Diagnostic imaging

Chapter 8


Diagnostic imaging



Diagnostic imaging forms an essential part of the investigation for many general and oncological diseases in cats. Radiology and ultrasound remains the mainstay for initial investigations for many conditions, but with the advent of easier accessibility to advanced imaging such as computed tomography (CT) and magnetic resonance imaging (MRI) these modalities are becoming used more frequently. The indications for each imaging modality for investigation of thoracic and abdominal disease and for imaging the brain will be covered in this section. For further general information on diagnostic imaging and specifically radiography of bone diseases, the reader is referred to other specialized texts1 and the imaging chapter in Feline Orthopedic Surgery and Musculoskeletal Disease.2



Diagnostic imaging modalities


The present chapter focuses on the species-specific radiographic differences in the feline thorax, abdomen and neurocranium. The use of advanced imaging modalities for these areas will be briefly discussed, giving some indications where they may give additional information not provided by radiography, particularly the use of MRI for imaging the brain.





Computed tomography


Due to an increasing availability and the advantages of multi-slice CT (fast acquisition with large anatomic coverage, thin slice thickness, increased spatial resolution), this technology has also become a highly valuable tool in veterinary medicine. As a cross-sectional imaging technique, CT avoids superimposition of adjacent structures by imaging a transverse slice. Consequently, soft tissue structures enveloped by bones e.g. the nasal cavity, brain, or pelvic organs, may be imaged well with CT whereas a radiographic image of these regions is of limited value. CT has also been increasingly used for imaging the lung, mediastinum and various abdominal organs. CT angiography is easy to perform with a peripheral intravenous injection of contrast medium (Fig. 8-1). With multi-slice CT, an arterial and venous phase, and for the liver a portal phase, can be imaged. In oncological diseases, CT angiography may indirectly help to identify and describe a neoplastic lesion on the basis of its relationship to adjacent arteries or veins (mass effect, compression and invasiveness), or the lesion itself may be outlined on the basis of its individual perfusion/contrast enhancement pattern.




Magnetic resonance imaging


MRI has long been the imaging modality of choice for neurologists and it is now also becoming a more important imaging modality for other clinical specialties. MRI has great potential for increasing use in other disease complexes, especially if soft tissue components need to be assessed, quantified and qualified. MR imaging has the potential to have a great impact in oncological and endocrinological disease in feline patients although relatively few studies exist in cats to date (Fig. 8-2).



MRI generates images without the use of X-rays. Instead it uses the magnetic properties of protons distributed in the patient’s body. Two MRI systems are currently in standard use in veterinary medicine, namely low-field systems (low magnetic field strength) using open scanners, and closed-bore systems with stronger magnetic fields. The open systems offer better access to the patient, the higher magnetic fields give better local resolution with slightly shorter scanning times. The higher the magnetic field, the more artefacts are possible. The high-field scanners (>1 Tesla) usually provide a large field of view (up to 50 cm) giving the possibility of performing whole-body imaging, which is excellent for tumor staging. Sequences used for staging include the fast turbo short tau inversion recovery (STIR) imaging,3 which is widely used in human medicine nowadays.


Different sequences can be used during MRI. The recorded signal is analyzed by a computer using a complicated calculation method and eventually presented to the radiologist in the form of a section image.4 Depending on the form, duration and intensity of the high-frequency impulses emitted into the region of interest (‘pulse sequence’), the tissue contrast in the image can be determined primarily through different T1- or T2-relaxation times, but also through the proton density alone. In this context, we refer to T1-, T2- or proton density (PD)-weighted sequences. Depending on these weightings, a tissue may be visualized with different signal intensities. For example, pure water shows low signal intensity in T1-weighted sequences, and high signal intensity in T2-weighted sequences. In contrast, fat shows up with high signal intensity in both weightings, whereas muscle shows medium signal intensity in T1-weighted images, but low signal intensity in the T2-weighted ones (Fig. 8-3). The tissue contrast can be manipulated even further using special sequences. An example is the STIR sequence allowing selective suppression of the fat signal with high signal visualization of fluids in basically a T2-weighted sequence. The same can be done to extract the signal of cerebrospinal fluid (CSF) in T2-weighted images of the central nervous system. This is done with FLAIR-sequences (fluid attenuation inversion recovery) (Fig. 8-3).



Like CT and ultrasound scanning, MRI is a tomographic (slice) method for producing a two-dimensional image. For CT the two-dimensional images are formed by reconstructing the sagittal and dorsal planes from the acquired transverse slices. MRI, in contrast, is a true three-dimensional modality, which can acquire each plane by itself without the need for post-scan reconstruction.5,6


The high tissue contrast in MRI is a major advantage compared with X-ray techniques (radiography, computed tomography). Also, tissues can be distinguished by the different intensities created by using different sequences, allowing far better assessment of soft tissue disorders (Table 8-1). MRI is a very sensitive modality, which still lacks specificity. However, the combination of different sequences, with and without contrast enhancement, provides a great tool for depiction and description of the extent of a lesion. This modality is useful for optimal surgery planning.




Thorax


High-quality survey radiographs can provide a lot of information on thoracic disease but there are some limitations. The use of other imaging modalities, particularly ultrasound and CT, can provide additional information, for example on heart structure and function, and earlier evidence of pulmonary metastases, respectively.



Radiology


Thoracic radiographs play an important role in the investigation of respiratory, cardiovascular and oncological diseases and in the trauma patient. They can provide a quick initial and cost-efficient general overview. However, radiographic findings are often non-specific and interpretation might be challenging, particularly of pulmonary lesions.





Interpretive principles and normal anatomy


A systematic radiographic approach is important to identify pathologies and to recognize possible life-threatening conditions. The thorax can be divided into four basic anatomic regions: extra-thoracic region, pleural space, lung parenchyma, and mediastinum.


The extra-thoracic region includes the thoracic skeleton and the soft tissue of the thoracic wall and diaphragm. These structures should be included in a thorough evaluation of thoracic radiographs, in particular following trauma. Each individual rib, vertebral body and any of the included forelimbs should be scrutinized to identify skeletal lesions. Incompletely and densely mineralized costal cartilages might mimic fractures in older cats (Fig. 8-4). The thoracic surface of the diaphragm is visualized as it projects against the air-filled lung; caudally it silhouettes with the liver. In most cats, the ventral portion of the diaphragm is seen on a lateral view as a thin, soft tissue structure, outlined by mediastinal fat cranially and falciform fat caudally.



The pleural space contains only a very small quantity of fluid that is not usually visible radiographically. Space-occupying abnormalities can distend the pleural space, e.g., fluid or air accumulation, pleural masses and herniated abdominal content. In cats, unilateral pleural diseases are more common than in the dog. In contrast to dogs, the caudodorsal lung fields are separated by a triangular-shaped soft tissue opacity from the spine on a lateral view. This represents the intrathoracic psoas muscles and should not be misinterpreted as pleural effusion (Fig. 8-5).



Pulmonary vasculature, bronchi up to their secondary divisions and some interstitial markings, are usually visualized radiographically in a normal lung. Interpretation of lung parenchyma is challenging, mainly due to the wide range in appearance of normal lungs and the overlap of radiographic features between diseases of different etiology. An initial practical approach is to identify any increase or decrease in lung opacity and to determine the distribution, location and severity of the detected abnormalities. The type of pulmonary pattern (bronchial, vascular, alveolar, and interstitial) and determination of the predominant pattern are commonly used to further classify lung abnormalities. It is useful to assess whether the airways (alveolar, bronchi) are involved to determine if additional investigations are required (e.g., bronchoalveolar lavage). The presence of a mediastinal shift on the DV/VD planes is a valuable indicator of changes in volume seen in hemithorax (lung collapse, pulmonary mass).


Mediastinal structures normally seen radiographically include the heart, trachea, aorta, caudal vena cava, sometimes the esophagus, and in young animals the thymus. The normal feline heart is more consistent in shape than in dogs, with no breed-associated variations. The maximum width of the cardiac silhouette should not exceed 2–2.5 intercostal spaces on a lateral view. The apex of the heart is usually closer to the midline on a DV/VD view. In cats, the left atrium has a more cranial position, which makes it more difficult to identify on a lateral view. In older cats, the heart commonly becomes more horizontal in position and a focal aortic bulge might be noted (Fig. 8-6). In obese animals, large deposits of pericardial and mediastinal fat may mimic an enlarged cardiac silhouette. The cranial mediastinum should normally not exceed the width of the superimposed thoracic spine on a DV/VD view. Sternal and tracheobronchial lymph nodes are not visualized unless enlarged.7–12




Computed tomography


Computed tomography may be useful in cats with thoracic disease where other diagnostic modalities such as radiography and ultrasonography have failed to identify the cause or extent of the disease (Fig. 8-7). CT has potential for providing additional information on a variety of thoracic conditions in cats, particularly the possibility of identifying pulmonary metastases that are not visible on thoracic radiographs (Box 8-1).



Box 8-1   Some potential indications for thoracic CT in cats








Specific thoracic conditions



Cardiomyopathy


A variety of cardiac diseases can affect the cat, but myocardial diseases are most commonly encountered. Hypertrophic cardiomyopathy (HCM) is the most common form of myocardial disease affecting cats. Many cases of hypertrophic cardiomyopathy are clinically asymptomatic. Detection of a heart murmur and gallop rhythm should therefore warrant further investigation, even in an apparently healthy cat.


Echocardiography is the most sensitive imaging modality to diagnose and to classify cardiomyopathy. Echocardiographic changes are characterized by focal or generalized increased thickness of the left ventricular free wall and interventricular septum. Left atrial enlargement can usually be recognized in symptomatic patients. Thoracic radiographs, however, are indicated and superior to identify signs of congestive heart failure, such as pulmonary venous distension and pulmonary edema. In cats, pulmonary edema has a variable appearance and distribution (Fig. 8-8), rather than being located in the perihilar region as with dogs.13 Pleural effusion is often present secondary to left-sided heart failure. The concentric hypertrophy of the ventricular wall is often not recognized on radiographs and only an abnormal lung pattern might be noted. Advanced left atrial enlargement is particularly obvious on the DV view, seen as a ‘valentine’ shaped heart.




Thoracic trauma


Patients who have suffered thoracic trauma must be stabilized prior to radiography. In dyspneic patients a DV view should be taken to avoid further respiratory compromise. Lesions that might be recognized radiographically include pulmonary contusion, pneumothorax, and rib fractures (Fig. 8-9).



Pulmonary contusion may be evident radiographically as complete lung lobe consolidation or as patchy areas of alveolar/interstitial infiltrates, commonly ipsilateral to the impact site. Radiographs taken immediately following trauma may not reveal these changes, which usually become apparent two-12 hours post-trauma.14


Post-traumatic rib fractures should be differentiated from older, hypertrophic non-union rib fractures, which occasionally can be seen in cats with chronic lower airway disease.


Diaphragmatic rupture is a common consequence of blunt external trauma. Identification of abdominal viscera in the thoracic cavity and loss of the diaphragmatic outline are conclusive signs for diaphragmatic rupture. However, pleural effusion may be present and obscure these structures. Cranial displacement and loss of abdominal organs on an abdominal radiograph are useful, indirect signs to identify a diaphragmatic rupture. In cats, only falciform fat and omentum may herniate, which may be recognized by the loss of the diaphragmatic line ventrally and by the relatively small fat pad ventral to the liver (Fig. 8-10).15 Positive-contrast gastrointestinal studies may aid the diagnosis.


Related posts:

  1. Introduction
  2. Spleen
  3. Skin tumors
  4. Larynx

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Sep 6, 2016 | Posted by in SUGERY, ORTHOPEDICS & ANESTHESIA | Comments Off on Diagnostic imaging

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