It would be enormously difficult to think about the diagnosis or treatment of cancer without the use of diagnostic imaging.
It would be enormously difficult to think about the diagnosis or treatment of cancer without the use of diagnostic imaging. From diagnosis, through staging, on to surgery and/or radiation therapy treatment planning, diagnostic imagery plays a role to one extent or another. Whether we choose routine radiographs, ultrasound, CT or MRI depends on many factors including area of interest studied to desired outcome of the test performed. As the availability of more complex imaging techniques becomes greater and as owners demand a more advanced level of care for their pets, diagnostic imagery's role will continue to grow within the care and diagnosis of the veterinary oncology patient.
Standard radiographs have been the baseline test in imaging for many years as they were generally accessible to all practitioners and cost-effective for most owners. We still use it today, however, more often than not, it has become a screening test; a "quick-ee". Radiographic images are made by differences in the absorption of x-rays as the primary x-ray beam passes through the patient. Some x-rays pass through and some are absorbed. In general, absorption is affected by density, thickness of the patient at that point of the body and the atomic number of the tissue the x-ray beam is passing through. Those x-rays that are not absorbed travel through and strike the x-ray the film, turning it black.
Radiography is most helpful for evaluating bones (e.g. appendicular skeleton). It is also a very good screening tool to evaluate abdominal and thoracic lymph nodes as in the case of lymphoma and mast cell tumor patients. Its' greatest weakness is that it allows for the superimposition of structures, one above or below another.
As CT is incorporates the use of x-ray, it relies on physical density differences as well, to create images. However, as these images are then manipulated by a computer and presented in slices, the issues of structural superimposition is no longer existent. It further noted for the same reason, that the grey scale (or difference in contrast) is superior with CT. Therefore, CT is better able to identify pulmonary nodules, mediastinal lymphadenopathy, as well as other mediastinal and pleural masses.
CT is very helpful for surgical planning and absolutely invaluable for radiotherapy planning. There is no image distortion with CT and this is why it, rather than MRI, is used for determining patient plans. However, patients may have CT/MRI fusion studies performed where both scans are overlaid by computer. Typically, a contrast agent is given to assist in demarcating abnormal or tumorous structures and their infiltrates. CT can also assist with guided biopsy techniques in some cases where ultrasound is unable to visualize masses. There are also guides in production for the potential to biopsy brain masses under CT guidance.
Ultrasound's great benefit is its ability to evaluate the internal structure of organs. Images are again produced relative to a material's density. However, in the case of ultrasound, it is the different absorption of sound (acoustic impedance) rather than x-rays that produces the images. Sound is sent from and reflected back to a transducer. It is this "echo" that becomes what is read as the diagnostic image.
Ultrasound images are of better diagnostic quality than radiographs when evaluating cases with pleural and/or peritoneal effusions due to the lack of detail the fluid will cause on radiographs. Biopsy samples can also be obtained with ultrasound guidance with minimal invasiveness. It is important to note that while ultrasound is a sensitive tool for lesion detection, it is unable to determine etiology of disease. Biopsy (or fine needle aspiration) tissue samples remain necessary for definitive diagnosis. A potential complication that should be considered, however, is seeding along the track when sampling transitional cell carcinoma of the prostate or bladder. As always, each individual case should be considered uniquely for the best possible outcome on a "per patient" basis.
MRI (Magnetic Resonance Imaging) is a three-dimensional imaging modality. Images are produced by way of interaction of hydrogen atoms placed in contact with a magnetic field. The primary use of MRI is to evaluate the central nervous system. As noted earlier, due to image distortion all radiation therapy treatment plans are calculated based on CT rather than MRI due to image distortion. Contrast enhancement is used and is helpful to evaluate perfusion and blood flow. It is information like this that assists surgeons in evaluating tumors for soft tissue invasion. This allows for discussion of limb salvage (sparing) versus limb amputation.
Scintigraphy uses the administration of radioactive isotopes that will localize to an area of the body. There is very little anatomic detail to be found in these studies. In general they are quite sensitive for lesion detection. In cases of bone metastases, abnormal sites can be found before symptoms manifest or lesions are detected on standard radiographs. As in the case of ultrasound, however, etiology remains nonspecific. Nuclear medicine tests commonly performed involve imaging of bone, thyroid, kidney, and liver.
The largest drawback to scintigraphy is the need for a department trained to dispense and deliver the isotopes to the patient. Even in the case of radiopharmaceuticals with relatively short "half-lives" (some have as little as 6 hours) strict care must taken to observe safe handling guidelines, for all involved.
As advances in imaging modalities in human medicine continue to be made, veterinary medicine will see those same advances. The more information we have to embark on a treatment plan, the better prepared we can be to assist our patients to a positive outcome.