display quality of different monitors in feline digital radiography

DISPLAY QUALITY OF DIFFERENT MONITORS IN FELINE DIGITAL

RADIOGRAPHY

EBERHARD LUDEWIG, CHRISTIAN BOELTZIG, KATRIN GaBLER, ANJA WERRMANN, GERHARD OECHTERING

In human medical imaging, the performance of the monitor used for image reporting has a substantial impact on

the diagnostic performance of the entire digital system. Our purpose was to compare the display quality of

different monitors used in veterinary practice. Two medical-grade gray scale monitors (one cathode-ray

tube [CRT], one liquid crystal display [LCD]) and two standard consumer-grade color monitors (one CRT,

one LCD) were compared in the ability to display anatomic structures in cats. Radiographs of the stifle joint and

the thorax of 30 normal domestic shorthair cats were acquired by use of a storage phosphor system.

Two anatomic features of the stifle joint and five anatomic structures of the thorax were evaluated. The

two medical-grade monitors had superior display quality compared with standard PC monitors. No differences

were seen between the monochrome monitors. In comparison with the color CRT, the ratings of the color

LCD were significantly worse. The ranking order was uniform for both the region and the criteria investigated.

Differences in monitor luminance, bit depth, and screen size were presumed to be the reasons for the observed

varying performance. The observed differences between monitors place an emphasis on the need for guide-

lines defining minimum requirements for the acceptance of monitors and for quality control in veterinary

radiography. r 2010 Veterinary Radiology & Ultrasound, Vol. 52, No. 1, 2011, pp 1–9.

Key words: cat, CRT, feline, LCD, monitor, stifle joint, thorax, visual grading analysis study.

Introduction

IN DIGITAL RADIOGRAPHY the imaging chain comprises

four separate technical steps: signal acquisition, signal

processing, image archiving, and image presentation. The

performance of a digital radiography system depends on

the interplay of those interdependent parts.1,2 There has

been a transition of image presentation from reviewing

images on film, so-called hard copy viewing, to reviewing

images on computer monitors, or soft copy viewing. Soft

copy viewing offers advantages over hard copy reading

since the image can be adjusted on-line. The option to use

the entire spectrum of attenuation differences recorded by

the detector means that more information is available.3–5

Furthermore, with soft copy viewing zooming or measure-

ment tools are available and the cost of film, film process-

ing, and hard-copy image storage and retrieval are

eliminated.3,4,6 In the human medical profession, the tran-

sition from hard copy to soft copy viewing was not

instantaneous and was based on substantial prior work in

the field of medical monitor displays and workstation

technology. Historically, soft copy viewing in the human

medical profession was affected by limited monitor perfor-

mance. New generations of monitors offered better display

properties. Gray scale cathode-ray tube (CRT) monitors

and, later on, gray scale liquid crystal displays (LCD)

became the display media of choice for medical images.

Differences in monitor performance can influence the dis-

play quality and consequently the overall final diagnosis.7,8

To ensure a high and consistent level of image display

quality in human medical practice, guidelines exist that

define the minimum technical prerequisites for monitors

and methods of quality assurance.1,9 To our knowledge,

comparable regulations for veterinary radiology do not

exist. Because the price of monitors specifically designed

for medical imaging can exceed the price of a standard

computer monitor by a factor of ten, consumer-grade

monitors often are used in veterinary practice. Considering

the corresponding radiation safety aspects, poor monitor

selection or inadequate calibration violate the ALARA

principle. At worst, veterinary personnel receive occupa-

tional exposure to create an image that cannot be evaluated

adequately due to an unacceptable monitor.

This study was motivated by the uncertainty of whether

specific information dealing with monitor evaluation in

the human medical profession is applicable to veterinary

radiology. Our aim was to compare the display quality of

selected monitors on the basis of subjective assessment of the

appearance of anatomic structures in feline radiographs.

We hypothesized that monitors recommended for primary

image interpretation in human radiology offer superior dis-

play properties in feline radiographs as well. Furthermore,

due to the variability in object contrast and size, differences

in the ratings between the selected criteria could be expected.

Address correspondence and reprint requests to Eberhard Ludewig, atthe above address. E-mail: [email protected] April 15, 2010; accepted for publication June 16, 2010.doi: 10.1111/j.1740-8261.2010.01733.x

From the Department of Small, Animal Medicine, University of Lei-pzig, An den Tierkliniken 23, D-04103 Leipzig, Germany.

1

mailto:[email protected]

Material and Methods

Four types of monitors were evaluated (Table 1). The

two monochrome displays represented medical-grade de-

vices consistent with national standards.10–12 The color

displays were standard consumer grade monitors. At the

beginning of each reading session the settings of the gray

scale monitors were rechecked. Brightness and contrast of

the color monitors were adjusted to the achievable opti-

mum with the help of a SMPTE RP-133 test pattern.13 The

monitors were controlled by the graphic card of the indi-

vidual computer.

Under identical exposure conditions 30 lateral stifle joint

radiographs and 30 right lateral thoracic radiographs of 30

anesthetized normal domestic shorthair cats older than 1

year were acquired. General anesthesia was required for

reasons other than for radiography, e.g. castration, re-

moval of orthopedic implant, and treatment of a dental

disease.

The radiographs were made using a storage-phosphor

system� on a Bucky tablew (Table 2). Uniform processing

was employed for both the stifle and the thoracic radio-

graphs. Dynamic range reconstruction algorithm and un-

sharp mask filtering was employed for the images of the

stifle joint and the thorax, respectively. In pre-studies, the

parameters of these processing algorithms were optimized

with regard to detail visibility (Table 3).

The investigation was designed as an observer perfor-

mance study. In an absolute visual grading analyses (VGA)

study the observer stated their opinion on the visibility of a

certain feature on the basis of an absolute scale without

reference pictures.14 The images were evaluated indepen-

dently on the various monitors by four observers with a

minimum of 3 years of experience with digital radiography

(one board-certified radiologist, three residents of a na-

tional specialization program). Two features of stifle

radiographs and the appearance of five anatomic struc-

tures of the thorax were scored on the basis of a four-point

scale (4, excellent; 3, average; 2, borderline sufficient; 1,

insufficient) (Fig. 1). The observers were trained for their

task using a separate set of images. Consistent with the

practical routine of image reading, the radiologists were

encouraged to apply the entire workstation functionalityzto record as much information as possible. Evaluation time

per image was unlimited. To ensure uniform ambient

conditions all workstations were placed in the same reading

room. The ambient light and other conditions of the

viewing environment fulfilled the requirements for medical

image interpretation.11,15 Lighting was indirect, and

illuminance at the monitor surface was o100 lx. Observ-

ers were unaware of the animal identification.

Table 1. Technical Specification of the Monitors

Type Gray Scale CRT Gray Scale LCD Color CRT Color LCD

Manufacturer labeling Philips 21 CY9� Totoku ME 181Lw ADI Microscan PD959z Fujitsu Siemens Amilo Ay (laptop)Physical size (in.) 21 18.1 19 15.1Matrix 1280 � 1024 (1.3MP) 1280 � 1024 (1.3MP) 1600 � 1200 (1.9MP) 1024 � 768 (0.8MP)Dot pitch (mm) 0.35 0.28 0.24 0.30Bit depth 10 10 8 8Maximum luminance (cd/m2) 650 700 120 200Operating luminance (cd/m2) 250 360 100 200Contrast ratio 450:1 400:1 400:1 400:1Graphic card (type) SUN 81-76 Matrox Millenium

P650 PCIe 128MNVIDIA GeForce 7300LE ATI IGP 320M

Calibration DICOM GSDF Yes Yes No No

�Philips Healthcare. wTotoku Electric Co.,Tokyo, Japan. zADI Corp., Taipei, Taiwan. yFujitsu-Siemens, Sommerda, Germany. CRT, cathode ray

tube; LCD, liquid crystal display; DICOM, Digital Imaging and Communications in Medicine; GSDF, Grayscale Display Function Standard.

Table 2. Exposure Technique

Parameter

X-ray systemType Philips Bucky DiagnostFiltration 2.5mm AlFocus size 0.6 � 0.6mm2

Storage phosphor systemScreen Fuji HR-VReader Philips AC 500Spatial frequency 5 lp/mmDetective quantum efficiency(70 kVp: 1 lp � mm-1) 21%

Exposure conditions Stifle joint ThoraxGrid No NoFocus-to-detector distance 110cm 110cmField size 10 � 8 cm2 15 � 21 cm2

Tube potential 44 kVp 52kVpTube current 8mAs 6.3mAsExposure time 36.0ms 21.6msDose–area product 0.9 cGy � cm2 3.5 cGy � cm2

�Fuji HR-V, Fujifilm Medical Systems, Tokyo, Japan; PCR AC 500,Philips Healthcare, Hamburg, Germany.wBucky Diagnost TH, Philips Healthcare, Hamburg, Germany.

zeFilm workstation, version 1.8.3, Merge Healthcare, Milwaukee,WI.

2 LUDEWIG ET AL 2011

Because the image quality and visibility of the anatomic

structures in this study were rated subjectively, an assess-

ment of consistency was desirable to get an impression of

the objectivity and reliability of the image evaluation.

Kappa statistics were not applicable because of the multi-

variate character of the data, caused by the number of

observers and rating categories. Instead, Spearman’s rank

correlation was performed for all criteria of all observer

combinations. The level of significance was calculated for

each correlation. A significant positive correlation indicates

a high level of consistency between observer ratings.

Median and average absolute deviation of the ratings

were calculated for each anatomic structure. Visual grading

characteristic (VGC) analysis was applied to analyze the

data of the VGA study. In principle, VGC analysis treats

the scale steps as ordinal with no assumptions of the dis-

tribution of the data being made. It handles visual grading

data in a fashion similar to ROC data. The area under the

curve (AUCVGC) is a measure of the difference in the image

quality between two modalities. A curve equal or close to

the diagonal, equivalent to an AUGVGC of about 0.5, in-

dicates equality between the monitors compared (Fig. 2).16

Results

The analysis was based on a total of 3360 individual

observer decisions. For the individual readers (E.L., A.W.,

C.B., K.G.) the mean values of the correlation coefficients

averaged over the two criteria of the stifle joint images were

Fig. 1. Definition of criteria for the depiction of diagnostically relevant anatomic structures in the evaluation of image quality

Radiological structures Anatomical criteria

(A) Stifle jointBone Identification of the subchondral borders (black arrows), discrimination between trabecular and

compact bone (black circles), delineation of the patella, fabella(e), and popliteal sesamoid (blackasterisks)

Soft tissue Demarcation of the infrapatellar fat pad (open white triangles), and extraarticular soft tissuestructures (closed white triangles: muscle contours; white arrows: patellar ligament)

(B) ThoraxTrachea Discrimination of trachea and principal bronchi from the adjacent mediastinumCranial lung field Visibility of small vessels (white arrows) in the cranial lung fieldSternum Visibility of the border and the architecture of the sternebraeCardiac silhouette Identification of the caudal border of the cardiac silhouetteCaudodorsal thoracic field Rendition of the aorta (open triangles), the caudal vena cava (closed triangles), pulmonary

vessels (arrows), and contour of the diaphragm

Table 3. Image Processing Parameters

Stifle jointProcessing algorithm: dynamic range reconstruction

Contrast equation 0.8 (Kernel size: 135)Contour sharpness 1.00 (Kernel size: 3,

Curve type: F)Thorax

Processing algorithm: unsharp mask filterGradient amount (GA) 1.17Gradient type (GT) EGradient center (GC) 1.80Gradient shift (GS) � 0.28Frequency rank (RN) 9Frequency type (RT) UFrequency enhancement (RE) 1.4Kernel size 5

Processing Workstation: Easy Vision Rad Release 4.2 L5 (Philips

Healthcare, Hamburg, Germany)

3DISPLAY QUALITY OF DIFFERENT MONITORSVol. 52, No. 1

0.73, 0.74, 0.66, and 0.64, respectively. The mean values

over the five structures of the thoracic images were 0.75,

0.67, 0.71, and 0.65, respectively. Because of the sustained

high level of significance of the underlying correlations

(P � 0.001) the subsequent VGC analysis was based upon

the pooled data of the four observers (Table 4).

The results of the evaluation of the image quality are

summarized in Table 5 and in Fig. 3. The data revealed a

complete uniform ranking order of the four monitors over

all features evaluated. The two medical-grade monitors

offered clear superior display quality. There were no sig-

nificantly different rating results noted between those two

modalities. In comparison, the display quality of the color

CRT was inferior. This was characterized by both lower

median values and significant differences in the corre-

sponding AUCVGC values based on the direct compari-

sons. The median of four out of the seven features of the

color LCD ratings was lower than those for the color CRT

ratings. Concerning the individual allocations in the color

LCD in five criteria an insufficient quality (grade 1) was

recorded. Using the color CRT this was seen only for

one out of the seven features. In the gray scale monitors

there was no grade 1 allocation and even grade 2 ratings

were documented only for a single feature.

Discussion

The quality of a digital radiographic image is limited by

the weakest part of the imaging chain. We found obvious

differences between various monitors with regard to the

rendering of anatomic detail in feline radiographs. Image

quality was better when using a monitor that met defined

criteria regarding primary image evaluation in human

medical practice.

A comparison of our findings with results from other

studies performed with either clinical images from humans

Table 4. Spearman’s Rank Correlation Coefficients of the IndividualObservers

Observer 1 2 3 4

Stifle jointBone 0.76 0.76 0.72 0.72

0.71–0.84 0.71–0.84 0.71–0.73 0.71–0.73Soft tissue 0.70 0.73 0.60 0.57

0.59–0.88 0.63–0.89 0.48–0.66 0.48–0.63All structures 0.73 0.74 0.66 0.64

0.59–0.88 0.63–0.89 0.48–0.73 0.48–0.73Thorax

Trachea 0.73 0.65 0.68 0.690.69–0.76 0.62–0.69 0.62–0.76 0.65–0.74

Cranial lungfield

0.83 0.78 0.77 0.570.82–0.85 0.73–0.82 0.73–0.83 0.75–0.85

Sternum 0.69 0.56 0.68 0.680.61–0.79 0.47–0.61 0.60–0.80 0.47–0.64

Cardiacsilhouette

0.71 0.67 0.66 0.550.63–0.76 0.53–0.75 0.50–0.76 0.50–0.63

Caudodorsalthoracic field

0.76 0.67 0.74 0.730.70–0.80 0.66–0.70 0.66–0.78 0.64–0.80

All structures 0.75 0.67 0.71 0.650.69–0.83 0.56–0.78 0.66–0.77 0.55–0.73

Top: mean value of the correlation coefficients (in relation to the other

three observer).Bottom: minimum and maximum of the correlation

coefficients (in relation to the other three observer).

Number of ratings (n)

A -Color CRT

60

70

1.0

40

50VGAB

0.6

0.8

10

20

30

0.470

B -Color LCD

0.240

50

60 AUCVGC = 0.66 ± 0.07(x ± 95%-CI)

0 0.2 0.4 0.6 0.8 1.00

10

20

30

VGAA

1 2 3 4 Score

Fig. 2. The visual grading characteristic (VGC) curve (right) from the data of the ratings for the criterion ‘‘cranial lung field’’ for the color cathode-ray tube(CRT) and the color liquid crystal display (LCD) (left). The boxes represent the operating points corresponding to the observer’s interpretation. The area underthe curve (AUCVGC) differs significantly from 0.5, indicating a superior display quality of the color CRT (A) in comparison with the (B) color LCD.


or phantoms was hampered for major reasons. One was

that the monitors investigated diverged substantially in

their technical properties. The second related to the vastly

different target structures. Thus, it is not surprising that a

number of human studies described differences in the

monitor performance8,9,17–19 while others confirm equal

display quality.20–23

The quality of a monitor is determined by the interplay

of several factors, such as screen size, pixel size, luminance,

contrast ratio, and bit depth. Further characteristics such

as phosphor type, gray scale- or color monitor, glare- and

reflection characteristics, and display calibration are im-

portant as well.1,9,24,25 The importance of brightness and

spatial resolution have been emphasized.9,22,23,25,26 It is

likely that the superior performance of medical-grade

monitors in our study was related primarily to the ability of

the monitors to display more shades of gray. Luminance

was three to five times higher and therefore different look-

up tables were applied. The main advantage of high lumi-

nance is that it is easier to see the entire gray scale from

white to black in an image. Brighter monitors always yield

better perceived contrast.1,25 Furthermore, the gray scale

monitors were able to display 1024 shades of gray com-

pared with 256 shades for the color monitors, which also

improved gray scale rendition. Additional benefits of the

medical-grade gray scale monitors were that they were

calibrated to the DICOM part 14 Grayscale Standard

Display Function (GSDF).27 The aim of the calibration

was to obtain consistent presentation on all displays by

distributing the total contrast of the display across the en-

tire gray scale. As a result, objects were presented with the

same contrast regardless of whether they were located in

dark or bright parts of the image.28

In general, the pixel size of a monitor is an important

quality factor. But, it is not very likely that the differences

in results found in this study were related the pixel size.

Basically, to ensure adequate resolution, the matrix of the

monitor should be as close as possible to the matrix of the

preprocessed image data. Alternatively, high resolution is

attainable with magnification function.1,22,29 The spatial

frequency of the applied storage phosphor system was

5 line-pairs/mm. According to the Nyquist theorem, this

corresponded to a detector pixel size of 0.1mm (100mm).

The pixel size of the monitors included in the study ranged

from 0.24 to 0.35mm. Consequently none of the monitors

was able to display the exposed field of the thoracic

Table 5. Tabulated Results of the Ratings

Stifle Joint Thorax

Bone Soft Tissue Trachea Cranial Lung Field Sternum Cardiac Silhouette Caudodorsal Thoracic Field

Gray scale CRTNumber of ratings

1 0 0 0 0 0 0 02 0 0 0 0 0 0 03 26 22 21 23 8 12 244 94 98 99 97 112 108 96

Median 4 4 4 4 4 4 4Average absolute deviation 0.22 0.18 0.17 0.19 0.07 0.10 0.20

Gray scale LCDNumber of ratings

1 0 0 0 0 0 0 02 0 0 0 4 0 0 03 33 24 36 30 17 22 374 87 96 84 86 103 98 83


Color CRTNumber of ratings

1 0 0 0 2 0 0 02 32 10 20 32 8 12 273 75 84 81 70 64 64 824 13 26 19 16 48 44 11


Color LCDNumber of ratings

1 9 0 9 11 2 0 42 57 36 53 53 17 37 673 51 74 57 54 92 60 484 3 10 1 2 9 23 1



radiographs of 15� 21 cm2 in the original resolution with-

out the use of the magnification function. Because the ob-

servers used the zooming function, it is unlikely that the

differences of the monitor pixel size had a significant in-

fluence on the results. However, generally such an influence

may not be ignored completely because the larger monitors

allowed for a higher magnification. Beyond that, other

factors, such as monitor technology (CRT vs. LCD; gray

scale vs. color) or graphic card could also have attributed

to the differences.18,24,30

For the study, monitors that are commonly used in vet-

erinary practice were chosen. They were selected on the

basis of different technologies (CRT vs. LCD; gray scale

vs. color) and physical properties (e.g. luminance, contrast

ratio, spatial resolution, bith depth). Because some vendors

advertise the use of notebook computers for primary in-

terpretation, such as in mobile practice, a standard laptop

display was included. There are significant price differences

between medical-grade and consumer-grade monitors,

making the cheaper standard PC monitors appear attrac-

Fig. 3. AUCVGC values (mean � 95% confidence interval) of the monitor comparisons. (A) Stifle joint. (B) Thorax.


tive. For example the current price of a 21 in. medical-

grade gray scale LCD monitor is approximately h4000,

whereas a consumer-grade color LCD of the same size

ranges between h120 and h400. Because of those price

differences, monitor recommendations for human medical

practice are based on the diagnostic purpose for which the

workstation should be used. Basically, expensive high

quality monitors were recommended for image interpreta-

tion, whereas less expensive monitors with a lower perfor-

mance can be used for image viewing without the need for

an immediate final diagnosis. Therefore two categories of

monitors have been distinguished currently: monitors for

interpretation of medical images for rendering a clinical

diagnosis, termed primary or diagnostic monitors, and

monitors for viewing medical images without the require-

ment for clinical interpretation, e.g. for viewing images by

medical staff or specialists other than radiologists after an

interpretive report has been rendered, termed secondary or

nondiagnostic monitors.11,12,31–34 Within both of these

categories, the minimum specification differs dependent on

the application, e.g. thorax, skeleton, and mammography.

Beyond that, it was proposed to expand this limited

classification range to match the full range of applications

more precisely.34 When a single workstation will be

used for multiple applications, the monitor specification

has to match the highest level needed.11 Accordingly, in

our study, the monitor would have to fulfill the require-

ments for reporting thoracic images. The two gray scale

monitors met the national requirements for primary image

interpretation for both thoracic and skeletal image inter-

pretation in the human medical field11 while the two con-

sumer-grade monitors did not. The quality of the color

CRT was acceptable for secondary viewing. The color

LCD was inadequate even for secondary image review.

Thoracic and joint structures of the cat were selected

because they are small objects with a wide spectrum of

attenuation differences. In the thorax, motion caused by

respiration has to be considered to avoid loss of evaluabil-

ity. In the human medical profession, comparable chal-

lenging conditions are restricted to neonatal radiography.35

As in pediatric radiology it was assumed that the chosen

regions placed high demands on all parts of the imaging

chain to display structures of interest with high diagnostic

quality.36 The rating differences seen were unrelated to

both the two regions and the target structures evaluated.

This was somewhat unexpected, because the display quality

of low-contrast structures such as the cranial lung field and

caudodorsal thoracic field in the thoracic radiographs, and

soft tissue in the stifle images, were theoretically more

dependent on monitor properties related to the display

of luminance and contrast than target structures with

higher attenuation differences.37 However, other results

from phantom20,23 and clinical human studies19,23 agree

with ours.

The major drawback of this study was the small number

of monitors investigated. We recognize that the entire

spectrum of monitor quality could not be addressed.

Furthermore, more sophisticated monitors are becoming

available continually. There has been a trend away from

CRT monitors to LCD flat panel displays.18 We did not

evaluate large screen color LCD monitors. Large screen

color LCD monitors with high brightness and resolu-

tion (display diameter: �20 in., maximum luminance: �200cd/m2, matrix: �2MP) can perform similar to gray

scale monitors in human medical practice.21–23 The ability

of large screen LCD monitors to display subtle struc-

tures in small animals has not been characterized. Also,

monitors cannot be evaluated without considering the

associated graphics card. Our study design was in-

adequate to assess the effect of the graphics card in-

dependently.

Another limitation might be that anatomic structures

were evaluated instead of pathologic lesions. It was

assumed that the ability to detect pathologic changes is

related to accurate anatomic presentation.14,38 In contrast

to pathologic structures, anatomic landmarks have a more

uniform appearance. Consequently it is easier to evaluate

the quality of their radiographic presentation for a mean-

ingful interpretation. In human medical practice, the

quality of reproduction of anatomic structures is the ba-

sis of established standards of quality assurance.39,40 In

observer performance studies dealing with comparative

evaluation of the quality of clinical images, these criteria

are reliable measurement instruments.41–43 Despite the lack

of comparable standards in veterinary radiology, quality

criteria can be deduced from generally accepted paradigms

of image interpretation.44–47 Once the requisite level of

radiographic rendition of a diagnostic relevant criteria has

been verbalized, the description can be applied to observer

performance studies in general and for VGA studies in

particular.36 However, some have argued that it is more

difficult to identify existing pathologic changes.19,48 Accord-

ingly the results of our study could be considered as too

optimistic. Such an interpretation underlines the need of

high-quality monitors even more strictly. Another limitation

was that it was not possible to hide the monitor type from

the observers. Consequently, preferences of individual ob-

servers could not be excluded. However the consistent pos-

itive correlation of ratings between observers weakens this

argument.

In summary, we have shown that the performance of the

monitor used for soft-copy interpretation influences image

interpretation significantly. Monitor quality is a critical el-

ement within the imaging chain in small animal radiology.

Deviation from high quality monitors is accompanied by a

loss of information. From the view of radiation safety

considerations such a loss may not be tolerated as it rep-

resents a violation of fundamental radiation safety princi-


ples. In consequence, guidelines are needed that define

minimum requirements for devices used for soft-copy in-

terpretation in veterinary radiology. Because of similarities

of many target structures and the needed quality for their

radiographic presentation, guidelines for acceptance and

quality testing of display devices in human medical imaging

could be followed in veterinary medicine.

ACKNOWLEDGMENTS

The authors would like to thank Prof. Dr. Joe Morgan for assistancewith the manuscript.

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