Page 50 - Feline diagnostic imaging

44 4 Nuclear Imaging
4.4 Portosystemic Shunt Detection is available and especially when the patient’s owners would
Scintigraphy not opt for surgery if PSS was present.

Portosystemic shunts are not common in cats but are poten-
tially life‐threatening when they occur. Effective treatment 4.5 Other Feline Scintigraphic
usually involves identification and surgical correction of Procedures
the vascular abnormalities. Clinical signs and biochemical
evaluation can lead to strong suspicion of PSS but imaging Sentinel node lymphoscintigraphy allows mapping of
is necessary for definitive diagnosis and identification of the lymph drainage from a tumor and identification of sentinel
location and nature of the shunting vessels. lymph nodes – the first lymph node or group of nodes
Diagnostic imaging can be accomplished by surgical receiving drainage from a tumor. The location of the senti-
portography with laparotomy, injection of radiographic nel node(s) can be identified for biopsy in order to deter-
contrast medium into mesenteric vessels and immediate mine the presence or absence of early metastasis. The
radiographs. Noninvasive diagnosis was originally done by procedure involves injecting a very small amount of 99m Tc
per rectal portal scintigraphy with 99m Tc pertechnetate sulfur colloid ( 99m Tc‐Sc) in and/or around the margins of a
introduced into the descending colon with the patient posi- tumor and making images of the body region to follow
tioned on the gamma camera. Pertechnetate is very rapidly lymph flow until the first node is identified. Subsequent
absorbed through the colon mucosa and into the venous biopsy of sentinel nodes can be used to determine the need
system. Serial 1‐second images are begun just before intro- for adjunctive therapy after tumor excision [26, 27].
ducing the radiopharmaceutical and continued for 2–3 Skeletal scintigraphy (bone scan) has been used for many
minutes to observe portal flow patterns and identify porto- years to identify sites of increased bone activity in order to
systemic shunting [22]. localize the site of lameness in animals. Bone scans are per-
Per rectal portal scintigraphy has subsequently been formed frequently in horses and less often in dogs and cats.
replaced by splenoportography in which 99m Tc pertech- 99m Tc‐labeled methylene diphosphonate ( 99m Tc MDP) or
netate or 99m Tc mebrofenin is percutaneously injected hydroxymethylene diphosphonate (Tc‐99 m HDP) is
into the spleen by ultrasound guidance with the patient injected intravenously. Subsequent soft tissue phase images
positioned for imaging and dynamic image acquisition of can be made 5–15 minutes after injection. Bone phase
portal blood flow allowing detailed assessment of portal images are made three hours after injection, allowing time
vascular patterns and classification of portosystemic for localization of the radiopharmaceutical in bone and
shunts [23, 24]. In normal cats, the liver is clearly per- clearance of soft tissue activity [28]. Typically, as many
fused before blood reaches the heart. In cats with PSS, images are made as necessary to include the entire skeleton.
portal blood flow can be seen bypassing the liver. 99m Tc Skeletal scintigraphy remains useful to identify obscure or
mebrofenin is normally extracted by the liver and can multiple sites of osseous lesions resulting in lameness in
also be used to assess hepatobiliary function and identify dogs and cats. It is also an ideal method to survey for skele-
biliary obstruction [24]. tal metastasis in patients with malignant neoplasia.
With advances in CT technology, diagnosis of portosys- Other scintigraphic procedures previously reported in
temic shunts at referral hospitals is most often done with feline patients include radiolabeled lymphocytes for evalu-
single‐phase or multiphase CT contrast portography which ation of pancreatitis [29, 30] and assessment of the rate of
is noninvasive and allows detailed diagnosis of portosys- gastric emptying by mixing 99m Tc sulfur colloid with food
temic abnormalities. The remarkable 3D reconstructed [31, 32].
images of the vessels afford surgeons excellent information While newer and more advanced systems for diagnostic
for presurgical planning [25]. Portal scintigraphy remains imaging continue to be developed and now employed in
faster and less expensive than CT portography and may be veterinary medicine, scintigraphy with the gamma camera
an option to confidently determine the presence or absence remains a useful and relatively inexpensive option for
of PSS in circumstances where gamma camera scintigraphy selected applications in feline patients

References

1 LeBlanc, A. and Peremans, K. (2014). PET and SPECT 2 Randall, E. (2016). PET‐computed tomography in
imaging in veterinary medicine. Semin. Nucl. Med. 44: veterinary medicine. Vet. Clin. North Am. Small Anim.
47–56. Pract. 46: 515–532.