Abstract

As advanced diagnostics in aquatic animal medicine continue to parallel those in domestic animal medicine, and with a temporal delay--human medicine, the increasing use of cross-sectional imaging is apparent. Within the IAAAM, there has been a steady rise in presentations referencing computed tomography and magnetic resonance imaging since these modalities were first introduced in the early 1970s and 1980s (Figure 1).⁶

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Figure 1 Usage of CT and MRI in IAAAM presentations over the last three decades.

The unique ability of CT and MRI studies to circumvent the limitations of tissue superimposition is invaluable¹²; however, such detailed information comes at a price. The burden of knowledge associated with both performing and interpreting these studies is substantial. The need for familiarity with species-specific anatomy is well understood, but without fundamental understanding of appropriate CT and MR image acquisition, image display parameters, and modality indications, a study can be incomplete at best or non-diagnostic at worst. Lesions can be entirely missed or erroneously "created" where none exist. The degree of logistical planning and manpower needed to facilitate such an out-of-water study for an aquatic animal only adds to the importance of getting it right the first time. The value of consultation with either a specialist in human radiology or preferably in veterinary radiology is real, and meaningful consideration should be given to this approach.

Prospective CT and MR studies of a live juvenile harbor seal (Phoca vitulina) will be conducted and corresponding images utilized to demonstrate the principles outlined.

Computed Tomography

A number of recent publications have detailed the clinical use of CT in aquatic animal species.^{1,3,5,7,9,11,14} Generally CT is the cross-sectional modality of choice in evaluating all osseous structures, the nasal cavity, the pharyngeal/laryngeal/cervical region, the thorax, and the abdomen. With computed tomographic imaging, vital parameters pertaining to image acquisition include setting the algorithm appropriate for the tissues being evaluated (i.e., a high-frequency algorithm for bone), choosing axial vs. helical scans as necessary to balance the need for detail with speed and coverage, determining the appropriate slice thickness and pitch (to maximize signal-to-noise ratio), the exposure parameters, the region of interest to include as relates to the pathophysiology of the disease process, and the exact dimensions of both the display field of view and the scan field of view prior to the start of the study. Important artifacts such as beam hardening and partial volume and their cause should be well-understood, easily recognized, and either avoided or corrected. Careful positioning of the patient and monitoring equipment must be executed to minimize these problems. Determining necessity of contrast administration is time-critical, typically occurring immediately after initial image acquisition, and is crucial to the diagnostic nature of the study. Post-processing techniques (such as 3D reconstructions and retro-recon processes) should be understood and employed where relevant. The evaluation of images both at the time of acquisition (to approve the end of each series and the entire study) and after-the-fact (to meticulously interpret the data) must be performed with usage of appropriate window-width and window-level settings. The clinician interpreting the study should be well-versed in the specific Hounsfield units (HU) that quantitatively apply to various tissue types and various stages of hemorrhage, as well as in methods used to obtain these numerical data.

Magnetic Resonance Imaging

MRI use in aquatic animals was initially limited to anatomical post-mortem studies¹³ but has been documented recently in live patients with increasing frequency^2,4-5,8 in both structural and functional applications. By far the most common use of veterinary MR is in neuroimaging; however, musculoskeletal and nasal MR are more commonly being performed in addition to more specialized techniques such as MR angiography (time-of-flight, phase contrast, and contrast-enhanced sequences) and cardiac gated MRI. Knowledge of how various pulse sequences are generated and how they relate to the evaluation of specific tissues (such as the use of FLAIR sequences to detect periventricular edema) is crucial to obtaining a diagnostic study. The physics that dictate the ability of these varied pulse sequences to alter tissue contrast is complicated and beyond the scope of this discussion; however, an MR study cannot be undertaken or interpreted without 1) knowledge of this information, or 2) consultation with an individual that specializes in this very knowledge.

References

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2.  Dennison SE, Forrest LJ, Fleetwood ML, et al. 2009. Concurrent occipital bone malformation and atlantoaxial subluxation in a neonatal harbor seal (Phoca vitulina). J Zoo Wildl Med 40:385-388.

3.  Dennison SE, Schwarz T 2008. Computed tomographic imaging of the normal immature California sea lion head (Zalophus californianus). Vet Radiol Ultrasound 49:557-563.

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6.  IAAAM Conference Proceedings Archive provided by the Veterinary Information Network (VIN)--10 Feb 2010 https://www.vin.com/proceedings/Proceedings.plx?CID=IAAAM2009&O=Generic

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14. Zucca P, Di Guardo G, Pozzi-Mucelli R, et al. 2004. Use of computed tomography for imaging of Crassicauda grampicola in a Risso's dolphin (Grampus griseus). J Zoo Wildl Med 35:391-394.