Introduction

Neoplasia almost always alters the normal spatial relationships of tissues and organs and diagnostic imaging is critical not only for the diagnosis of cancer but also for staging patients and for following response to therapy. At present the most frequently used imaging modalities for the diagnosis of cancer are conventional radiography and ultrasonography, but magnetic resonance imaging (MRI), computed tomography (CT) and nuclear medicine (Nuc Med) are increasing in availability and usage. Even with these advances it is necessary to emphasise that it is not possible to make a tissue diagnosis based on the image and that relevant clinical information, as well as cytology or histopathology, are necessary to make a definitive diagnosis. Ultrasound, CT, MRI, and Nuc Med have not replaced conventional radiographs but serve to complement them. These newer imaging modalities have made assessing tumour margins, evaluating for metastasis and determining the degree of invasiveness more reliable. These techniques are quite expensive but lend more information to diagnosis and staging (extent of tumour burden and degree of invasion). In the long run this may decrease cost and patient suffering. They may also aid in narrowing the list of differentials and prevent unnecessary surgery. The newer imaging modalities will also help to guide deep needle aspirations or biopsies, as well as to plan open biopsies and surgical resections.

Imaging Modalities

Conventional Diagnostic Radiographs

These still remain a valuable diagnostic tool in the practice of veterinary oncology. Availability and cost are a major advantage. Radiographs provide good spatial resolution but have poor contrast discrimination between normal and neoplastic soft tissues. Invasiveness and tissue architecture cannot be assessed. One advantage to the use of conventional radiographs is their ability to cover a large area such as the abdomen or thorax in two images (lateral and ventrodorsal (VD)/dorsoventral (DV)). They can provide information about the size, shape, margination, opacity and position of organs and organ displacement, more easily than other imaging modalities. Conventional radiographs are a good screening procedure and guide to further diagnostic and imaging procedures.

Ultrasonography

Improved ultrasound technology and increased clinical applications have made ultrasound an important modality for diagnosis and evaluation of therapeutic response, in the veterinary cancer patient. The affordable cost and availability of ultrasound equipment has resulted in a tremendous increase in the use of ultrasound. Ultrasonography excels when discrimination of cystic versus solid masses, or evaluation of a body cavity filled with fluid (abdominal or pleural effusion) is needed, or when knowledge of internal architecture of an abdominal organ is needed, or when guidance of fine needle aspiration (FNA) or tissue core biopsy is required. Ultrasonography's ability to find tissue abnormalities (focal and diffuse) is excellent but definitive diagnosis still depends upon cytology or histopathology.

Computed Tomography

This is the production of tomographic images (cross-sectional slice images) using X-rays and computers. With CT scanning the recorded tissue densities have the same significance as the film densities on conventional radiographs. CT compared to conventional radiographs has excellent contrast discrimination and the ability to separate deep structures without the superimposition of overlying tissues (visualise brain without the inconvenience of the surrounding skull). Soft tissues must be significantly altered by disease before changes can be visualised with conventional radiographs; with CT changes can be appreciated earlier and more accurately. The appearance of a tumour may be compared with adjacent normal tissues to provide information on the degree of invasiveness.

Magnetic Resonance Imaging

This can be easily obtained in any plane (axial, sagittal and coronal). The contrast between different soft tissues is superior to all other imaging modalities. Standard MRI scans take a relatively long time (minutes rather than the seconds required in a CT; therefore body, respiratory, cardiac and intestinal motion are a potential problem. General anaesthesia is required and monitoring of patients can be difficult depending on the type of MRI unit utilised. MRI can be very expensive and availability is not as universal as CT, but benefit versus risk shows that for imaging of the central nervous system nothing is superior.

Imaging Body Systems

Pulmonary

Conventional radiographs represent the most valuable and cost-effective primary screening of neoplasia of the thorax. Radiographs should be taken in all patients suspected of having cancer and during treatment periods to monitor response to therapy. Taking three views of the thorax (right and left laterals as well as DV or VD) should act to increase sensitivity for finding lesions.

CT, by virtue of its cross-sectional display of anatomy, and superior contrast and anatomical resolution, can differentiate pulmonary lesions from overlying normal superimposed structures. CT is clearly superior to conventional radiography in demonstrating the extent of the primary lesion, invasion of the hilus or mediastinum, body wall or pleural space involvement and the presence of enlarged lymph nodes. Iodinated intravenous contrast can help to distinguish between vascular and nonvascular abnormalities. The most sensitive modality at present in the diagnosis and staging of pulmonary metastatic disease is CT.

Skeletal

A diagnosis of skeletal neoplasia can often be supported on conventional radiographs. Bone scintigraphy may be used to screen for additional lesions, or to evaluate the extent of lesions diagnosed radiographically. MRI and CT are not used to obtain a diagnosis but rather to supply additional information about location and extent of the tumour. Definitive diagnosis is made based on biopsy and histopathology.

It is impossible to make a definitive diagnosis of neoplasia versus an infectious or benign bone lesion by radiographic means alone. By integrating signalment, history, physical and laboratory examination along with the radiographic changes a prioritised differential diagnosis can be made with a high degree of accuracy. Radiographic changes may include adjacent soft tissue swelling and aggressive radiographic changes (poorly demarcated long transition zone between normal and abnormal bone; cortical lysis; periosteal reaction (amorphous or sunburst); pathological fracture). Primary bone tumours usually begin in the metaphysis and extend into the epiphysis and diaphysis secondarily. Uncommonly they may cross joints or invade adjacent bones, but this usually occurs late in the disease. Multifocal areas of increased medullary opacity (bone infarction) within the medullary cavity may be seen.

MRI provides a highly sensitive means to document or exclude pathology. MRI is superior to all other imaging modalities for delineating the margins of a tumour with respect to adjacent normal tissues.

Central Nervous System

The brain and spinal cord is one of the few places where conventional radiography is of little value unless there is adjacent bone involvement. It provides little information other than location of lesion.

CT provides diagnostic information for many brain and spinal lesions. Two scan series are usually performed with CT, the first one being a survey and the second one after intravenous iodinated contrast injection. Intravascular contrast agents usually enhance masses, demonstrate vascular alterations and areas of disruption of the blood-brain barrier. Certain canine brain tumours may have distinguishing features on CT images based on location and pattern of contrast enhancement.

MRI is the technique of choice for imaging brain and spinal cord neoplasia. MRI is ideally suited for imaging of intracranial neoplasia because of its inherent property of producing excellent contrast between normal and abnormal tissue. Intravenous gadodiamide contrast may also be valuable to show blood-brain barrier disruption.

Abdominal

Conventional abdominal radiographs are still an excellent screening technique for suspected abdominal neoplasia. Radiographic differentiation of abdominal masses depends on a good working knowledge of normal radiographic anatomy. A palpable abdominal mass is not always visualised on survey radiographs directly, but indirectly may demonstrate displacement of bowel or other abdominal organs. Conventional radiographs help to guide the diagnostic workup as well as determine the use of other imaging modalities. Before resorting to contrast radiography to enhance visualisation of a poorly demarcated mass one should consider positional or compression radiography. Positional radiography may consist simply of obtaining the opposite lateral, a DV or an oblique. Positional radiography may also include horizontal beam projections with erect and decubitus patient positioning. Compression radiography creates anatomical separation of adjacent abdominal structures. This may help define the appearance of the mass as well as the organ of origin. Ultrasonography is an excellent follow-up as it can define the organ of origin and internal architecture, and evaluate the rest of the abdomen for metastasis. This includes other abdominal organs as well as lymph nodes. Abdominal effusion, which may be present with cancer (carcinomatosis), usually doesn't hamper and can even enhance the ability to diagnose neoplasia using ultrasonography. It is also very helpful in the guidance for FNA and biopsy. CT also provides excellent abdominal detail, but cost and the necessity of general anaesthesia limits its current use.

Biopsy Techniques

Biopsy techniques can be used with fluoroscopy (thorax), ultrasonography (anywhere ultrasound can provide information) or CT (anywhere). At present they are most often performed blindly or with ultrasound guidance. In many instances neoplasia is just part of the differential diagnosis and a definitive diagnosis is necessary for prognosis and accurate treatment. Percutaneous ultrasound-guided FNA and tissue core biopsy are minimally invasive and cost-effective techniques for obtaining diagnosis. Risks that should be considered include: neoplastic dissemination, infection and haemorrhage. Ultrasound-guided tissue sampling has yielded few risks and even fewer life-threatening risks. Benefit versus risk dictates that these high-yield low-risk procedures provide helpful information necessary for the diagnosis and treatment of cancer in veterinary patients.