside imaging imaging volume are used to volume are used to control position of control position of collimatorscollimators

DOSE INCREASES

Signals from outside detector elements are used to monitor X-ray

output

X-ray beam in red contributes to dose

but not images

collimator


GEs Dynamic RollersGEs Dynamic Rollers

Computer controls rollers to adjust position

of collimator laterally maintaining constant X-

ray flux over central image detector rows

Signals from outside rows of detector elements fed to

computer


Rotate - RotateRotate - Rotate 3rd generation3rd generation

Fan XFan X--ray beamray beam XX--ray tube & detector ray tube & detector

array rotate as one about array rotate as one about patientpatient

600600--1000 elements1000 elements Whole body subsecond Whole body subsecond

single slice scanssingle slice scans PrePre--patient collimation patient collimation

defines slice width ~1defines slice width ~1--10 10 mmmm

X-ray tube

Detector array

Bow tie filter

PatientProfile


Multi-slice Helical CT (3)Multi-slice Helical CT (3)


Multi-slice Helical CT (4)Multi-slice Helical CT (4) Now use Cone Beam Reconstruction:Now use Cone Beam Reconstruction:

Corrects for finite angle of cone beamCorrects for finite angle of cone beam

In reconstructing a particular axial image slice uses In reconstructing a particular axial image slice uses only those detector elements that instantaneously view only those detector elements that instantaneously view the slice of interest the slice of interest not ones viewing adjacent tissues.not ones viewing adjacent tissues.

As table advances and detector rotates different detector As table advances and detector rotates different detector elements will contribute to the image sliceelements will contribute to the image slice

Improved imaging in sagittal, coronal & oblique Improved imaging in sagittal, coronal & oblique planesplanes

Improved volume imaging allowing CTImproved volume imaging allowing CT--angiography, ECG gated cardiac studies etc.angiography, ECG gated cardiac studies etc.


Detectors & X-ray Beam Width

Routine abdo scan

Detectors & X-ray Beam Width

Routine abdo scan


Pitch & Table FeedRoutine abdo scan

Pitch & Table FeedRoutine abdo scan

Pitch Pitch = table feed per rotation / beam collimation= table feed per rotation / beam collimation

= 36 / 24 = 1.5= 36 / 24 = 1.5


Effective mAsEffective mAs

Effective mAs = mA * s / pitchEffective mAs = mA * s / pitch

= mA * s * beam collimation / table feed= mA * s * beam collimation / table feed


Effective mAs & DoseEffective mAs & Dose

Dose index (CTDIDose index (CTDIvolvol) i s a measure) i s a measure

of the average absorbed dose to of the average absorbed dose to

one slice of the patient assuming one slice of the patient assuming

typical patient dimensionstypical patient dimensions

CTDICTDIvolvol effective mAseffective mAs= 8.4 mGy= 8.4 mGy

Double the effective mAs & Double the effective mAs & CTDICTDIvolvol doublesdoubles


Care Dose & CareDose 4DCare Dose & CareDose 4D

This is Siemens version of AEC This is Siemens version of AEC

Incorporates rotational and ZIncorporates rotational and Z--axis axis

modulation of the tube currentmodulation of the tube current

The The Eff mAsEff mAs is the reference is the reference

mAs that has been chosen from mAs that has been chosen from

clinical experience to be clinical experience to be

appropriate for imaging the average appropriate for imaging the average

adult patient adult patient


Multi-slice Helical CT (2)Multi-slice Helical CT (2)

Cone beamFan beam


Care DoseCare Dose

Constant current

Attenuation correctionfrom topogram

Shoulders

Chest

Abdo

Pelvis

Modulation on the fly

as it rotates

Current (mA)

Distance z-axis


Helical ScanningHelical Scanning

Patient motion

Path o f continuously rotating X-ray tube relative to patient

33rdrd generation generation plus: Slip ring technology Slip ring technology

allows for continuously allows for continuously rotating Xrotating X--ray tube with ray tube with transfer of low voltage transfer of low voltage and data from/to outside and data from/to outside worldworld

Continuous advance of Continuous advance of patient couch @ 1patient couch @ 1--10 10 mm/smm/s

Single breath hold Single breath hold imaging (~60 s)imaging (~60 s)


MDCTMDCT Helical plus:Helical plus:

2 to 64 discrete 2 to 64 discrete detector rowsdetector rows

Reconstructed Reconstructed image slices of image slices of between 0.5 & between 0.5 & 10 mm10 mm

Improved Improved volume imaging volume imaging AngiographyAngiography

Calcium Calcium scoring etc.scoring etc.

RANZCR QUDI MMC July 2010(from Kalendar 2005)


Siemens Sensation 16 (with CareDose 4D)


Cerebrum: Reference Effective mAs = 260

0

5

10

15

20

25

30

180 190 200 210 220 230 240 250 260 >260

Effective mAs

F

r

e

q

u

e

n

c

y




Abdo / Pelvis contrast: Reference Effective mAs = 150

0

1

2

3

4

5

6

7

8

70 80 90 100 110 120 130 140 150 160 170 >180

Effective mAs

F

r

e

q

u

e

n

c

y


Siemens Sensation 16 (with CareDose)


Abdomen/pelvis with Contrast

0

5

10

15

20

25

30

35

40

250 300 350 400 450 500 550 600 650 >650

DLP (mGy.cm)

F

r

e

q

u

e

n

c

y

Ref effective mAs = 180Ref effective mAs = 120




Lumbar Spine

0

5

10

15

20

25

300 350 400 450 500 550 600 650 700 >700

DLP (mGy.cm)

F

r

r

e

q

u

e

n

c

y

Fixed effective mAs = 360Ref effective mAs = 300


MDCT Doses DLP (mGy.cm) MDCT Doses DLP (mGy.cm) Examination (Adult) CDose1

Brain 960

Chest 220

with contrast 485

CAP with contrast 765

AP with contrast 555

triple phase 1155

Lumbar spine 560


MDCT Doses DLP (mGy.cm) MDCT Doses DLP (mGy.cm) Examination (Adult) CDose1 CDose2

Brain 960 660

Chest 220 195

with contrast 485 370

CAP with contrast 765 560

AP with contrast 555 345

triple phase 1155 725

Lumbar spine 560 455455


MDCT Doses DLP (mGy.cm) MDCT Doses DLP (mGy.cm) Examination (Adult) CDose1 CDose2 CD4D

Brain 960 660 605

Chest 220 195 190

with contrast 485 370 370

CAP with contrast 765 560 555

AP with contrast 555 345 325

triple phase 1155 725 700

Lumbar spine 560 455455 420420

CT Dose Optimisation

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