CHAPTER 21 Radiographic Considerations of the Young Patient
Radiography of kittens and puppies is technically demanding because of the small patient size. Interpretation of radiographic images can be an even greater challenge, primarily because of unfamiliarity with the immature, developing skeletal system. Use of the basic principles of radiographic interpretation is essential, and doing so at least in part alleviates the uncertainties invariably encountered. This chapter will focus on the important unique radiographic features of puppies and kittens, application of radiographic principles, and interpretation advice; specific diseases are left to the appropriate dedicated chapters.
The basic principles of radiographic interpretation include size, shape, location, number, margination, and opacity of structures imaged. Recall that the five basic radiographic opacities, in increasing order, are air, fat, soft tissue, bone, and metal. The term summation refers to the two-dimensional (2D) additive effects of three-dimensional (3D) structures, such as the organs within the abdomen. For example, the caudal pole of the right kidney often overlaps the cranial pole of the left kidney, the area of overlap yielding an increase in soft tissue opacity relative to the nonoverlapping portions. The term silhouette sign (or border effacement) is used to describe the effacement of the borders of two touching structures of the same radiographic opacity. For example, if the spleen touches the border of the liver without fat between, the margins of the two organs cannot be distinguished.
Because there are relatively few anatomic and radiographic resources of the neonatal and juvenile dog and cat to use to compare, we must rely on laterality comparison (right vs. left) and on littermates or other age-matched patients for comparison. Further, such resources are of limited value because of the great differences in radiographic appearance that can occur in a relatively short period, not to mention breed differences. Although the neonatal puppy or kitten poses the most difficult interpretation challenge, fortunately these ill or injured patients are often presented by the breeder, who may have access to littermates. Radiographing a littermate for direct comparison can be the most important criterion for the practitioner when deciding normal from abnormal.
Radiographic examination of neonatal puppies or kittens is a supreme challenge in patience. Safe manual restraint is nearly impossible when using leaded gloves trying to hold tiny appendages. Because sedation or anesthesia is not routinely performed in such young patients, restraint methods using positioning devices such as foam sponges and tape are needed. Experience has shown us that a well-fed kitten or puppy is the most amenable to radiography. Exercise and play will quickly tire a neonate, which allows the patient to be restrained using positioning foam and tape. Cooing and soothing the patient help but require the radiographer’s patience. Your best kitten or puppy “whisperer” may be needed! However, even under the best of circumstances, perfect positioning can be taxing, and it may be necessary to accept less-than-perfect radiographic images, depending on the specific indication for radiography. For example, critical assessment of the cardiac silhouette may be compromised by rotation of a ventrodorsal (VD) or dorsoventral (DV) radiograph; mild patient rotation probably will not hinder a diagnosis of heart failure.
For the youngest puppies and kittens, the lowest radiographic technique that the x-ray equipment is capable of will be necessary, as you might use for a bird or guinea pig. This requires a very low kilovoltage potential (kVp) and corresponding relatively low milliamperage-seconds (mAs). For example, a kVp of 40 or 45 is used for the smallest patients, which is the lowest allowable setting on diagnostic x-ray units. The mAs used depends on the speed of the intensifying screen/film combination. A 400 or 800 speed system may use 1 to 2 mAs, whereas a slower speed higher detail system (e.g., a 100 speed system) may require 4 to 8 mAs to achieve proper film density (blackness).
Given a choice, a slower speed system (e.g., 100 vs. 400 speed) should be used to achieve the best image detail. Use of a higher speed (more efficient) system will noticeably reduce image detail and in some instances may not be capable of yielding a diagnostic image because even the lowest machine setting will be too high and result in overexposed radiographs. In this circumstance, the focal-film distance (FFD) can be increased, effectively reducing exposure. For example, the conventionally used 40-inch FFD can be increased to 50 inches or more by raising the x-ray tube height. Although a grid is not typically used for imaging of small patients or body parts less than 10 inches in thickness, a grid can be used to effectively reduce x-ray film exposure; this effect can be dramatic. A grid with an 8:1 ratio will reduce x-ray exposure by a factor of 3; a 12:1 grid can do so by as much as a factor of 4.
If digital radiography is used, the same basic aforementioned considerations apply. Use the lowest kVp and mAs that allows enough x-ray (photon) density to achieve a mottle-free image. Many digital x-ray units require more radiographic exposure to achieve a diagnostic image than conventional film-screen radiography. Depending on the digital system used, image quality of such small patients may be worse compared with conventional film-screen radiography, exacerbated by magnification (zooming) of images on the viewing monitor.
For neonatal radiography, “whole body” images are routinely made and are acceptable. In larger puppies and kittens, as in adult patients, thorax- or abdomen-targeted radiographs are ideal, based on requirements of patient size, geometric distortion, and conventional film radiography technique differences used to maximize detail of thoracic or abdominal structures (i.e., high kVp technique to maximize image latitude and lung parenchymal detail, with lower kVp and higher mAs technique used to enhance fat and soft tissue contrast for maximum abdominal radiographic detail).
Either VD or DV images are acceptable. In practice, VD images of the thorax/abdomen are usually easiest to obtain. For the lateral image(s), right or left recumbency positions are equally diagnostic. As for radiography of adults, recumbency may be dictated by the type or laterality of the disease process in question. For example, if the VD/DV radiograph indicates left-sided pulmonary consolidation (such as pneumonia), a right lateral recumbent radiograph is indicated to maximize radiographic visualization of the diseased lung. Recall that the nonrecumbent lung is more aerated than the recumbent lung. It is the aerated portions of lung that provide contrast with lung pathology. Left-sided pulmonary consolidation may be completely invisible on a left lateral radiograph as a result of recumbency-induced left lung atelectasis (collapse).
Radiographic assessment of the appendages is perhaps the most challenging aspect of neonatal radiography. In the smallest patients, whole body images may be sufficient to assess the limbs. In the youngest patients, isolating a limb and obtaining two views may be truly challenging. This becomes easier in older, larger patients. The use of tape or loops of gauze to extend and isolate the limb works well. Care must be exercised because the limbs are relatively vulnerable. Although experience has shown that kittens and puppies are often restless and at first intolerant of these procedures, a gentle hand and patience can usually result in acquisition of diagnostic images.
In a 3-day-old puppy (Figure 21-1), the heart looks relatively larger than in an adult (increased cardiothoracic ratio), as the lungs are not fully developed and well aerated as they will be later in life. As a result, the normal lung parenchyma shows a mild, diffuse, and unstructured interstitial pattern compared with an adult. Increased cranial thoracic soft tissue opacity is caused by overlying forelimb musculature and the thymus (see Figure 21-1, A). The abdomen of this puppy is normal and differs markedly from an adult. There is essentially no abdominal serosal detail, principally because of lack of abdominal fat to provide contrast with the soft tissue abdominal organs. Gastrointestinal gas provides the only contrast visible. The kidneys are not visible because of lack of retroperitoneal fat. The pendulous appearance of the abdomen is also a normal finding in puppies and kittens, caused by relative lack of abdominal wall muscle tone and a relatively large liver. In an adult, this appearance would be diagnostic for abdominal fluid.
Figure 21-1 A 3-day-old puppy. No sedation was used, relying only on positioning devices, calming and cooing, and patience. A, Left lateral whole body radiograph. The puppy was placed in left lateral recumbency, calmed until she was still. B, Ventrodorsal whole body radiograph. The puppy was positioned in dorsal recumbency, secured laterally by foam sponges. The thorax is well positioned, except for the overlying forelimbs. Overlying hindlimbs compromise abdominal assessment. If critical assessment of the peripheral lung parenchyma or abdomen were necessary, the overlying paws would need to be repositioned. C, Ventrodorsal whole body radiograph. The same puppy as in B was repositioned. The abdomen is perfectly positioned, although the thorax is now rotated. D, Lateral radiograph of the left forelimb. Tape and a small pad have been used to position the limb for radiography while gently restraining the puppy’s body. Note the small ossified clavicular remnant cranial to the shoulder. E, Lateral radiograph of the left hindlimb. The limb was passively positioned, extending it while gently restraining the puppy’s body. The thin linear radiopaque structures surrounding the pes are tape, used to position the limb for radiography. F, Ventrodorsal pelvis.
Musculoskeletal items of note include open fontanel of the skull, separate centers of ossification of the wings of the first cervical vertebra (atlas), ossification of the hyoid apparatus, and upright and rectangular shape of the vertebrae with apparent widening of the intervertebral disc spaces because of nonossification of the vertebral endplates (epiphyses). The sternum has a similar appearance.
A VD whole body radiograph (see Figure 21-1, B) allows adequate visualization of the heart, lungs, and abdomen. There is increased width of the cranial mediastinum because of the presence of the immature thymus. When the puppy is repositioned to remove the overlying paws (see Figure 21-1, C), note the shortened, rectangular shape of the vertebral bodies created by nonossified vertebral endplates (epiphyses). The ilial and ischial ossification centers of the pelvis can be seen; the femoral heads are not ossified at this age.
A lateral radiograph of the left forelimb (see Figure 21-1, D) shows that the humeral, radial, ulnar, metacarpal, and phalangeal diaphyses are ossified. The humeral head, distal humeral condyle (the lateral portion, the humeral capitulum, and the medial portion, the humeral trochlea, are separate centers, eventually fusing by approximately 2 months of age to become the humeral condyle), radial and ulnar epiphyses, metacarpal and phalangeal epiphyses, and all the carpal cuboidal bones are not ossified at this early age. Radiographs of the hindlimb (see Figure 21-1, E) show that the femoral, tibial, fibular, metatarsal, and first and second phalangeal diaphyses are ossified, as are the calcaneus and talus. The third phalanges are barely perceptible. All the epiphyseal centers are still cartilage models (soft tissue radiographic opacity) and not yet ossified. Note that the patellar, fabellar, and tarsal cuboidal bones are not visible as bony structures. In Figure 21-1, D and E, the lack of ossification creates a radiographic image of “widened” joints and soft tissue swelling. Therefore assessment for potential trauma or infection is challenging and may require comparison with the contralateral limb.
In a VD pelvic view (see Figure 21-1, F), the ilial, ischial, and pubic ossification centers are present, although the acetabular bones and femoral heads are not ossified. The stifle joints appear wide due to lack of epiphyseal ossification; the patellae and fabellae are also not ossified. The tarsi have a similar appearance, due to lack of epiphyseal and cuboidal bone ossification.
In the 12-day-old puppy (Figure 21-2), phalangeal and talus ossification has progressed. The primary difference in the radiographic appearance between the 3-day-old and 12-day-old puppy is identification of early ossification of accessory carpal bones (see Figure 21-2, C), the fourth tarsal bones (see Figure 21-2, D), and very faint ossification of the central tarsal bones. The cardiothoracic ratio is considered normal (identical to an adult) with better aeration of the lungs; although poor abdominal detail persists, a very thin stripe of abdominal fat can be seen separating the ventral abdominal body wall from the ventral liver margin, and the abdomen is less pendulous.
Figure 21-2 A 12-day-old puppy. No sedation was used to make these images. A, Left lateral whole body radiograph. B, Dorsoventral whole body radiograph. After calming the patient, the dorsoventral radiograph was made by simply positioning the puppy in sternal recumbency. There is now identification of the accessory and fourth tarsal bones and mild progression of ossification of the talus and phalanges. C, Close-up lateral view of the carpi showing early ossification of the accessory carpal bone (white arrows). D, Close-up view of the tarsus showing the earliest ossification of the fourth tarsal bone (white arrow).
A lateral whole body radiograph of a 26-day-old puppy shows further skeletal ossification (Figure 21-3, A). The vertebral endplates (epiphyses) are now identified. Abdominal detail is still poor, but the thin layer of fat between the abdominal wall and the ventral margin of the liver is thicker and more evident than at day 12. In Figure 21-3, B, note visualization of carpal bones, many long bone epiphyses, vertebral endplates (epiphyses), and maturation of ossification centers noted 2 weeks earlier. The cranial mediastinum is widened by the thymus. Abdominal detail remains poor, but the margins of the spleen can be faintly seen along the cranial to mid-left abdominal wall. Partial ossification of the humeral head, distal humeral condyle (two parts), distal radial epiphysis, radiocarpal and distal row carpal bones, distal metacarpal epiphyses, and proximal epiphyses of the first phalanges has occurred in the forelimbs (Figure 21-3, C). The femoral head, distal femoral epiphysis, and proximal and distal tibial epiphyses are now clearly evident in the hindlimbs (Figure 21-3, D). The distal metatarsal and proximal first phalangeal epiphyses are faintly seen as tiny focal areas of ossification. Tarsal bone ossification has progressed.
Figure 21-3 A 26-day-old puppy. No sedation was used to make these images. A, Left lateral whole body radiograph. Note the concavity of the vertebral endplates (epiphyses). Positioning of the patient is less than ideal for critical evaluation of the cranial thorax, obscured by forelimb musculature. B, Ventrodorsal whole body radiograph. The patient was positioned in dorsal recumbency using laterally placed large foam positioning devices. C, Lateral radiograph of the left forelimb. D, Lateral radiograph of the left hindlimb.
In a lateral whole body radiograph of a 34-day-old puppy (Figure 21-4, A), the lungs are not fully inflated, resulting in a diffuse increased soft tissue interstitial pulmonary pattern, not to be confused with pneumonia. Cranial thoracic soft tissue opacity from the thymus and overlying forelimb musculature can be seen. Abdominal detail remains poor, but a small amount of intraabdominal fat is seen along the ventral body wall, separating several loops of small intestine. A VD whole body radiograph shows further skeletal ossification (Figure 21-4, B). Cranial mediastinal widening is caused by the presence of the thymus. Abdominal detail remains poor, similar to 1 week earlier. In the forelimbs (Figure 21-4, C), the humeral head is similar to what its mature shape will be. The distal medial and lateral portions of the humeral condyle (capitulum and trochlea) are readily identified as separate centers of ossification that will fuse to each other by approximately 2 months of age. The proximal radial epiphysis is beginning to ossify. Progressive ossification of the distal radial epiphysis, the carpal cuboidal bones, the metacarpus, and phalanges is noted. In the paws (Figure 21-4, D), continued maturation of the metacarpal bones and phalanges is evident, but the distal ulnar epiphysis is not yet ossified. Progressive ossification of the diaphyseal and epiphyseal long bones and the cuboidal bones of the tarsus has occurred in the hindlimbs (Figure 21-4, E). The apophysis of the calcaneus (calcaneal tuber) is now evident. Still not ossified are the patella, fabellae, and tibial tuberosity. A VD pelvic view (Figure 21-4, F) shows that the acetabular bones are not yet ossified, nor are greater trochanters, patella, and fabellae. The coxofemoral joints appear widened, as do all the joints imaged.
Figure 21-4 A 34-day-old puppy. No sedation was used to make these images. A, Left lateral whole body radiograph. B, Ventrodorsal whole body radiograph. C, Lateral radiograph left forelimb. D, Close-up dorsopalmar view right forepaw. The cuboidal bones are distinctly ossified but far from their mature appearance. E, Lateral radiograph of the left hindlimb. F, Ventrodorsal pelvic radiograph. Skeletal maturation has progressed.
A lateral whole body radiograph of a 42-day-old puppy (Figure 21-5, A) shows how abdominal serosal detail continues to improve. Note the thin fat stripe between the ventral abdominal body wall and the ventral margin of the liver, as well as increased visualization of the serosal surfaces of the small intestinal loops, separated by mesenteric fat. The liver margin extends well beyond the costal arch, with smooth and sharply defined margins. Whereas in an adult this would indicate liver enlargement, this is a normal finding in puppies and kittens. Note the subcutaneous fat accumulation when compared with the earlier images.
Figure 21-5 A 42-day-old puppy. No sedation was used to make these images. A, Left lateral whole body radiograph. Mineral ingesta are noted incidentally in the stomach and colon. Motion artifact is noted with one of the forepaws. B, Ventrodorsal whole body radiograph. The nonsedated puppy is securely positioned using laterally placed foam sponges. Bony ingesta are noted in the stomach and colon. C, Left lateral forelimb. D, Left lateral hindlimb. Note the irregularity of the distal femoral condyle ossification. This irregularity is a normal pattern of ossification; it can be confused with lytic change associated with osteomyelitis.
Progressive ossification of skeletal structures has occurred. Note the vertebrae have assumed an adult shape (except the open physes); the intervertebral disc spaces are also near normal in width, without the concavity noted in the earlier studies.
In Figure 21-5, B, the thoracic portion of the puppy is mildly rotated with the sternum projected to the left. The thymus can be identified as a roughly triangular soft tissue opacity in the cranial left thorax. Abdominal detail continues to improve. Note the spleen is well visualized along the left lateral body wall by the thin fat opacity along its mesenteric margin.
Joints are becoming more adult-like in appearance as the epiphyseal bone continues to ossify (Figure 21-5, C). The distal ulnar epiphysis is now visualized; a tiny focus of ossification of the medial humeral epicondyle is now seen. The joints of the pes are becoming more adult-like in appearance (Figure 21-5, D). The stifle lags in ossification; the patella, fabellae, and tibial tuberosity have not yet begun to ossify.
In a 49-day-old puppy, progressive skeletal maturation has occurred. The thorax and abdomen (Figure 21-6, A to C) are otherwise very similar in appearance to the preceding week. In the forelimbs (Figure 21-6, D), the medial epicondylar apophysis of the distal humerus and distal ulnar epiphysis have significantly ossified during the past week. The remaining osseous structures are progressively maturing. Progressive ossification of the bony structures of the hindlimb has also occurred (Figure 21-6, E); however, the patella, fabellae, and tibial tuberosity are still not apparent radiographically.
Figure 21-6 A 49-day-old puppy. No sedation was used to make these images. A, Lateral thoracic radiograph. This image was made on a small image plate. B, Lateral abdominal radiograph. This image was made on a small image plate. C, Ventrodorsal whole body radiograph. This image was made on a 14 × 17-in image plate, allowing the entire body to be imaged in one exposure. D, Left lateral forelimb. E, Left lateral hindlimb.
At 55 days of age, the thorax is normal (Figure 21-7, A). The vertebrae and other osseous structures continue to mature. A VD view (Figure 21-7, B) shows persistent widening of the cranial mediastinum caused by the thymus. Abdominal detail is still vastly different from an adult, but progressive fat deposition has resulted in a striking increase in abdominal serosal detail when compared with earlier images, although a typical puppy “pot belly” appearance remains (Figure 21-7, A and C). In the past 6 days, the olecranon (proximal ulnar apophysis) has been ossifying in the forelimbs. The supraglenoid tubercle ossification center is now visualized within the cranial portion of the scapulohumeral joint, overlying the manubrium (Figure 21-7, D). Within the same period, the patella and tibial tuberosity and the head of the fibula also now begin to ossify. Continued development of the remaining bony structures is evident. A VD pelvic view (Figure 21-7, F) will show that the acetabula continue to develop, although considerable ossification will occur during the next 2 months. The femoral heads are nearly completely ossified and have the shape of a mature dog. The patellae are radiographically evident.
Figure 21-7 A 55-day-old puppy. No sedation was used to make these images. A, Left lateral whole body radiograph. Increased opacity noted in the cranial thorax on the lateral view is from the thymus, seen as a soft tissue opacity in the cranial mediastinum on the ventrodorsal view. B, Ventrodorsal whole body radiograph. C, Left lateral abdominal radiograph. This image is processed to maximize abdominal detail by increasing the radiographic contrast between the abdominal fat and soft tissues of the abdominal organs. D, Left lateral forelimb. E, Left lateral hindlimb. F, Ventrodorsal pelvis. Note the patellae are now evident.
Once the puppy reaches 62 days of age, it is now large enough to justify a dedicated thoracic radiograph (Figure 21-8, A). The thorax resembles an adult in lateral views, with the exception of the increased opacity of the cranial mediastinum and the immature skeleton. The appearance of the intrathoracic structures closely resembles an adult dog on VD thoracic views (Figure 21-8, B) as well. The liver is sharply marginated and extends beyond the costal arch, a normal finding in a puppy or kitten (Figure 21-8, C). It is well delineated from the ventral abdominal body wall by an increasing layer of relatively radiolucent fat. Note that the serosal margins of various small intestinal loops are readily seen by surrounding mesenteric fat. There is not yet enough retroperitoneal fat to allow visualization of the kidneys, but there has been a progressive increase in subcutaneous fat layer along the puppy’s back. In a VD view, the serosal detail of the abdomen is not as apparent (Figure 21-8, D), and the kidneys cannot be seen. The supraglenoid tubercle of the scapula, the medial humeral epicondyle, the olecranon, and the distal ulnar epiphysis have enlarged and continued to ossify (Figure 21-8, E). The cuboidal bones appear very well ossified in a dorsopalmar view of the forelimbs (Figure 21-8, F). The paired palmar sesamoids of the metacarpophalangeal joints are now visible. The conical conformation of the distal ulnar physis is clearly apparent, as are the normal “flared” appearances of the distal radial and ulnar metaphyseal bone. Rapid maturation of the tibial tuberosity can be noted in the hindlimb (Figure 21-8, G), and the patella has developed a mature shape.
Figure 21-8 A 62-day-old puppy. No sedation was used to make these images. A, Left lateral thorax. B, Ventrodorsal thorax. C, Left lateral abdomen. The stomach is full and contains multiple bone fragments (the puppy had been recently fed). The radiolucent linear structure over the dorsal aspect of the coxofemoral joints represents normal acetabular physes; they can be confused with fractures. D, Ventrodorsal abdomen. A markedly distended stomach occupies approximately one half of the abdomen in this recently fed puppy. Note the lucent physis between each ilium and acetabular bone, corresponding to those noted on the lateral radiograph. E, Lateral forelimb. The conical conformation of the distal ulnar physis is clearly evident. F, Dorsopalmar forelimb (manus). G, Lateral hindlimb. Note the patellar ligament and the relatively radiolucent infrapatellar fat pad.