Introduction

Imaging is a useful diagnostic tool in the medical field, both human and veterinary. Imaging modalities used in human medicine are not always feasible options in veterinary medicine due to cost, but radiography, ultrasound, computed tomography and magnetic resonance imaging are routinely used in many specialty practices. Diagnostic nuclear medicine is used less commonly due to cost and availability of machinery and facilities but still proves to be a useful tool when feasible. Consideration must be given when determining which imaging modality will be best for various scenarios dependent on their individual strengths and weaknesses. These modalities are significant in oncology to assist with diagnosing, staging and monitoring cancerous processes.

Radiography

Radiography is the most common imaging technique used in veterinary medicine and is the oldest of all imaging techniques. A radiograph, commonly called an x-ray, is made when x-rays are produced by a generator and directed through an object and onto film or a digital detector. The object in question will absorb the x-rays in differing fashion depending on the density of the constituents of the object. This absorption depends on the thickness, physical density, and effective atomic number of the tissue in question.¹ Metal will absorb differently than plastic, bone differently than soft tissue. The x-rays produced pass through the tissues in question, some of which are absorbed by the tissue, and those that reach the x-ray film or digital processor determine the blackness and gray scale of the image.¹ This process forms a 2-dimensional image and is called projectional radiography. These standard projectional radiographs have been used to evaluate bone fractures, lung/thorax pathology, intra-abdominal pathology and more. Bones and lungs image very well with projectional radiography.

In veterinary oncology, radiography is used mostly commonly to evaluate the thorax for evidence of any neoplasia or pulmonary metastasis from distant cancers. Radiography is also commonly used to evaluate probable boney lesions suspected to be cancerous. Abdominal radiographs are typically superseded by abdominal ultrasound which is much more sensitive for evaluating internal structure of the various organs in the abdomen.

Standard radiographs do have their limits, however. They are not sensitive enough to distinguish differences in soft tissue in a detailed manner that allows the images to be diagnostic in some cases. For example, to detect pulmonary metastasis with projectional radiography the nodules themselves must be bigger than 7–9 mm in diameter. In contrast computed tomography can detect pulmonary metastasis as small as 1 mm in diameter.

Radiographs are a useful diagnostic tool for staging of pulmonary metastasis or neoplasia and evaluation of boney changes as related to cancerous processes.

Ultrasonography

Ultrasonography utilizes ultrasonic waves that are directed via probe into the area of interest. These waves are absorbed or reflected by different tissues based on the tissue’s acoustic impedance. Acoustic impedance describes the density of a tissue and the velocity of sound in that tissue. As the ultrasonic waves pass through tissue, they inherently reflect the opposite direction based on the acoustic impedance differences throughout the tissue. These reflections are detected by the transducer in the probe and form an image after being processed. This process of image formation is best utilized in the thoracic and abdominal cavities. Evaluation of organ, lymph node and mass/lesion size, shape, margins, heterogeneity and vascularity are possible with ultrasound with a high level of detail and comparatively low cost to other modalities.

Thoracic Ultrasonography

The use of thoracic ultrasound in oncology typically applies to the evaluation of mediastinal, cardiac, or pulmonary neoplasia and lymph nodes. Imaging of mediastinal and pulmonary masses can assist in obtaining ultrasound guided fine needle aspirates or biopsy samples for diagnosing a neoplastic process. Echocardiography, ultrasound of the heart, can detect macroscopic evidence of cardiac neoplasia and often determine the location of the mass on the heart assisting with diagnosis.

Abdominal Ultrasonography

Abdominal ultrasound is primarily used for evaluation of liver, kidney, spleen, intestine, bladder and lymph node abnormalities. Many cancers and their metastatic processes affect the abdominal organs or drain to regional lymph nodes within the cavity. Ultrasonography excels at evaluating the presence of lymphadenopathy within the abdominal cavity, although radiography can detect it with less sensitivity. Visual evaluation via ultrasound of enlarged lymph nodes can often allow the clinician to extrapolate if a malignancy is the cause of the enlargement. The same applies for ultrasonographic evaluation of the major organs and/or known mass/lesions. Measurements of intestinal or gastric wall thickening can be a strong indicator of GI lymphoma. Monitoring of splenic nodules can be obtained. Progression of a prostatic carcinoma can be evaluated. Abdominal ultrasound also allows for the precise sampling of areas of interest via guided fine needle aspirate or biopsy. Sampling within the abdomen would be unattainable without appropriate visual guidance provided by ultrasound.

Effusion

Effusions of both the thoracic and abdominal cavities can be well visualized with ultrasonography. Ultrasound guidance can allow both diagnostic and therapeutic centesis while minimizing risk to organs or vasculature in the immediate area.

Computed Tomography

Computed tomography (CT) is a more detailed diagnostic tool when evaluating soft tissue areas of the body. CT utilizes x-rays like radiography but provides a 2-dimensional “slice” of the patient that can be reconstructed into a 3-dimensional image. Radiography utilizes a fixed x-ray tube while CT uses a motorized, rotating x-ray tube that shoots out narrow beams of x-rays to obtain each slice. Each rotation produces a single slice image, with the thickness of the slice varying dependent on the situation and desired results. Image slices are taken of the area of interest for the patient and those images can be combined to form a 3-dimensional image of the entire area.

CT can provide more detailed information than radiography and ultrasound in many regards. CT is excellent for imaging the sinonasal region, orbits, and ear canals as it can assess both the soft tissue and bone for neoplastic changes. As noted above, CT is superior to radiography regarding pulmonary metastasis detection as it can detect significantly smaller nodules than radiographs. CT can help narrow down the tissue of origin for an abdominal mass or the lung lobe that a pulmonary tumor stems from. CT can also evaluate the relationship of a mass to the surrounding structures, such as with infiltrative lipomas or sarcomas. Contrast-enhanced CTs can assist with surgical and radiation planning while evaluating margins and presence of infiltrative disease. CT-guided biopsies can be used to obtain samples from thoracic, brain, spinal and skeletal lesions that ultrasound would be less suited for.

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI), like CT, produces images forming a 3-dimensional picture of the area of interest. Living tissues contain abundant amounts of protons. Protons inherently spin, creating a magnetic charge. MRI uses powerful magnets that create a strong magnetic field causing the protons within the body to align with that field. A specific radiofrequency pulse is passed through the body, changing the alignment of the protons to a 90 or 180-degree angle from the magnetic alignment. Once the radiofrequency pulse is turned off, the protons realign with the original magnetic field produced by the magnets. The protons release electromagnetic energy during this realignment which is detected and processed to form an image. The MRI machine can differentiate between tissues by how quickly they release the electromagnetic energy allowing for superior soft tissue imaging. CT is often used as it does give detailed soft tissue imaging suitable for most of our needs and is less costly to the client and the facility.

MRI is heavily used for neuroimaging in veterinary medicine. Evaluation of brain and spinal neoplasia is the most common use for oncology. MRI can assist with lesion localization and assessment of any bone infiltration which can help facilitate surgery or radiation. MRI use in humans spans a wider base regarding its oncologic uses and will likely become more broadly used in veterinary medicine in the future.

Nuclear Medicine

Scintigraphy

Scintigraphy involves the administration of radiopharmaceuticals that are drawn to an area of interest by physiologic processes. Scintigraphy imaging studies do not provide anatomic detail like other modalities but provide functional information. The radiopharmaceuticals administered are processed by the body and will collect in various areas of the body dependent on many factors. How the specific tissues process the drug determines how much of the radioactive substance is held in that tissue. Many of the radiopharmaceuticals used for cancer imaging or detection are known to collect in malignant cells offering a view of the locations of these cells. This can allow for early detection of metastasis, to more thoroughly evaluate the margins of a boney lesion or to aid in selection of a biopsy site.

SPECT and PET

Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) are more advanced nuclear imaging modalities. They are used extensively in human medicine but are still used rarely in veterinary clinical practice compared to other imaging modalities. SPECT and PET are more advanced than standard scintigraphy and use different sourced gamma ray-emitting radionuclides to obtain the images.

PET/CT

PET/CT is a combination of PET and CT providing the anatomic imaging to be combined with the physiologic imaging. This provides a very specific look at the location of malignant cells as identified by PET to better assist with diagnosis, staging, and treatment.

Imaging in Oncology

Imaging plays a large role in oncology. Without it we would be unable to concretely diagnose, stage, treat and monitor many neoplastic diseases. Consideration must be taken when selecting an imaging modality for not only its use, but for its cost and convenience (Table 1).

Table 1. Comparison of Imaging Modalities Used in Veterinary Medicine

*Adapted from Imaging in Oncology (Forrest, 2013, p. 99)

In daily practice radiography and ultrasound are most used, followed by CT and MRI, and lastly nuclear medicine. The integration of the imaging modalities is key to our ability to diagnose and treat various neoplastic processes.

References

1.  Forrest L, Kraft S. Imaging in oncology. In: Withrow S, Vail D, Page R, eds. Small Animal Clinical Oncology, 5th ed. St. Louis, MO: Elsevier; 2013:98–110.

2.  LeBlanc AK, Daniel GB. Advanced imaging for veterinary cancer patients. Vet Clin North Am Small Anim Pract. 2007;37(6):1059–1077.

3.  Mattoon JS, Bryan JN. The future of imaging in veterinary oncology: learning from human medicine. Vet J. 2013;197(3):541–552.