Carpal Tunnel Ultrasound and Injection
What are the generally accepted indications for a carpal tunnel ultrasound and injection? There are two main indications for carpal…Read more
DMSA, or dimercaptosuccinic acid, is bound to technetium 99m to form the radiopharmaceutical, 99m-Tc DMSA. After intravenous injection, the isotope is allowed to circulate through the body anywhere between 90 minutes and 4 hours. During this time, 95% of the radiopharmaceutical is bound to the renal cortex and the remaining 5% is usually excreted into the urine. Modern gamma cameras, particularly those capable of performing single-photon emission tomography (SPECT), are able to reconstruct 3-dimensional images of the kidneys.
In children, common indications include indentifying renal scars as a result of an insult (usually recurrent urinary tract infections) or reflux-associated nephropathy.
In adults, common indications include determining the relative excretory function of each of the kidneys and indentifying renal scars. Segmental function of an individual kidney (i.e. upper, mid and lower pole) can also be estimated when considering partial nephrectomy or if part of the kidney lies within the radiotherapy treatment field.
No prerequisites required.
There are no absolute contraindications.
Please see relative contraindications below.
Pregnancy. A DMSA scan can still be carried out during pregnancy, but it is recommended that you discuss the indication, benefits and the timing of the study with the nuclear medicine physician. Although there is a small risk of radiation exposure to both the mother and foetus, this may be acceptable depending on the clinical indications for the study. In some patients, the scan can be delayed until the pregnancy is completed.
Breast-feeding. Some of the radiopharmaceutical is excreted into breast milk. A DMSA scan can still be carried out while breast-feeding. The patient can express and store breast milk, and may require another carer to help in looking after the child in order to reduce prolonged close contact with their child and minimise ionising radiation exposure. Your nuclear medicine department will provide you with a protocol for patients who are breast-feeding. If you require more information, then contact your nuclear medicine physician.
Recent or current pyelonephritis may demonstrate a defect in the renal cortex, which may be mistaken as a scar. It is generally advised to wait a minimum of 6–8 weeks after the infection before imaging. If there is a history of recent infection at the time of the scan, a repeat DMSA scan in 6–8 weeks is usually sufficient to confirm.
Weight limits apply to scanner equipment. Most scanning beds have weight limits ranging from 160 kg to 220 kg. Some imaging can be carried out with the patient sitting or standing, or on a hospital bed.
Other recent nuclear medicine scans (e.g. bone scan) or treatment may confound the DMSA scan images, as most radiopharmaceuticals are excreted through the kidneys. Please discuss the appropriate timing of the scan with the nuclear medicine physician, so as to allow complete excretion of the prior nuclear medicine radiopharmaceutical before commencing the DMSA scan.
All nuclear medicine tests involve ionising radiation. Clinically, the benefits of the test should outweigh the potential risks (see InsideRadiology: Radiation risk of medical imaging for adults and children). The radiation dose of a DMSA scan is less than that of an X-ray of the spine and is equivalent to approximately 4 months of natural background radiation.
The scan involves intravenous injection of the radiopharmaceutical, which may be complicated by extravasation. This is usually self-limiting. A cold compression bandage and elevation of the limb is usually all the treatment required. Intravenous injection in children may also be challenging and stressful both for the child and their carer (see InsideRadiology: Making my child’s test or procedure less stressful).
Before the procedure, patients including children can eat and drink normally. In young babies, the feed can be delayed so that the “feed and wrap” technique can be used to ensure that the baby remains still during the test.
There is generally no post-procedural care required. It is however, recommended that the patient drink plenty of fluid to increase urination, as this is the main way the radiopharmaceutical is excreted from the body. Young children who are unable to keep still for the scan may require sedation. In this case, standard post-sedation observations will be carried out before the child leaves the department.
Other modalities, such as ultrasound, CT and MRI, can also provide information regarding renal scars or renal differential function, but DMSA is considered the gold standard in both regards.
Ultrasound provides information on kidney size and the presence of renal tract dilatation, but is operator dependent and challenging, particularly in non-compliant patients or in those with large body habitus. Some scars or areas of acute pyelonephritis can look normal or be difficult to see on ultrasound imaging. However, ultrasound is often the first and most appropriate imaging modality in assessment of renal disease. CT involves higher ionising radiation and usually also requires intravenous contrast injection (except for investigation of radiopaque calculi). MRI is promising, but requires the patient to stay very still, requires the patient to be MRI safe and not claustrophobic.
Page last modified on 14/5/2018.
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