Linear Tomography in Veterinary Practice

CHRISTINE GIBBS’

INTRODUCTION

Linear tomography has provided useful information in the radiological investiga- tion of certain conditions encountered in veterinary practice. The procedure has the advantage of carrying a very small risk to the patient.

LITERATURE REVIEW The use of layer radiography as an aid

to radiological interpretation has been ac- cepted in the field of medical radiodiagnosis for many years, but its application to vet- erinary radiological investigations has re- ceived relatively little attention. The basic principles, and the use of simple ap- paratus have been described (4, 7), to- gether with suggested indications, and il- lustrations of certain cases. Italian work- ers have used tomography in post-mortem studies of the hypophyseal fossa in the dog (8, 9), the investigation of lung and medi- astinal tumors (lo), and in relation to the diagnosis of trichomoniasis in racing pig- eons (1). In more recent publications tomographs have been used to demonstrate cavitating pulmonary metastases (6) and a tumor of the petrous temporal bone (5) in dogs.

MATERIALS AND METHODS

The purpose of tomography is to produce a sharp image of a single narrow plane within the body by blurring out the images of structures above and beneath it. This is achieved by synchronous movements of the x-ray tube and film about a pivot dur- ing exposure. The plane in focus lies at the level of the pivot. The efficiency of blur dispersal and, therefore, the degree of

sharpness of the plane in focus is related to the distance traveled by the tube when moving through any particular angle or arc.

The simplest form of movement is recti- linear, in which the tube and film move in an arc or straight line in opposite directions to each other. This system has limita- tions, because dense structures which have a dimension parallel to the direction of tube movement produce linear blur shad- ows on the tomograph which inevitably detract from the sharpness of the plane in focus. However, the equipment required is simple and can often be added to stan- dard radiographic installations. The elimi- nation of blur by electronic or photographic subtraction methods has recently been re- ported and may have a place in improving the resolution of tomographs (3).

Elliptical and circular tube movements have been evolved to reduce the effect of linear blur, but the most complex and effi- cient form of movement is hypocycloid, in which the tube and film move in two ec- centric circles, producing maximum blur dispersal and a very thin plane in focus. The distance traveled, and, therefore, the exposure time is long which makes the technic suitable only for parts which can be immobilized for prolonged periods, i.e., the skeleton. In medical practice it has particular application in tomography of the petrous temporal bone to demonstrate structures of the middle and inner ear (2). Because blur dispersal is so efficient, the background of hypocycloid tomographs is a homogeneous grey and if the blurred structures are of similar radiographic den- sity t o those in the plane in focus, a rather low range of contrast is achieved. This may be an additional limiting factor of the technic.

Lecturer, Department of Veterinary Surgery, University of Bristol, Langford House, Langford, Bristol, England

37

38 C. GIBBS 1973

Fig. 1. Apparatus setup for vertical ventrodorsal tomography of the canine skull. The X-ray tube (A) is connected to the back of the Potter Bucky tray (B) by a rigid metal rod (C). The level of the pivot is shown on the scale (D), and its height can b e altered by adjusting the screw (El. The tube is angled to the left ready for an exposure to be made.

The apparatus for rectilinear tomog- raphy in use a t the Bristol Veterinary School is illustrated in Fig. 1. It consists of a simple bar attachment connecting the overcouch tube of a Phillips Rotopulse unit to the bucky tray of a Diagnost 502 couch. The tube is driven by an electric motor mounted on the ceiling crane, and moves in a straight line, but angulates to maintain centering. The distance of the pivot from the table top, which governs the layer in focus, is adjustable, and is measured in centimeters or inches on the scale.

The equipment can be adapted for lateral tomography of the extremities of large animals. Most of the body weight of the patient is supported by a trolley the same

Phillips Electrical Ltd.

height as the x-ray table, and the part to be tomographed is positioned under the tube.

As the conventional bucky tray allows only one film to be carried a t a time, a t each examination it is necessary t o make several exposures in order to obtain tomographs of the required number of layers. Bucky trays for multilayer cassettes are available and have obvious advantages. It is pos- sible, however, t o obtain multiple sections of thin parts by using several envelope wrapped nonscreen films stacked together.

Since exposure has to continue for the duration of tube movement, times of up to 3 seconds must be used. This means that full general anesthesia is essential for all tomographic examinations. For tomog- raphy of the body cavities, respiratory movements must be controlled by the anesthetist.

The effect of linear blur must be taken into account when selecting the position of the part to be tomographed in relation to the direction of tube movement. The choice depends on the orientation of struc- tures at all levels in the radiographic field. For example, in tomography of the chest, vertical movement is used. The images of the ribs, which tend to lie a t right angles to the direction of movement, are satis- factorily blurred out leaving the lung fields clear, while the vertebral column, which is vertically orientated, produces dense linear blur. Oblique movement has been found useful for some investigations of the equine skull.

By comparing the effects of vertical and horizontal tube movements (Fig. 2), it can be seen that different structures are high- lighted on each tomograph. The direction of movement selected for a clinical ex- amination would therefore depend on the location and type of lesion involved.

A further radiographic factor to be con- sidered is the thickness of the plane in focus. This is dependent on the angle through which the tube moves: as the angle is increased, layer thickness de- creases. The maximum angle for recti- linear movement is of the order of 45", the limitation being the degree of obliquity

VOL. XIV LINEAR TOMOGRAPHY IN VETERINARY PRACTICE 39

Fig. 2. positioned at an angle of 45' to the table top.

Canine Skull (Alsatian). Comparison between vertical (left) and horizontal (right) ventrodorsal tomographs. The skull was

of the x-ray beam at the extremities of the angle of swing. The layer thickness using

this angle is approximately 5 mm. For large objects and lesions, smaller angles and thicker layers may be indicated when ex- posure times would be correspondingly re- duced.

Before beginning a tomographic examina- tion, the location of the lesion to be in- vestigated must be identified in two planes on conventional radiographs. From these it is possible to assess the approximate size and depth of the lesion and to decide on the most suitable position. The radio- graphic field should be as small as possible, and an extending cone is useful to reduce divergent rays to a minimum. The num- ber of sections cut and the distance be- tween layers depends on the area to be examined, but a series of tomographs should be made to include the entire lesion and any related anatomical features. Tomography has made a useful contribution to the radio- logical investigation of the following groups of conditions:

1. Diseases of the head affecting the nasal cavity, sinuses, middle ear and tem- peromandibular joints and, in particular, dental abnormalities in the horse.

2. Fractures, infective lesions and con- the middle ear cavity (arrowed) and ossicles genital malformations of the vertebral

Fig. 3. Foal, 6 months. Vertical ventradorsal tomograph through the petrous temporal bone showing a section through

40 C. GIBBS 1973

Fig. 4. Alsatian, 8 years. On the survey radiograph (above), extensive new bone formation in the region of the left tempora- mandibular joint obscures the outlines o f the vertical mandibular rami and articulations. The caudal port of the petrous temporal bone and tympanic bullae are not involved. A lateral tomograph through the left temporo-mandibular joint (below) shows the articular surfaces in normal apposition thus demonstrating that pathological changes do not extend into the joint.

column in small animals and of the cervical spine in larger species.

3. Arthritic or periarthritic changes of certain limb joints. 4. Calculi, and other calcified renal le-

sions. 5. Intrathoracic masses. The procedure also has a place in the

study of the normal radiographic anatomy of complex skeletal parts, such as the temporal region of the skull (Fig. 3).

Fig. 5. Thoroughbred Yearling. Maxilla. The lateral survey radiograph (above) shows absence of the unerupted 3rd premolar. The structure of the resulting lesion cannot be determined because the area is overlain b y the contralateral tooth. An oblique tomagraph (below) shows the sclerotic margin (arrow) of a cystic cavity.

CASE HISTORIES The following case histories illustrate

some of the indications for tomographic investigation listed previously.

In order to obtain the maximum in- formation about a lesion, it is desirable to examine the series of tomographs collec- tively, side by side. Since it is not possible to reproduce them in this manner, most of the figures show a selected tomograph clearly demonstrating the lesion alongside a conventional radiograph of the same part. The numbers, where present, indicate the depth of cut measured in centimeters or inches from the table top.

An 8-year-old Alsatian was presented with wasting of the temporal muscles, dif-

VOL. XIV LINEAR TOMOGRAPHY IN VETERINARY PRACTICE 41

Fig. 6. which are demonstrated by a ventrodorsal tomograph (left). and there is an abnormality of the right atlanto-occipital joint which is concave cranially and convex caudally. atlas is irregular in outline.

Cat, 11 months. A deformity of the right side of the atlas can be identified on the survey radiograph (right), the details of A vertical fissure separates the two halves of the body of the atlas,

The right wing of

ficulty in opening the mouth and a bony swelling over the left temporomandibular joint (Fig. 4). Tomography confirmed that the lesion was confined to periarticular structures.

The use of tomography in equine dental examinations is demonstrated in a yearling which had shown a facial swelling and per- sistent nasal discharge for several months (Fig. 5 ) .

The congenital anomaly of bipartite atlas in a young cat was asymptomatic (Fig. 6). The defect appeared as an in- cidental finding on radiographic examina- tion of the skull. A vertebral abscess in a calf was associated with clinical signs of hindquarter ataxia (Fig. 7).

Stacked envelope wrapped films were used to obtain a series of tomographs of the elbow joint of a Labrador which show extensive periarticular new bone formation (Fig. 8).

Fig. 7. Friesian Calf, 1 month. The lateral survey radiograph shows a destructive lesion between the bodies of the 1 st and 2nd lumbar vertebrae over which shadows of rumen contents are superimposed. Details of the lesion are demonstrated clearly on the lateral tomograph (below).

42 C. GIBES 1973

Fig. 8. Labrador, 6 years. The survey radiograph (above left) shows extensive periostial new bone formation around the elbow joint. In this view there appears to be some opacity in the joint space adjacent to the junction of the radial and ulnar articulations. 2, 3 and 4 show a series of tomographic sections through the ioint space which do not confirm the presence of an intra-articular lesion. In 4 the anconeal process is seen to be intact.

Nephrotomography shows renal calculi in a 7-year-old female Dachshund (Fig. 9). Tomography of an intrathoracic mass in a 10-year-old Crossbred Collie demonstrated that it arose from the diaphragm (Fig. 10). The tumor was a fibrosarcoma.

SUMMARY

The veterinary literature relating to the general principles of tomography and its use in clinical radiological investigations is reviewed. Basic principles involved in the production of tomographs are discussed with particular reference to the efficiency of blur dispersal using different, forms of tube movement.

A simple type of linear tomographic apparatus which can be fitted to many standard x-ray installations is described. It consists of a rigid bar connecting a motor driven x-ray tube t o a Potter Bucky grid

Fig. 9. On the survey radiograph (left) the outline of the left kidney is obscured by fecal material in the colon, the ventral border of the spleen and the last rib, and the renal calculi shown on the tomograph (right) cannot be distinguished from bowel contents.

Dachshund, 7 years.

VOL. XIV LINEAR TOMOGRAPHY IN VETERINARY PRACTICE 43

Fig. 10. Crossbred Collie, 10 years. The survey radiograph (above) shows a radiopaque mass in the ventral thorax which extends from the region of the diaphragm to overlie the caudal border of the heart. In a lateral tomograph (below) the lesion i s outlined clearly and arises from the diaphragm.

tray which moves about an adjustable pivot. This apparatus can be used for small animals and the extremities of large animals. Attention is drawn to the im- portance of selecting the position of the patient to minimize the effects of linear blur.

Tomography has provided useful in- formation in the investigation of lesions of the skull, vertebral column, limbs and kidneys, and in the differentiation of intra- thoracic masses. It also has a place in studies of the normal radiographic anat- omy of certain structures.

A series of case reports illustrate some of the conditions in which tomography has been used as an aid to radiological diag- nosis. University of Bristol Dept. of Veterinary Surgery Langford, Bristol, England

ACKNOWLEDGMENTS: I am indebted to col- leagues in the Clinical Departments of the Uni- versity of Bristol for making available the clinical material used in these studies and to Professor A. Messervy for providing the facilities for this work to be carried out.

REFERENCES 1. Ballarini, G.: Clinical Examination of the

Liver in Birds. Application of Radiography (Tomography and Aerocystography) to the Diag- nosis of Trichomonad Hepatitis in Racing Pigeons Clin. Vet. Milano, 91: 185-199,1968.

2. Carter, S. J., Martin, J. J., Middlemiss, J. €5. and Ross, F. G. M.: Polytome Tomography. Clin. Radiol. 14: 405-413,1963.

Elimination of Blur in Linear Tomography. Acta Radiol.

4. Geary, J. C.: Veterinary Tomography.

5 . Griffiths, 1. R. and Lee, R.: Opthalmo- plegia in the Dog and the Use of Cavernous Sinus Venography as an Aid to Diagnosis. JAVRS

6. Reif, J. F., Snider, W. R., Kelly, D. F. and Brodey, R.: Cavitating Pulmonary Metastases in a Dog.

7. Schlaff, S.: Special Techniques in X-ray Diagnosis and their use in Dogs (I. Tomography). Mh. Vet. Med. 18: 473-478,1963.

8. Tarocco, C.: X-ray Study of the Sella Turcica in the Dog. Nuova Vet. 39: 120-129, 1963.

9. Tarocco, C.: Radiography of the Sella Turcica for the Diagnosis of Tumors of the Pitu- itary Gland in the Dog. Acta Med. Vet. Napoli.

Stratigraphy in the Diagnosis of Lung and Mediastinal Tumours in the Dog. Nuova Vet. 43: 394-411,1967.

3. Edholm, P. and Quidling, L.:

10: 441-447,1970.

JAVRS 8: 32-38,1967.

12: 22-28,1971.

JAVRS 10: 12-17,1969.

9: 83-91,1963. 10. Trenti, F.:

ZUSAMMENFASSUNG Es wird die Ubersicht uber die veterinarmedi-

zinische Literatur uber die generellen Gesicht- spunkte der Tomographie und ihre Verwendung bei klinisch-radiologischen Untersuchungen gegeben. Die grundlegenden Prinzipien in der Durch- fuhrung von Tomogrammen werden besprochen, vor allem was die Wirksamkeit der Verwischung bei Verwendung verschiedener Rohrenbewegungen anbetrifft. Ein einfacher linearer tomographischer Apparat, der vielen radiologischen Normeinrich- tungen angepasst werden kann, wird beschrieben. Er besteht aus einer Stange, die eine motor- betriebene Rontgenrohre mit einer Bucky Blende verbindet, die sich um einen regulierbaren Dreh- punkt bewegt. Dieser Apparat kann sowohl fur Kleintiere als auch fur die Extremitaten von grosseren Tieren verwendet werden. Es wird auf die Wichtigkeit der Lage des Patienten auf- merksam gemacht, um die Folgen der Verwis- chung auf ein Minimum zu reduzieren. Die

44 C. GIBBS 1973

Tomographie hat erlaubt brauchbare Informa- tionen in der Untersuchung von Lasionen am Schadel, an der Wirbelsaule, den Gliedern und Nieren sowie in der Differenzierung von intra- thorakalen Tumoren zu sammeln. Eine Reihe von Fallen illustriert einige der klinischen Zus- tande in denen Tomographie als Hilfsmittel fur die radiologische Diagnose verwendet wurde.

RESUME La literature v6t6rinaire traitant des principes

generaux de la tomographie et de son utilisation dans des investigations cliniques et radiologiques sont revis6e. On discute les principes de base int6ressant l’execution de tomographies en se r6fBrant particulibrement B l’efficacit6 dans la dispersion des effacements en utilisant des mouve- ments de tube diff6rents. On d6crit un appareil

simple de tomographie lin6aire lequel peut Gtre adapt6 B de nombreuses installations radiologiques standards. I1 consiste en une barre rigide joig- nant un tube radiologique aliment6 par un moteur B une grille Potter Bucky tournant autour d’un pivot ajustable. Cet appareillage peut Stre utilis6 pour des animaux de petite taille ainsi que pour les extr6mit6s d’animaux de grande taille. L’attention est attir6e sur l’importance du choix de la position du patient pour r6duire au minimum l’effet des effacements lin6aires. La tomographie a permis d’obtenir des informations utiles dans l’examen de l6sions du c r h e , de la colonne ver- thbrale, des membres et des reins ainsi que dans la diffhrentiation de masses intrathoraciques. Elle a Bgalement trouv6 sa place dans les Btudes de l’anatomie radiographique normale de certaines structures. Une s6rie de cas cliniques illustre cer- taines conditions pour lesquelles la tomographie a Bt6 utilis6e pour aider B pr6ciser le diagnostic radiologique.