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Portosystemic shunt: scintigraphy

ISSN 2398-2977

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Synonym(s): Portosystemic shunt imaging

Introduction

  • When administered in high concentration per rectum, a proportion of  99mTc-pertechnetate is rapidly absorbed across the colonic mucosa.
  • Its normal transit through the portal vessels to the liver and shortly thereafter the heart can be imaged by dynamic scintigraphic studies.
  • 99mTc-pertechnetate is not actively taken up by the liver or heart and the study is therefore essentially a nuclear angiogram of the portal system.
  • The normal time delay between detection of the radiopharmaceutical in the liver and heart is related to patient size.  When portosystemic shunting is present, some of the administered radiopharmaceutical reaches the heart at the same time as, or before, it is detected in the liver.
  • During the first pass of 99mTc-pertechnetate, the ratio of integrated heart to liver counts can be used to estimate the percentage of portal blood flow which has bypassed the liver through the shunt.

Radiopharmaceutical

  • 99mTc-pertechnetate  (99mTcO4-).

Dosage

  • 37-74 MBq/kg has been used in small animals and is suggested as a guideline for dosage in foals. As less than 15% is initially absorbed through the colonic mucosa, the dose should be concentrated in 1-2 ml saline.

Uses

  • This is a simple and inexpensive screening test for the presence of  a portosystemic shunt and may be indicated in foals where more common causes of neurological abnormalities such as trauma, vertebral body abscesses, brain abscesses and meningitis, have been ruled out.
  • Estimation of the degree of shunting by calculating the shunt fraction.
  • Follow up evaluation of  surgical  intervention by comparison of pre and post-operative shunt fraction calculations. The comparison may not be reliable because of interstudy variability.

Alternative techniques

  • Other aids in the diagnosis of portosystemic shunts include:

Time required

Preparation

  • The camera should undergo quality control checks daily, this will take 20-30 min.

Procedure

  • Acquisition is usually completed 180 sec following bolus injection of radiopharmaceutical per-rectum.

Decision taking

Criteria for choosing test

  • A provisional or differential diagnosis of a portosystemic shunt in a foal will have been made based on clinical signs and serum biochemistry.

Interpretation

  • A normal portoangiogram reveals uptake of radiopharmaceutical in the portal vessels within 20 sec following administration.  Liver uptake is subsequently seen and after a further delay (portal hepatic transit time), uptake is recorded in the heart.
  • When portosystemic shunts exist, radioactivity appears in the heart at the same time or prior to its appearance in the liver.
  • Anatomic localization of the heart and liver can be made prior to the study using ultrasonography. Lead markers can be placed for subsequent identification following the scan. Alternatively the structures can be identified on composite images of the entire study.
  • Time-activity curves for the liver and heart are created from the data after the first appearance of the radiopharmaceutical in either organ.
  • Integration of the curves over a time interval representing the normal liver-to-heart delay allows calculation of the shunt fraction.

Requirements

Personnel

Veterinarian expertise

  • High experience needed.

Nursing expertise

  • Medium nursing experience needed.

Materials required

Minimum equipment

  • Gamma camera
  • Dedicated room with heating/air conditioning to avoid temperature fluctuations that may damage the crystal in the camera.
  • Gantry/mounting system for flexibility in obtaining views.
  • Dedicated computer and associated nuclear medicine software.
  • Low energy, parallel hole, general purpose or high resolution collimator. 
  • Image recording device.
  • Lead lined container for storing waste.
  • Syringe shields.
  • Handling forceps.

Ideal equipment

  • Large field of view camera, ie 50 cm.
  • Dose calibrator.

Minimum consumables

  • Radiopharmaceutical.
  • 12 French soft rubber catheter.
  • 3 way stopcock.
  • Syringes/needles.
  • Sedation.
  • Disposable gloves.
  • Radioactive decontamination kit.
  • Cleaning materials.

Other requirements

  • Prior to acquisition of images the field uniformity of the gamma camera should be checked.
  • The camera needs to be tuned to the photopeak of the radionuclide being used, eg 140 KeV for technetium 99m.

Preparation

Pre-medication

  • Jugular catheter inserted   Blood: collection  .
  • An experienced horse handler.
  • Foal is sedated and placed in left lateral recumbency on a padded surface.
  • A 12 French soft rubber catheter is inserted into the rectum and cautiously advanced a short distance into the caudal colon.

Technique

Approach

Step 1 - Gamma camera preparation

  • A large field of view gamma camera fitted with a general purpose, low energy, parallel hole collimator is used.
  • The gamma camera is lowered to center over the cranial abdomen on the right side.

Step 2 - Radiopharmaceutical administration

  • The radiopharmaceutical is administered through the catheter via one limb of an attached 3-way stop-cock.
  • The catheter is immediately flushed with 10-15 ml of room air via the other limb of the stop-cock. 
  • Room air is used to avoid dilution of the radiopharmaceutical.
  • A dynamic acquisition is started immediately prior to administration of the radiopharmaceutical: 180 x 1 sec frames or 90 x 2 sec frames are acquired using a 64 x 64 or a 128 x 128 matrix size.

Aftercare

Immediate Aftercare

Special precautions

  • Horses that have been administered a radiopharmaceutical constitute a radiation hazard.
  • The foal is normally stabled for the following 24 h and a temporary controlled area must be demarcated. The necessary radiation protection legislation must be complied with.
  • Current radiation protection guidelines for release to public transport are based on risk assessment and dose constraints (0.3 mSv/year to a member of the public making some realistic assumptions).
  • Half-life for 99mTechnetium is 6 h.

Long term Aftercare

Follow up

  • Inter-study variability of Shunt Fraction calculation poses question over its validity for follow-up studies.

Outcomes

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Samii V F, Kyles A E, Long C D et al (2001) Evaluation of interoperator variance in shunt fraction calculation after transcolonic scintigraphy for diagnosis of portosystemic shunts in dogs and cats. JAVMA 218 (7), 1116-1119 PubMed.
  • Daniel G B, Bright R, Ollis P et al (1991) Per-rectal portal scintigraphy using 99mTechnetium pertechnetate to diagnose portosystemic shunts in dogs and cats. J Vet Intern Med 5 (1), 23-27 PubMed.
  • Buonanno A M, Carlson G P, Kantrowitz B (1988) Clinical and diagnostic features of portosystemic shunt in a foal. JAVMA 192 (3), 387-389 PubMed.
  • Lindsay W A, Ryder J K, Beck K A et al (1988) Hepatic encephalopathy caused by a portocaval shunt in a foal. Vet Med 83 (8), 798 VetMedResource.

Other sources of information

  • Koblik P D, Hornof W J (1996) Portosystemic shunt imaging. In: Handbook of Veterinary Nuclear Medicine. Eds: Berry C R & Daniel G B. North Carolina State University, Raleigh, NC, USA. pp 97-105.