a neonatal phantom for cr and dr quality...

$: A Neonatal Phantom for CR and DR Quality Assurancechapter.aapm.org/nccaapm/z_meetings/2008-04-25/...Apr 25, 2008 · 0.60 0.70 0.80 0.90 1.00 40 50 60 70 80 90 100 kVp fractional$
25 april 2008 ljrutz ncaapm 1

A Neonatal Phantom for CR and DR Quality Assurance


Acknowledgements

Research supported by an NIH STTRGrant # 2R42HD040747 Phases 1&2

Gammex, Inc. Grantee Washington University, St. Louis, MO


Washington University

Steven Don – Pediatric Radiologist, PIBruce Whiting – Imaging Physicist, Co-investigator

Parinaz Massoumzadah – Imaging PhysicistGlenn Fletcher – Medical Physicist

Charles Hildebolt – StatisticianRuth Holdener – Technologist

Summer students


Participating University Investigators

Duke UniversityDon Frush – Pediatric RadiologistEhsan Samei – Imaging Physicist

Henry FordMichael Flynn – Imaging Physicist

Oregon Health Science UniversityKatharine Hopkins – Pediatric Radiologist

Paul Brown – Medical Physicist


Gammex, Inc.

R&D, Engineering & Production Staff

Disclosure• L. Rutz currently contracted consultant for Gammex, Inc


Promise of Computed Radiography

• Exposure Reduction– High sensitivity of the image plates would allow reduced patient exposure.– Retakes resulting from under and over exposures would be eliminated.– CR maps the exposure to the proper gray level.

• Improved Image Quality– Intelligent image processing would allow “optimization” of image information.– Image processing could be customized to the preference of the radiologist.– Spatial frequency enhancement would improve visibility of fine details.– Contrast enhancement and selection of display parameters would optimize

visualization of low contrast information.

Great Appeal to Pediatric Radiologists


Computed radiography is different from screen/film radiography.

• The CR system uses a-priori knowledge of the anatomy to optimize the image.

• Application of gray-scale mapping and spatial frequency enhancement.

• Histogram analysis plays a key role in the image processing.– Selection of data to be treated as anatomy.– Scaling of data into proper display range.


Grayscale Mapping


Histogram Based Data Selection


Effect on Routine Quality Assurance

• Image quality assessment now includes the effects of image processing.

• Image processing depends on a-priori knowledge of the anatomy being imaged.– Tissue attenuation characteristics.– Size– Expected histogram shape.

25 april 2008 ljrutz ncaapm11

More creative anthropomorphic Phantoms

Hard to use more than once.

Require test protocolsStylized, does match histogram.

Semi-anthropomorphic. Technical Phantoms


To test all aspects of the system, including the effect of the image processing….

We need a phantom that can mimic both anatomic structure and radiographic properties of the tissue composition.

To see this. You need this.

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25 april 2008 ljrutz ncaapm 13Solution?


OK, Back to the Drawing Board

• Design a New Phantom• Build a Prototype• Test against Physics Requirements• Test against Clinical Requirements.

SteveBruce

Lois

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Why build a Neonatal chest phantom

• Importance of CR for dose reduction in Pediatric Radiology

• Technical difficulty of imaging neonates with CR

– Small size of structures, and similar tissue attenuation characteristics, challenge the technical limits of the system.

– Image quality factors of resolution and signal to noise ratio are stressed in neonatal imaging.

• If a system tests well for neonatal imaging it should be capable of good image quality in general.


Requirements

• Histogram must mimic a real neonate at all clinically relevant beam qualities.

• Size, shape and structures must be close enough to real neonate to give radiologists and technologists a visual reference.

• Should have the ability to demonstrate image quality using clinical protocols, not test settings.

• Should provide a means of demonstrating the effects of changes to input and image processing parameters.

• There should be some imaging challenge so that image degradation or relative inferiority could be determined.


Creating the Physical Model

• Used CR images and CT scans of cadavers.– established sizes and major tissue types.

• Used anatomical drawings, film images and reference materials to refine the structures’ appearances, especially the bronchial tree.


m ate rialde nsi ty in

g/cc

%H %C %N %O %Na %Mg %S i %P %S %C l %K %C a Total

newborn cortical

bone 1.72 4.4 15.3 4.1 47.8 0.1 0.2 8.5 0.2 0.1 19.4 100.1newborn

femur1.18

8.7 10.5 2.5 69.1 0.2 0.1 2.6 0.1 0.3 5.9 100fetal lung 1.04 10.6 7.6 1.8 79.2 0.2 0.2 0.1 0.2 0.1 100adult lung 0.26 10.3 10.5 3.1 74.9 0.2 0.2 0.3 0.3 0.2 100newborn muscle 1.05 10.4 10.3 2.4 76.2 0.1 0.1 0.1 0.2 0.2 100

Inner Bone 456

1.1336.67 55.64 1.96 23.52 3.23 0.11 8.86 100

CB2-50% 1.56 4.77 41.63 1.52 32 0.08 20.02 100LN450 0.45 8.47 59.57 1.97 18.11 11.21 0.58 0.1 100

LN300 0.3 8.46 59.38 1.96 18.14 11.19 0.78 0.1 100Muscle 452

1.05 8.1 67.17 2.42 19.85 0.14 2.32 100

Chemical Constituents

Creating the Tissue Types• Selection or creation of tissue mimicking materials.

Used published references for neonate: e.g. ICRP 23


Predicted Transmission Fractions as a function of kVp

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LN450Adult lungICRU fetal lung


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newborn femur(ICRU)newborn corticalbone (ICRU)IB456


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452 musclenewborn muscle

Computational Comparisons


Muscle at 90 kVp

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90 kvp calc90 kvp meas

Lung material 90 kVp y = 0.9869e-0.0112x

y = 0.9711e-0.0119x0.000.100.200.300.400.500.600.700.800.901.00

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calcmeasExpon. (calc)Expon. (meas)

Bone at 90 kVp

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Validation of Computation


Lung material transmission spectra at 90 kVp

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lung, m uscle, rib

lung, m uscle, riblung, m uscle, rib

m uscle, air

m uscle, vertebra,air

unattenuated spectrum

Predicted transmission comparison for selected sections of the neonatal phantom using LN450 and NNLN4

Sections are denoted as AA, BB etc to correspond to dimensions of draft drawings. Section C'C' is the cross section half waybetween DD and EE.

The assumption is that the NNLN4 will have a physical density of 0.45.

added filtration

Aluminum LN450 NNLN4 Muscle Bone Air Total

cm^2/gm cm^2/gm cm^2/gm cm^2/gm cm^2/gm cm^2/gm2.700 0.45 0.45 1.05 1.560 0.0012t in cm t in cm t in cm t in cm t in cm t in cm

lung, muscle, rib AA 0 2.9 2.9 2.2 0.3 0 5.4lung, muscle, rib BB 0 3.485 3.48 1.62 0.3 0 5.4lung, muscle, rib CC 0 3.2 3.2 0.9 0.3 0 4.4muscle,air DD 0 0 0 3.4 0 2 5.4muscle, vertebra C'C' 0 0 0 2.8 0.5 2.1 5.4

Refining the Model

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Phantom 1.3

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Phantom 1.3

Phantom 1.3 Phantom 1.3Phantom 1.3

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Phantom 1.3 and Cadavers

Green plus = 1985-2590 gramsYellow circle = 960-1380 gramsRed dot = 484-640 grams

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P hantom 1.3

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P hantom 1.3 and Cadavers

G reen plus = 1985-2590 gram sY ellow c ircle = 960-1380 gram sR ed dot = 484-640 gram s

Validating the Fit

• Early Prototypes


Validation: Phantom adequately represents a 1kg to 2kg neonate both anatomically and

radiographically.

• Thirty clinical images were compared to phantom images obtained using identical technique factors at three sites on three different CR systems.

• The raw image data were analyzed using ImageJ .• Pixel values of the whole body, right lung base, heart, liver,

stomach bubble, humeral metaphysis, muscle, subcutaneous fat and pneumothorax were normalized relative to direct exposure (air) for each image.

• The results were plotted against the weight distribution of the patients.


Patient 3

Region Area Mean StdDev Min Max mR AttenuationBackground 9.033 2468.258 4.084 2456 2481 2.94 0.00lead marker 0.677 1354.222 7.965 1341 1376 0.23 2.57

right lung base 1.807 2143.995 6.648 2123 2161 1.39 0.75heart 1.449 1917.409 7.971 1899 1936 0.83 1.27liver 0.979 1859.221 5.152 1841 1870 0.72 1.40

stomach bubble 0.979 1859.221 5.152 1841 1870 0.72 1.40humeral metaphysis 0.931 2100.192 5.07 2089 2111 1.26 0.85

muscle 1.694 2242.844 4.943 2229 2253 1.75 0.52subcutaneous fat 1.035 2347.727 6.03 2332 2359 2.23 0.28

pneumothorax

Phantom 1.1Region Area Mean StdDev Min Max mR Attenuation

bkground 177.077 2476.883 4.798 2458 2497 3.00 0.00t5 vertebra 6.436 1942.572 7.334 1910 1965 0.88 1.23

stomach bubble 14.076 2047.932 6.654 2027 2070 1.12 0.99heart by t6 7.151 1973.524 7.052 1951 1996 0.94 1.16

t6 rib over lung 0.79 2060.786 9.023 2036 2078 1.15 0.96t6 rib over heart 2.785 1966.828 7.363 1946 1986 0.93 1.17liver uner rtlung 4.573 1971.547 7.128 1951 1997 0.94 1.16

last vertebra, stom bub 6.737 2016.089 6.676 1993 2037 1.04 1.06lung/t3-4 5.57 2077.48 9.247 2051 2102 1.20 0.92

Final Design Validation

Kodak Whole Body

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PNTXNormalRDSFit Normal

Fit PNTXFit RDSFit All

Validating the Fit

Phantom

• Later versions, larger data set.


Phantom Weight (in kg)*

Center Heart Thorax T3 rib Humerus

Wash U 1.1 1.1 1.4 1.7

OHSU 1.0 1.3 1.5 1.5

Duke 1.2 1.2 1.6 1.5

*based on log neonate weight and log size


Validating the Histogram Equivalence

• Whole body histograms made from clinical images.– Use ImageJ.– Distributions of size and technique factors.– Included best approximation of thorax area to

match phantom.

comparison of histogram for 10 normal neonates and phantom

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Histogram Comparisons

normalized histogram comparison for 10 normal neonates and phantom

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Nearly Final Phantom

Torso with heart, stomach bubble, fat layer.

Rib, spine, clavicle and part of humerus

Bronchial tree

Normal Lungs: with pneumothorax

Hyaline Membrane Disease Lungs without pneumothorax.


Adding Clinical Relevance to the QA Process

• Simulated disease processes are used as indicators of system performance.– pneumothorax for resolution.– hyaline membrane disease for noise

• Both disease states are of particular concern in neonatal imaging.

• Relates quality assurance testing to diagnostic value.


Pneumothorax

• Common problem in the NICU.• Signs are a lucent (black) area against the lung.

– Can be any size or shape, from a fine border along the edge to a balloon sized shadow.

• Often also shows a very fine opaque (white) line along the lung border.

• This is the pleural thickening line that results from the compression of the lung as the air pushes against it.

• Typical size of the line is on the order of 0.2 mm.


Pneumothorax Test Object


Hyaline Membrane Disease

• Radiologists have difficulty distinguishing between healthy and hyaline membrane disease at low exposures.– ALARA is important.– Attempts to avoid retakes for noise resulted in Dose Creep.

• Use of the HMD Lung to assess lower limit of exposure has been demonstrated.– As noise limit is approached, the HMD lung texture becomes

indistinguishable from the normal lung and muscle tissue.


a: 1mR b: 0.25mR c: 0.1mR

Demonstration of use of the neonatal phantom to determine minimum radiation exposure.


Summary

• Simple and effective phantom for routine quality assurance of CR.– Capable of full system assessment using clinical

imaging protocols.• Anticipate other applications.

– Teaching– Acceptance Testing– Evaluation of new imaging techniques, e.g.

filtration, processing parameter selection.

a neonatal phantom for cr and dr quality...

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