Application of Multislice CT for the Diagnosis of Diseases in Small Animal Patients
John S. Mattoon, DVM, DACVR
Professor and Chief of Radiology, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Washington State University, Pullman, WA, USA
Multidetector CT has been extensively used in human medicine for a number of years, largely replacing single slice and single detector-row spiral CT scanners. We are now gradually seeing this trend in veterinary medicine. As human imaging departments upgrade to units with higher capabilities, many veterinary practices are now able to afford refurbished equipment that was state-of-the-art only several years ago!
Multidetector (multislice) CT has several main advantages over single slice axial and helical CT: Speed of acquisition and impressive reconstruction capabilities, including 3-D and virtual CT. Of course, multidetector CT scanners can do everything conventional CT can do, but it does it better and faster.1
Multidetector-row spiral CT scanners make use of multiple adjacent detector rows that acquire scan data simultaneously. The detector rows are available in various thicknesses, depending on the manufacturer and model, as thin as 0.5 mm or less. Available are 4, 8, 16, 32, 64 and even 312 multidetector-row units! The more rows of detectors, the more area that can be scanned per revolution.
How Multidetector CT Changed Our CT Caseload
While single slice axial CT has been available at Washington State University for over 15 years, MRI has been the favored cross-sectional imaging modality, outpacing CT by nearly 5:1. Our 2009 CT total was approximately 100 small animal cases, averaging 8 per month. In May of 2010, our 16-slice CT unit was installed. From May 2010 through April 2011 our CT caseload has increased to an average of 38 per month, with over 450 small animal CT scans over the 12 month period. Why did this happen?
Speed of Acquisition
Multidetector CT allows phenomenally fast data collection. For example, a cat thorax can be imaged in only a few seconds. With sixteen 0.5 mm isotropic slices with each ½ second revolution, it is easy to appreciate the speed of data acquisition. We have actually slowed down our scan rotation just a bit, and routinely use 0.75 second per scan revolution for better contrast resolution. Typical cases, including IV contrast injections and post-contrast scans take 10–15 minutes total! It generally takes longer to situate the patient in CT than to scan it.
FAST acquisition times have allowed the use of sedation or gentle manual restraint for CT imaging of many, if not the majority of our small animal patients. General anesthesia is not always needed, nor desired. This is one of the primary reasons for an increase in CT caseload. Less risk to the patient, less cost to the owner, faster throughput for all.
The majority of non-anesthetized small animal patients are restrained with tape, Velcro, and foam wedges. There is a new positioning device available that places a cat or small dog into a Plexiglas "tube", the VetMouseTrapTM.2
Rapid acquisition speed also allows a more efficient use of contrast agents with CT studies. We are now able to obtain arterial phase scans in addition to venous exams and the quality of our contrast studies, angiography in particular, have become state-of-the-art.
Reconstruction of Isotropic Data
Sagittal and dorsal plane reconstructions from axial (transverse) image acquisition data is of course available for all CT scanners. One of the extremely beneficial advantages of multidetector CT is the acquisition of isotropic data, i.e., a voxel of equal dimensions, a cube. This allows reconstructions without loss of information. Reconstructed images are no longer "notchy". Further, reconstruction can be made in any plane desired, beyond conventional sagittal or dorsal images.
3-D presentation of data becomes a reality. Including virtual imaging.3 All of this adds up to more complete and accurate depiction of anatomy and pathology.
Types of CT Cases Now Requested and Performed
Oncology orders the majority or our CT cases, perhaps 75%. They routinely scan tumor sites to evaluate extent and margins of tumors for radiation therapy planning. Any tumor of the head is placed in a bite block, or fiducial markers will be placed. CT planning is directly off the CT images.
Pulmonary metastasis checks are routinely made with the patient awake and constitute a large number of CT cases.
The two most commonly scanned areas are dog elbows and tarsi. These studies are generally made with the patient under general anesthesia, as they proceed directly to surgery after CT. Stifle and carpal examinations are sometimes performed4-6, but MRI is preferred for the stifle at our institution.
As mentioned, pulmonary metastasis checks are done nearly daily. These may be made without or without prior thoracic radiographs. Many are performed without general anesthesia.
Because of the speed of multidetector CT, its use for thoracic trauma has grown tremendously. This was rarely done with our old scanner. Pulmonary vascular studies are now routinely performed.7
Multidetector CT has raised the bar for imaging of suspected portosystemic shunts, and it has become our standard imaging procedure.8,9 The amount of vascular detail that can be achieved, in addition to reliable arterial and portal phases has truly changed our ability to accurately diagnose shunts but also provide our surgeons with detailed vascular maps.
We commonly use our CT for suspected ectopic ureters, where again the detail provided is spectacular.
Any Downside to Multidetector CT?
Yes, there are a few downsides to the use of multidetector CT.
One is the use of sedation or manual restraint. In some cases, there is deleterious patient motion that compromises the exam. In these cases, anesthesia may become necessary. Initially we feared that maintaining lung volume would be a challenge with sedated patients, but this is really not a major issue. In fact, we have a bigger problem with anesthetized patients brought to the CT suite in lateral recumbency, with resultant atelectasis. Sometimes our sedated pulmonary exams exceed the quality of anesthetized exams!
Our biggest frustration is the expectation of our clinicians for an immediate diagnosis. While data acquisition is unbelievably fast, it takes time to reconstruct the data, and to reconstruct the amount and type of data that our colleagues have become accustom to. Equally important is that with so much data to review, and such highly detailed images (especially vascular studies such as shunts), it takes a considerable amount of time and concentrated effort to properly review the images.
These minor quibbles are tiny compared to the benefits afforded by rapid acquisitions and phenomenal detail afforded by multidetector CT. Highly recommended!
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8. Echandi RL, Morandi F, Daniel WT, et al. Comparison of transplenic multidetector CT portography to multidetector CT-angiography in normal dogs. Vet Radiol Ultrasound 2007;48(1):38–44.
9. Schwarz T, Rossi F, Wray JD, et al. Computed tomographic and magnetic resonance imaging features of canine segmental caudal vena aplasia. J Small Anim Pract 2009;50(7):341–349.