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Volume 3, Number 2 June 1983 RadioGraphics 325

Index terms:Radiography

digital

High qualitydigital radiographic images:Improved detection of low-contrast objectsand preliminary clinical studies

Masamitsu Ishida, M.Sc.t

Paul H. Frank, M.D.

Kunio Doi, Ph.D.

James L. Lehr, M.D.

The visibility ofdetail appears to be improved by the use of digitalunsharp masking, a technique that may have beneficial applicationin a variety of digital imaging systems.

THIS EXHIBIT, A SELECTION OF THE BA-DIATION PHYSICS PANEL, WAS DISPLAYEDAT THE 68Th SCIENTIFIC ASSEMBLY ANDANNUAL MEETING OF THE RADIOLOGICALSOCIETY OF NORTH AMERICA, NOVEMBER28-DECEMBER 3, 1982, CHICAGO, ILLI-NOIS.

From the Kurt Rossmann Labora-tories for Radiologic Image Research,Department of Radiology, The Uni-versity of Chicago, Chicago, Illinois.

t Presently, Fuji Photo Film Co.,Ltd., Technology Development Center,Miyanodai Kaiseimachi, Ashigaraka-

migun, Kanagawa 258, Japan.Address reprint requests to Kunio

Doi, Ph.D., Department of Radiology,The University of Chicago, 950 East59th Street, Chicago, IL 60637

* Supported in part by USPHSGrant CA 24806.

Introduction

The major goal in radiographic imaging is to assist the radiologist in arrivingat an accurate diagnosis. The detection of abnormalities is a fundamental task re-quired for accurate diagnosis. For improved detection, it is important to provideradiographic images of superior quality that have high contrast, high resolution,and low noise. It is equally important, however, that the production of such imagesrequire no increase in patient exposure. A compromise among these parameters isnecessarily involved in the design of improved radiographic imaging systems,whether the systems are conventional or digital.

We have developed a high quality digital image processing and simulationsystem with which we can investigate the effect of physical parameters and imageprocessing techniques in radiographic imaging systems. A conventional radiographis used as an input, and a high quality digital film image is produced as theoutput.

In this paper, we demonstrate that the detectability of low-contrast radiographicpatterns can be improved with digital image processing techniques. We also showthe potential use of these techniques to improve diagnostic certainty in clinical ra-diographs without any increase in patient exposure.

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Digital radiographic images Ishida et at

326 RadioGraphics June 1983 Volume 3, Number 2

Figure 1Schematic diagram of the digitalimage processing - and simulationsystem. An original radiographmounted on the transparent readdrum is scanned, and the image sig-nals are digitized by an analog-to-digital converter. After digital imageprocessing, the image data are con-verted back to analog signals by adigital-to-analog converter. Finally, aphotographic film is exposed to aglow tube, the light output of which ismodulated by the processed imagesignals. This system is a powerful re-search tool for studies of the potentialof digital radiography, because manyparameters can be varied indepen-dently over a wide range.

Figure2Specifications for the digital imageprocessing system. The importantfeatures of the system include: (1)high density and high spatial resolu-tion, (2) the production of hardradiographs as the output, (3) a largeimage format, and (4) real-time, on-line operation for overall contrastmanipulation and /or unsharp maskfiltering

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327

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Detection Studies

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Figure 5Nonlinear unsharp mask filtering isalso implemented with the system.The weighting factor K is a functionof the local density in the originalimage. The frequency content of thenonlinearly enhanced radiograph willbe relatively low in low-density areasand high in high-density areas.Therefore, quantum noise is sup-pressed in the low-density areas ofthe nonlinearly enhanced radiographwithout impairing the advantages ofthe unsharp mask filtering. This non-linear technique helps to reduce thenoisy appearance of processed im-ages when a large weighting factor isdesired for an increased enhance-ment.

Figure 7Receiver operating characteristic(ROC) curves for detection of 5 mmlow-contrast disk patterns in radio-graphs made with the X-OmaticRegular/XRP system, with and with-out digital image processing. TheROC curves were determined with 5rating categories. It is apparent thatdetectability is significantly improvedby the use of unsharp mask fil-tering.

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Ishida et al Digital radiographic images


Figures 6A, B & CComparison of original and processedimages containing low-contrast ra-diographic patterns of circular disks5 mm in diameter. The original image(A) at three different backgrounddensities (0.5, 1.0, and 1.5) was ob-tamed by exposure of X0matic#{174}Regular screens (Eastman KodakCo.) with XRP film. The processedimages were enhanced by two typesof image processing techniques: (B)windowing, or overall contrast en-hancement by a factor of 4, in whichthe average density of the processedimage in the center was kept equal tothat of the original image at 1 .0; and(C) linear unsharp mask filtering witha mask size of 12 mm and a weightingfactor of 3.0. Note that the windowingtechnique cannot reproduce thewhole density range because of thelimited dynamic range of the displaysystem. The unsharp masking tech-nique, however, increases only thelocal contrast, so that a wide range ofbackground densities can be repro-duced. Therefore, unsharp mask fil-tering is superior to windowing whenapplied to clinical images. It shouldalso be noted that, although the pro-cessed images appear to be noisy,the circular disks are easily detectedin the center image of (B) and in eachimage of (C).

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Figure 8AFigure 8B

RadioGraphics June 1983 Volume 3, Number 2


330

Clinical Studies

A preliminary clinical evaluation of high-quality digitalimage processing was performed on mammograms, tomo-grams, angiograms, as well as chest and double-contrastgastrointestinal radiographs. An unsharp masking techniquewas used for clinical radiographs because of its advantagescompared to overall contrast enhancement or windowingtechniques as shown in Figure 6.

Ishida et at Digital radiographic images


Figure 8C

Clinical Studies

Figures 8A, B & COriginal chest image (A), and processed images obtained with linear (B)and nonlinear (C) unsharp masking techniques with a 6 mm mask. Thevisibility of lung details in the high-density areas is enhanced in both (B)and (C). In low-density areas, however, where relatively few x-ray quantaare absorbed, the nonlinearly processed image appears to be less noisythan the linearly processed image.

Figure 9A

Figure 9B



Figures 9A & BComparison of an angiogram ob-tamed with a conventional film sub-traction technique (A) and a digitallyprocessed image (B) that was madeby density reversal and high-fre-quency enhancement of the angio-gram without subtraction. Althoughthe overall appearance of the twoimages is similar, the clarity of bloodvessels in the filtered image seems toindicate the potential utility of high-frequency filtering for improvementof angiographic images.

Clinical Studies

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Clinical Studies

Figures 1OA & BAn underexposed abdominal angiogram (A) is restored by overall densitymanipulation and unsharp mask filtering. The restored image (B) pro-vides much more detail in the paravertebral venous plexus, withoutrequiring another patient exposure.

Figure 11B

Digital radiographic images Ishida et al


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Clinical Studies

Figures llA & B(A) Original mammogram. The processed mammogram (B) is enhancedby the use of an unsharp mask 3 mm in size. Multiple small calcifications,which are characteristic of breast carcinoma, and soft tissue structuresare seen much more clearly in the processed image than in the originalmammogram.

Clinical Studies

Figure 12A Figure l2B


Volume 3, Number 2 May 1983 RadioGraphics 335

Figures l2A & BUnprocessed (A) and processed (B) radiographs of a patient with mild,active ulcerative colitis. The disease is present distal to the distaltransverse colon. The processed radiograph shows the en face ap-pearance of mucosal ulcerations more clearly than the nonprocessedradiograph. The most proximal area of disease, characterized by mu-cosal nodularity in the transverse colon, is seen more readily in theprocessed image.

Figure 13A



Figure 13B

Clinical Studies

Figures 13A & BThe original image (A) is a 1 mm polytomographic slice through a tem-poral bone. The processed image (B) shows the bone detail much betterthan does the original tomogram. This is due to relative suppression oflow frequencies, which tends to remove the tomographic blur, therebyimproving the detail visibility of the tomographic image in the focalplane.

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I Ishida et at Digital radiographic Images

Volume 3, Number 2 May 1983 RadioGraphics

Clinical Studies

Figures 14A & BIn an intravenous cholangiogram, the extrahepatic and cystic ducts arebarely visible in the original image (A), but are clearly shown in the pro-cessed image (B). The image processing parameters used for this low-contrast radiograph are the same as those used in the detection studies(a 12 mm mask and K = 3.0).

337



Conclusions

This digital image processing and simulation systemis a useful toot for investigating the effect of image pro-cessing techniques on both basic detection studies andclinical images. The detectability of simulated low-contrastradiographic patterns has been increased significantly byunsharp mask filtering. This technique has a unique ad-vantage compared to windowing or overall contrast en-

hancement, because unsharp masking increases the localcontrast while maintaining the wide dynamic range of theoriginal image. The visibility of lesions and anatomic detailsin conventional radiographs also appears to be improved bythe use of digital unsharp masking. This technique may beof great advantage when applied to other forms of digitalsystems such as digital fluoroscopy.

Readings

1. Ishida M, Kato H, Doi K, Frank PH: Development of a new digital radiographic imageprocessing system. Proc of SPIE 1982; 347:(42-48).

2. Schreiber WF: Wirephoto quality improvement by unsharp masking. J Pattern Recognition1970; 2:117-121.

3. Green DM, Swets JA: Signal detection theory and psychophysics. Krieger Publishing Co.,New York, 1973.

We are grateful to Chien-Tai Lu, Ph.D., Eugene E. Duda, M.D., and Heber MacMahon,M.D., for supplying ctinical radiographs and for their discussions with us; and to Y. Higashida,M. Carlin and Y. Kodera for observing the simulated images.

Response to Dr. Grays letter, (Page 324)

Dr. Grays points are well-taken and correct. Unfor-tunatety, radiologic technologists are taught to convert fromsingle phase to three phase techniques by simply dividingmAs in half which disregards the change in beam quality.The effective kilovoltage for single phase is approximately70 percent of peak, about 90 percent for three phase, sixpulse equipment. For a 70 peak kilovoltage exposure theeffective kilovoltage is 49 for single phase and 63 forthree-phase, six pulse equipment. Exposures in this rangecan provide a significant contrast change.

In response to Dr. Grays second point concerning x-ray

tubes, there is no question that heavy duty radiographictubes with the smallest appropriate focal spots should beobtained. For most applications, the appropriate tube wouldbe a 0.6/1.2 with twelve degree target. In these days mostradiologists have converted to film-screen combinations with

relative speeds of 200-400 which produce one-half to one-fourth of the heat into the x-ray tube. With less heat loading,smaller focal spots can be used. But, I would question theneed for high speed rotation of the anode. The wear factoron the bearings of the x-ray tube at high speed rotation isabout nine times normal speed. Prolonged use of high speedrotation can lead to shortened tube life. I am of the opinionthat high speed rotation for generators less than 500 mAcapacity is not warranted and if necessary, should be auto-matic.

I think that Dr. Grays comments further illustrate theneed for considerable thought to be given before the pur-chase of x-ray generators.

Thomas T. Thompson, M.D.Professor of Radiology

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