A systematic evaluation was conducted of certain digital image enhancement techniques performed in image space. Three types of images were used, computer generated phantoms, tomograms of a synthetic phantom, and axial tomograms of human anatomy containing images of lesions, artificially introduced into the tomograms. Several types of smoothing, sharpening, and histogram modification were explored. It was concluded that the most useful enhancement techniques are a selective smoothing of singular picture elements, combined with contrast manipulation. The most useful tool in applying these techniques is the gray-scale histogram

Roberts, E., Jr.

English

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NAS. Technical Memorandum 78974
DIGITAL ENHANCEMENT OF COMPUTERIZED AXIALTOMOGRAMS
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Ernest Roberts, Jr.
Lewis Research Center
Cleveland, Ohio
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TECEI	 AL PAPER to be presented :it the	 "j^ F4Cft^h
+	 Fifth Aruival Computers in Cardiology Conference C
cosponsored by the Institute of Electrical and
Flectronics Engineers, the National Instituter of
Health, and the Stanford University School of Medicine
Stanford, California, September 12-14, 1978
1
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kDIGITAL ENHANCEMENT OF COMPUTIMIZED AXIAL T'ONOMWIS
grneat Roberto, Jr.
NARA-Lewin Deneareh Center
21000 grookpork Rand
Cleveland, Ohio 44135
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Abstract
A systematic evaluation line been conducted of
certain digital inwgo enoincenlant tochniquea psr-
femmed lit image upaco. Threo typea of imagco have
linen used, computer generated phantoma, tomogrtinin
of a oynthatic phantom, and axial tomagramo of
human anatomy containing images of Icaiella, arti-
fieiallly introduced into the tomogramn.
Several types of smoothing, sharpening, slid
trlstogram modification havo been explored. It ban
been concluded that the most uocrul onhancennt
tecbniqueu tiro n salsctive smoothing of singular
picture elomento, combined with contrast manipula-
tion. The mout uuefel tool in applying tbese tech-
nlqucu In tilene gray-scale hiatogram.
Summ nry
NASA developed tecluology line hsen applied to
the enhancement of computerized axial tamegrame of
human anatomy. The work wall purfornad oat an Image
analysis ayatum developed for the inspection of
auranpace atructuron. The tomograma wore oupplied
by the Cleveland Clinic Foundation in the form of
computer readable magnetic ttpeu.
A dyntenntie evaluation 14119 conductud of w
repertory of digital enhancement technlquen per-
formed in lounge apace. Three tYPee of Ttageo were
unedt computer generated pbnntonm, tomograms of it
synthetic phantom, and tenograms of human anatomy
containing imagen of lesions, artificially intro . -
duced Into the tomograms.
The ropertory of enhancement functions included
oaveral typed each of smoothing, nhurponing and
hiatogram mnnipulatiau. The funetionu were applied
In an orderly manner to several of ench of the
three types of imagan. 'Tile input parameters were
varied syntemutically,
The following conclusions were reached for the
type of Images investigated lit 	 program:
1. SCICCCIVe anmothing of ningulnr picture cle-
emnts is mout useful for reducing Ini"o 9piltou.
2, Contrast menipulmt'iml is the moat effective en-
luueement method.
3. Tile gray-scale hintogrmn in the most useful
tool in applying these technlqucn,
Acknow.lodrementn
The author extends bin guttitudo to Thonms F.
Neanoy, N.D., Chalrumn, Division of Radiology, and
Jeffry N. Arnold, 11 h.0.,
 
(formerly) good, Selontl-
ric Computing, of the Cleveland Clinic Foundation,
for providing the tomogrann used In thin work. A
special word of thanks lit 	 to lit. Sanford F.
Felder, '1'vchnology 141.l.izatisn E"gAmor, NASA Lewin
Rencar0i Center, file coordinating the Program.
Introduction
Tim raw imngen produced by computerized axial
tomography voancns certain uhartcomWila. Fine
structure is frequently obacnxd by 11mi.tod reno-
lution mud union introduced by both the X-ray sea-
earn and the reconstruction algorithm. '1'heue short-
comingu are shared with tisane of radiographic lnmgoe
produced lit 	 nondestructive inspection of nm-
terialo and utrueturon. The ronulution of nucln prob-
lonu In neroupnco applications in currently under
inventigatiou . ,.t NASA, Lewin lie"earch Center. A
real-time computerized radiographic inspection fa-
cility ban been developedl,
To explore common problema, a cooperative pro-
Penn' between the Nondestructive Evaluation Suction
of Lewin Ronenrch Center and the pivinion of Radl-
ol.ogy of the Cleveland Clinic Foundation lilt" been
Initiated, It is coordinated by O ut Technology
Utilization Office of Lewis Reaaarch Center, The
work in being conducted by NASA personnel in the
laboratories of _Ise NandLatructIvo Evaluation Sec-
tion,
Axial tomogrem" of human anatomy tae supplied
by Cleveland Clinic. 'they are transferred :Lit the
form of computer readable magnetic tape. They are
proccaned oil
	 NDE section computer, using it
repertory of onhnncemont functions developed for
on-line radiographic inspection, Currently, the
inveatigation In limited to operations in image
space. It in plannad to be extended to fouilur
filtering and analysis lit
	 epltce.
The tomographic imageu supplied by Cleveland
Clinic consist of 256 Sines of 256 square picture
elanunta. The lnmge analysis hardware proctaaun
lnmgeu or 470 lines of 512 rectangular picture Ole-
etentn,. Tile elaments have all
	 ratio of 4:3,
the brondenet television standard, For dinplity
purpaaee, it in necessary to trasfortn the Cleveland
Clinic images by cyclically repenting picture ele-
mmnt. However, they are stated ill
	
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mass memory, and processed, as 256 X 256 arrayn.
The image analysis hardware uses a range of
brightness values from 0 (black) to 255 (white).
The tapes supplied by Cleveland Clinic use arbi-
trary X-ray absorption values. They are trans-
formed on reading to "delta numbers" from =1000
to represent the X-ray film density of air (black)
through +1000 to represent bone (white).
Procedure
Three classes of enhancement functions were
explored: image smoothing, image sharpening, and
histogram modification. These techniques are des-
cribed more fully elsewhere 2 . Four types of image
smonthing functions were used:
1. Weighted gray value averaging over a neigh-
borhood, 3 picture elements wide by 3 high. The
weight given each element is an input parameter.
2. A weighted median otherwise identical to item
1.
3. A replacement of a neighborhood of picture ele-
ment.+ by the mean of the neighborhood. The size
may be made 2X2, 4X4, 8X8, or 16 X 16,
4, Selective averaging, the criterion being that
the central point in a 3 X 3 neighborhood be out
of a specified range. If the element is not out of
range, it is not averaged.
Two types of sharpening functions were explored:
1. Laplacian,
2. Root-mean-square gradient.
Two types of histogram modification were inves-
tigated:
1. Contrast expansion, in which a specified input
range of gray values is transformed to a different
specified output range. Picture elements falling
outside the input range are transformed to the
minimum or maximum value.
2. Cray value segmentation in which an arbitrary
number of gray value boundaries is specified, and
any given input valueis transformed to the mean.
of the range in which it falls.
These functions were systematically applied to
3 types of images:
1. Computer generated phantoms having irregular
boundar.tes. The boundaries, gray values, and
superimposed noise are input parameters. A typi-
cal phantom image had an average gray value on one
aide of the boundary of 100, and 104 on the other
aide (in terms of the hardware values). Random
noise having a range of f2 was superimposed on the
image, producing a range of 98 to 102 on one side,
and 102 to 106 on the other. This image, contrast
expanded to full scale, is shown in figure 1.
Figure 1. - Computer generated phantom.
2. Several tomograma of a synthetic phantom.
This was a nylon body of irregular shape immersed
in varying strength solutions of potassium iodide
and water. one of these images is shown, contrast
expanded to full scale, in figure 2.
Figure 2. Tomogram of synthetic phantom,
3. Several tomograma of human anatomy, to which
images of lesions were artificially introduced.
One of these is shown full scale in fig •,re 3, with
the locations of the artificial, lesions indicated
by the labels a, b, c, d, e, f, and S.
In all cases, the anamolies were not immediate-
ly apparent to the unaided human eye.
Several sets of input parameters were tested
for each function. Each function was, in turn,
applied to several of the 3 types of images. Com-
binations of functions were used when that appeared
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,A.ac (QUALITY
Figure 3. - Tom, , ,m.,m of human anatom%. Pointers
Indicate locatfon of Icslons.
to be a logical procedure. The output images were
Inspected visually in an :attempt to locate the
known boundaties or anamolles. When necessary,
histograms of the various images were produced.
The grav-value histogram, presented on the v ►deo
tmmnitor, is a plot of the number of picture ele-
ments corresponding to each grav value. it is taken
over a rectangular region defined by a cursor dis-
played on the monitor. Typically, It covers ap-
proximatel y 40 percent of the total image area. It
Is shown as a white rectangle In figure J. The
histogram is normalized to a maximum ordinate value
of 100. The number of picture elements at 0 (black)
may he omitted from the normalization computation
in response to at 1rompt by the computer program.
The plot illustrated in figure 4 uses the bright-
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ness values produced by the naoagr analysis hard-
ware (0 to 155).
The histograms were used both to dellne the
values of Input parameters to the various functions,
and to analyze their effect on the output lmsse.
Investigations conducted on the computer Son-
erated phantotas revealed that random noise reduces
the ettectiveness of the two types of image sharp-
ening functions. In the absence of noise, both of
these functions clearly reveal the boundary he-
tween areas of differing gray value, even when they
ore virtually indistinguishable by the unaided eye.
A thresholding, process IN especially effective.
In this case, a histogram is generated of the
gradient image. rhr histogram is used to define a
threshold value. All values greater than this
threshold are Net to black, and all others set to
white. This binar y ltttage is thin logically summed
with the original Image, often with dramatic re-
sulta.
However, the presence of random noise intro-
duces a speckle pattern superimposed over the gra-
dit-it inuige. This pattern can he minimized either
by smtxmthing the original image, or b y selectively
discarding isolated black picture elements on the
binary gradient image. But both procedures tend to
obscure the boundary. When the range of noise is
such that there IN an overlap of gray values be-
tween the two regions, the methods fall completely.
This is the case with the computer generated phan-
tom described herein, and iM apparently the case
with the tomamgrams.
The various smoothing procedures reduce the
Isolated IN.iee spikeM in the images. However, the
more effective the procedure in reducing noise, the
more it obscures boundaries. The only one of these
functions which leaves recognizable boundaries to a
selective averaging procedure. in this procedure,
each picture element is compared with the range of
its I X 3 neighbors. If it falls outside a de-
fined multiple of that range, it is replaced by the
mean of the neighborhood. Otherwise. it is ig-
nored.	 It can be :applied either before or after a
mtra:•m expansion, and can be reapplied several
times in succession. :! produces a very clean
looking image, with little apparent ill effect on
the boundaries. However, the cleaning process may
possibly eliminate texture, which may be usefua to
the experienced eye in detecting anamolfes.
Expansions of limited ranges of gray values
are most effective. The histogram of the raw image
is useful in defining the ranges to be expanded.
Several successive expansions revealed all of the
known anamolies in the Images under investigation.
The histogram of figure 3 is shown in figure 4,
and two expansions are Nhown in figures 5 and 6.
In figure 5, the values between 127 and 137
have been expanded to full scale. Note that this
makes the lesions labeled "e" and "f" in figure 3
visible. Similarl y . in figure 6, the values he-
,reen 140 and 150 have been expanded to full scale,
making the lesion labeled "b" visible.
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Figure 4. - Histogram of figute i
References
1. E. Foberts, Jr., "Real-Tim. , Radiographic In-
spection Facility," NASA TN U-H529, 1977,
2. R. C. Gonzalez and 1 1 . Wlntz, Itigital Image
Processing. Resding, Mass.: Addison-Wesely, 1977.
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4UAL rY
4
Conclusions
A NyNtematic Investigation of several image
t	 enhancement functions with varying input parameters
applied to three types of frrwk,es produced the follow-
Ing conclusions:
E —► 	 _	 ^
1. The application of certain digital image en-
i .+	 hancement techniques substantially lncreaxes the
visibility of anamolies in tomograms of human
anatomy.
2. Image xherptnlng is of little value, apparently
because of the low signal to noise ratio and the
small variations in the regions of interest.
%,^ ,;, ` i ,	 3. Most of the smoothing technlquer "smear out" too
much f Lu structure. Selective averaging of sin-
gular picture elements produces a clearer image, but
may possibly obscure texture which is useful in
Figure S. - Contrast expanr.l,ur of figure s. Pointers 	 identifying anamolies.
Indicate locations of lesions.
4. Contrast expansion 1s of greatest value. How-
ever, Its use must he guided by use of the gray-
scale histogram.
Figure 6. - Contrast expansion of figure 7. Pointer
indicates location of lesion.
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1. Melon No	
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2 Goventmon( Accrosmn No a	 Iletipimu+ Catalog No
NASA TM-79974
4-'role and Suballe b	 Ilepon hale
DIGITAL GNIIANCLMrNT OF COMPUrERIZGD --	 -	 ---	 -6 Pedmnnng clipuu+abon Cade
AXIAL TOMOGRAMS
7. Aulhorlsl
	
^_ 6	 Pedmnnng otguura6an Ilepurt Np
Ernest Roberts, Jr. E-97 ,19	 `
m	 -	 -16 1Yrnk Unit No
6. Perlormmg o(tlanuation Name and Addrau
National Aeronautics and Space Administration
It	 Contractt or Grant No
Lewis Research Center
Cleveland, Ohlo 44139 to Type at nnpart and Periodcdatved
Technical Memorandum12. Spomonng Agency Noma and Address
National Aeronautics mid Space Administration -	 -- -•_-	 -- ----
10 "inner Me Agency Code
Washington, D.C.	 2Q549
16. Sup plementary Notes
16. AburoCt	 i
A systematic evaluation has been conducted of certain digital Wings enhancement techniques
performed in imago space.	 Throo types of inmges have been used, computer generated
phantoms, lomogrmns of it synthetic phanlmn, mid axial tomogrni ns of human mudoury contain-
Ing inmges of lesions, artificially introduced into the tontogranis. 	 Save" types of smoothing,
sharpening, mid histogram moditication have been explored. 	 It has been concluded that the
most useful enhancement techniques tire a selective seloolhhtg of shnguhu• picture elements,
connbhned with contrast manipulation.	 The most useful tool in applying these techniques is
the gray-scale Iniologrtmn.
17, Key Words (Suggested by Authodo) 16	 Dlatriblltlen Statement
Image cnhmicemcnt Unclassified - unllmited
Radiology STAR Category fit
19. Security Cloult. lot On' report) 20	 Security Minor. lot Itua h, gal 21. No. of Pages 22,	 Price'
Unclassified ^— Unclassified
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i'
I	 For sale by the National Technical Inl6lmabon Service, Spnnghehl. Vugnua 22161 I
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Digital enhancement of computerized axial tomograms

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