Many radiographic systems are available including conventional film-screen systems and digital radiography (CR - computed radiography, and DR - direct radiography). The main difference between these systems without delving deep into physics, is the image receptor. Regardless of image receptor, the image is produced by the passage of x-rays through the patient and is a 2D representation of the 3D volume included in the collimated area. Contrast can be used to enhance structures or to subjectively, or sometimes objectively, evaluate specific system function.

Computed Tomography or CT also uses x-rays to produce an image. In this case, each image is a thin slice of the patient, thus perceived spatial resolution is improved as superimposition is abolished. Contrast resolution is superior to radiography resulting in images that are much more pleasing to the eye. Benefits of CT include the ability to select different reconstruction algorithms to accentuate different structures eg. bone vs. soft tissue and to perform image reformatting in different imaging planes eg. sagittal vs. transverse vs. dorsal. Vascular studies and enhancement of abnormal structures are achieved by the administration of intravenous iodine contrast. Intrathecal administration of specific iodine contrast media permits a myelographic CT, which has recently been advocated as a superior study to conventional radiographic myelograms for identification of subtle extramedullary spinal lesions.

CT is not as widely available as radiography, and specialist help should be sought prior to undertaking CT studies to ensure that the study performed is optimized.

Magnetic Resonance Imaging or MRI is a complex imaging modality which uses a strong magnetic field to align protons in water contained within tissues, then the application of radio waves to disrupt the alignment. Recovery of the protons, each performing as a small magnet, from the disruption results in the production of a signal that is detected by a receiver coil. The signal is then manipulated mathematically resulting in an image. Different sequences are available and multiple sequences and orientations are usually needed to fully characterize a lesion or confirm normalcy. MRI has less spatial resolution than CT or radiography but superior contrast resolution, resulting in very pleasing images. MRI also permits subtle differences between, for example, gray and white matter of the brain to be seen, thus internal organ structure can be evaluated. Pre- and post contrast images can be used to further define abnormalities. Vascular studies can also be performed.

Access to MRI can be limited and the cost may be prohibitive in many circumstances. Specialist help should be sought to determine whether or not MRI is the best modality for a specific study, which sequences and orientations should be requested. Given the complexities, MRI studies should be interpreted by an experienced radiologist. While some lesions are very obvious, other more subtle but significant lesions can easily be missed. Additionally, MRI artifacts can be mistaken for lesions.

Ultrasound transducers produce sound waves that are projected into the patient. When sound waves reach different tissues, they may be reflected, refracted or pass through. The amount of each interaction varies depending on the tissues concerned. Different tissues have different patterns of scatter and reflection, and disruption of the normal pattern indicates abnormalities. Ultrasound has the benefit of permitting evaluation of organ internal structure, but function cannot usually be determined. Ultrasound contrast media exist and are currently being investigated to determine the full spectrum of application. Contrast media are currently used to investigate cardiac anomalies (patent foramen ovale, right-to-left shunting patent ductus arteriosus), metastatic disease and tissue perfusion.

Nuclear medicine involves the use of radiopharmaceuticals that are either injected or ingested by the patient. Gamma rays are emitted by the radiopharmaceutical from within the patient and detected by a gamma camera. Radiopharmaceuticals are designed to mimic physiological functions and as a result accumulate in specific anatomic locations, for example the thyroid gland. Increased or decreased uptake may be identified, or uptake may be seen in an unexpected location, indicating disease.

Nuclear medicine studies are limited in marine mammals to studies in turtles due to problems associated with restricting radioactive contamination and control of animal waste. Research utilizing PET (Positron Emission Tomography) has been performed in cetaceans.

Technical details regarding imaging hardware and software are beyond the scope of this site. However, an excellent site with this information can be reached at www.animalinsides.com. Animal Insides offers tutorials on different modalities as well as a consultancy service if you are planning to purchase equipment. The site also provides instruction for developing technique charts to optimize currently owned conventional equipment. I highly recommend taking a look.