Techniques and technology for dose reduction in CT - ImPACT CT ...

Techniques and technology for dose reduction in CT 

Nick Keat, ImPACT 

St George’s HospitalI 

London 

Dose Reduction in Diagnostic Radiology

About ImPACT 

• Based at St George’s 

Hospital, Tooting, London 

• Employed by Medicines and 

Healthcare products 

Regulatory Agency (MHRA*) 

to evaluate CT scanners for 

the NHS 

*formerly MDA 


Introduction – techniques and technologies 

• Is there a problem? 

• Where do opportunities for dose reduction exist? 

• What can we do now? Techniques 

• What is in development? Technologies 


Is there a problem? 

Jan 22, 2001 

CT scans in children linked to cancer 

By Steve Sternberg, USA TODAY 

Each year, about 1.6 million children in the USA get CT scans to the head and abdomen 

— and about 1,500 of those will die later in life of radiation-induced cancer, according to 

research out today. 

What's more, CT or computed tomography scans given to kids are typically calibrated 

for adults, so children absorb two to six times the radiation needed to produce clear 

images , a second study shows. These doses are "way bigger than the sorts of doses that 

people at Three Mile Island were getting," David Brenner of Columbia University says. 

"Most people got a tenth or a hundredth of the dose of a CT." 

Dose Reduction in Diagnostic Radiology 

Brenner DJ et al Estimated risks of radiation-induced 

fatal cancer from pediatric CT. AJR 176:289-296 (2001)


• CT is inherently a high dose examination, and dose is rising 

• Numbers are relatively low : ~4% of examinations 1 

• BUT total contribution to doses from medical exams is high: 

~40% of total 1 

1989 1999 

2% 

20% 

4% 

40% 

Exams 

Dose 

Exams 

Dose 


1 

NRPB estimates


• The amount of information in a CT exam is far greater than 

an equivalent plane x-ray 

• Huge amount of diagnostic information available 

• Can’t get ‘something for nothing’ 

CT head ~ 2 mSv 

CT chest ~ 8 mSv 

CT abdomen ~ 10 mSv 

CT pelvis ~ 10 mSv 

X-ray skull ~ 0.03 mSv 

X-ray chest ~ 0.02 mSv 

X-ray abdomen ~ 0.7 mSv 

X-ray pelvis ~ 0.7 mSv 

• CT uses digital detectors that do not over-range like film 

• An ‘over-exposure’ is rewarded with better image quality 

• Until recently, no form of AEC was available for CT 

• More about that later 


Things to bear in mind 

• Diagnostic image quality is overriding concern 

• No point reducing dose until image is useless – 100% wasted 

dose! 

• Some dose savings can be made without affecting image 

quality at all 

• The remainder comes down to dose and image quality 

optimisation 


Where do opportunities exist? 

• Eliminate unnecessary exams 

• Other diagnostic tests may be available 

• Ensure exams are targeted to necessary organs 

• Chestabdomenpelvis is not a single word! 


Where do opportunities exist? 

• Optimise scan protocols 

• Is a pre-contrast scan necessary? 

• Are multiple phases all necessary? 

• kV / collimation / pitch / mAs 

• Tailored exposure parameters for the individual 

• Use gantry tilt 

• Shielding of sensitive organs 

• Comparison with reference doses 


Techniques : Scan protocol optimisation 

• Process of ensuring that the scan and exposure parameters 

provide the best balance in terms of 

• Image quality 

• Radiation dose 

• Other factors, e.g. total scan time 

• Adjustable parameters 

• kV 

• Collimation / slice thickness 

• Pitch / feed per rotation 

• mAs 


Scan protocol optimisation : kV 

• Generally standardised at around 120 kV 

• Lower kV : 80 / 100 can be used 

• Higher dose for same image noise 

• Better contrast than at 120kV 

• Iodine – soft tissue contrast is better, so may be optimal for some 

angiographic head exams 

• Can be used in paediatric protocols to reduce beam intensity 

• Higher kV : 135 / 140 sometimes used 

• Marginally lower dose for same image noise 

• Contrast is slightly poorer 

• Penetration is greater 

• Can be used for obese patients or dense body sections 


Scan protocol optimisation : collimation 

• In multi slice CT, narrow collimations result in higher dose 

due to ‘overbeaming’ in z-axis 

• Avoid imaging using the penumbra of the beam 

• Effect is less significant at wider collimations, and 

for 16 slice than 4 slice at same image width 

• e.g. GE LightSpeed, LightSpeed Ultra and 

LightSpeed 16 

Collimation 

4 x 1.25 (5 mm) 

8 x 1.25 (10 mm) 

16 x 1.25 (20 mm) 

Z-axis geometric 

efficiency (%) 

67 

83 

97 

z-axis 


Scan protocol optimisation : slice width 

• Reconstructed slice width doesn’t directly affect dose 

• Available slice widths defined by collimation 

• Can’t reconstruct a 1 mm slice from 2 mm data 

• Thinner slices are more noisy 

• Slice width affects contrast : partial volume effect 

• Optimum when slice width = z-axis length of object 

Noise ∝ 1/( slice) 

More noisy 


Less contrast 

Contrast ∝ 1/(slice)* 

* Due to partial volume effect

Scan protocol optimisation : helical pitch 

• Higher pitch gives proportionally lower patient dose 

• At the expense of slice width and contrast with single slice 

• At the expense of noise or/and slice width in multi-slice 

• Some manufacturers increase mA to compensate for pitch 

‘effective mAs’ or ‘mAs per slice’ 

• Higher pitch generally gives more helical artefacts 

• But also leads to faster scan times 

• Compromise is necessary 

• Priorities depend on application 


Scan protocol optimisation : mAs 

• Dose is proportional to mAs 

• Image noise decreases with increasing mAs 

• noise ∝ 1/( 

mAs) 

• Optimal mAs is very difficult to define 

• Always have to err on the side of over-exposure to avoid 

repeat exams. But by how much? 

• Variety of techniques can be used: addition of simulated noise, 

studies with cadavers 


Techniques : tailored exposure parameters 

• A protocol mAs is generally defined for ‘standard’ sized 

patient 

• Need to adjust this for larger and smaller patients to ensure 

consistent image quality 


Tailored exposure parameters for the individual 

• X-ray intensity halved approx. every 5 cm of patient 

• Could double/halve mAs for patients 5 cm larger or smaller 

than standard 

• mA adjustment usually more conservative than this 

• Image quality requirements may be different for different sized 

patients; generally want lower noise for small patients 

• Suggested to double/halve mAs for patients 10 cm larger or 

smaller than standard* 


*Dose optimization in CT: implementation and clinical 

acceptance of size-based technique charts, 

CH McCollough, RSNA 2002

Tailoring exposure parameters to the individual 

• e.g. for abdomen scans, measure lateral patient width from 

AP scout at liver. Standard mAs is set for 40cm* 

Patient Width (cm) 

Relative mAs 

> 21 - 26 

> 26 - 31 

> 31 - 36 

> 36 - 41 

> 41 - 46 

> 46 - 51 

0.4 

0.5 

0.7 

1.0 

1.4 

2.0 


*Dose optimization in CT: implementation and clinical 

acceptance of size-based technique charts, 

CH McCollough, RSNA 2002

Weight / age based paediatric protocols 

• Pre-programmed paediatric protocols available on most 

systems 

• Siemens & Philips 

• age-based for heads 

• weight-based for bodies 

• GE colour-coded system 

• assigns scan parameters according to weight-zone colour 

e.g. 6 - 7.4kg, 7.5 - 8.4kg, 8.5 - 11.4kg, 11.5 - 14.4kg … 

• ties in with hospital-wide Broselow-Luten, ‘Rainbow’ System 

which covers drug doses, needle sizes etc 


Techniques : Gantry tilt 

• Has the potential to reduce dose to sensitive organs, e.g. 

the eye lenses in head scanning 

• Dose levels are too low for cataract formation 

~ 50-100 mGy to lenses from head CT 

cataract threshold ~ 500 (acute) – 1000’s mGy 


Techniques : shielding sensitive organs 

• Attenuating bismuth based shields are available for 

shielding of superficial sensitive organs 

• Can reduce the dose to the lens of the eyes, thyroid or breast 

• Re-usable, but sterilisation can be an issue 


Radioprotection to the Eye During CT Scanning, 

Hopper et al, AJNR 22:1194-1198 (6 2001)

Techniques : extra rotations in helical scanning 

• To reconstruct the first and last image in a helical series, 

data is gathered from outside the nominal scan volume 

• Irradiation outside the scan volume can depend on various 

factors (collimation, slice width, pitch, recon. technique) 

• More significant for short multi-slice scans, with small no. of 

rotations 

• Worth considering different collimation or axial scanning in 

these situations 


Techniques : comparison with reference doses 

• Reference doses are available, giving CTDI w and DLP for a 

range of common examinations 

• See http://www.drs.dk/guidelines/ct/quality/index.htm 

• Data based mainly on NRPB survey data from 1990 

• NRPB, in association with the CT Users Group and ImPACT 

are updating this data. Survey data has been returned, and 

is being analysed 

• CTDI w and DLP values 

are displayed on modern 

scanner consoles 

for comparison 

CTDIw 2.28mGy 


Technology : Multi-slice scanners and dose 

• Four slice scanners first introduced in 1998 by four major 

manufacturers 

• 16 slice systems 

now widely available 

• Possible to buy a 

1, 2, 4, 6, 8,10 or 16 

slice system 


Multi-slice scanners and dose 

• Have already covered collimation issues 

• Apart from that, there are no significant differences in basic 

dosimetry between single and multi-slice 

• Main issues relate to how the scanners are used 

• Multi-slice scanners are more flexible 

• Can use their advantages to give faster scanning, better z-axis 

resolution or greater coverage 


Multi-slice scanners and dose 

• Greater flexibility afforded by MS CT can tempt users to 

• scan larger volumes 

• scan more phases in multi-phase studies 

• increase mAs to keep noise down in thin slices 


What is being developed : technology 

• Multi-slice is the most significant development since helical 

• Main technology being developed in CT affecting patient 

dose is automated tube current adjustment 


How the systems work 

• In order to set mA, systems need knowledge of patient 

attenuation. This comes from: 

• SPR (scout, pilot, topogram or scanogram) views and/or 

• Knowledge of the attenuation from the previous rotation 

• To use the system, the operator needs to be able to set the 

desired exposure level by use of: 

• A ‘standard’ mAs 

• A reference image of the desired image quality 

• A desired image noise level 


Technology : automatic mA adjustment 

• Can work on 3 levels, adjusting for attenuation differences 

• From patient to patient 

• Along patient length – z-axis 

• In AP and lateral directions – rotational 

attenuation 

low attenuation 

high attenuation 

• CT has finally caught up with planar x-ray and has the 

equivalent of an AEC 

• In CT, tube current, not exposure time is being controlled 


Automatic mA adjustment 

• Tube current is adjusted during scanning to compensate for 

attenuation differences – dose applied to patient only where 

needed, avoiding dose where it isn’t 

mA 

position 


Advantages of automatic mA control 

• More constant signal to detectors 

• Image quality is maintained at constant level 

• Tube heat capacity conserved 

• Reduction in ‘photon starvation’ streak artefact 

• Dose optimisation 


What’s available now 

• All systems still being developed 

• The following shows current availability – all manufacturers 

say more will be available in future versions 

Patient size mA 

adjustment 

Z-axis 

adjustment 

Rotational 

adjustment 

GE 

Philips 

Siemens 

DoseRight ACS 

Auto mA 

DoseRight DOM 

CareDose 

Toshiba 

Real EC 


Results from automatic mA systems 

• ImPACT have tested auto mA systems on 16 slice scanners 

from all manufacturers, using an elliptical cone phantom 

Noise (%) 

28 

24 

20 

16 

12 

automA off 

Noise Index 16 

Noise Index 12 

Noise Index 8 

Noise Index 4 

8 

4 

0 

-150 -100 -50 0 50 100 150 

Z-position (mm) 


Results from GE LightSpeed 16

Summary 

• Patient doses from CT are high despite the relatively low 

number of examinations 

• The cost in dose is rewarded with extra diagnostic 

information 

• Techniques with existing equipment to reduce dose are the 

usual ones : justification and optimisation 

• The technology is advancing with systems that have 

automatic tube current control 

• Improves the dose – image quality trade off 

• Aids optimisation of technique 

Slides from this talk will be available at 

www.impactscan.org next week

Techniques and technology for dose reduction in CT - ImPACT CT ...

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