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Department of Medical Imaging, Lapeer Regional Hospital, Lapeer, Michigan
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Introduction |
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CT has a low sensitivity and specificity for detection of parathyroid lesions and is not routinely used for this purpose. High-resolution US combined with Doppler studies is effective in experienced hands and for parathyroid adenomas with a cervical location; its use is limited in cases of intrathoracic lesions.
In the early 1980s, Ferlin et al (1) introduced a new method of radionuclide localization based on computer subtraction of thallium-201 chloride from Tc-99m pertechnetate images. In the late 1980s, Coakley et al (2) introduced Tc-99m sestamibi as an agent for parathyroid imaging. Subtraction techniques that made use of Tc-99m sestamibi and Tc-99m pertechnetate or I-123 increased the sensitivity of localization from 68% to over 90% (3,4).
In 1992, Taillefer et al (5) introduced double-phase scintigraphy with Tc-99 sestamibi, which proved to be reliably accurate and easy to perform. Its reported sensitivity in the preoperative localization of parathyroid lesions is 89%–95% (6–9) in patients undergoing initial surgery for primary hyperparathyroidism but only 59% in those undergoing repeat surgery (10). The simplicity of performance and high accuracy of the double-phase technique have made it the predominant technique used in clinical nuclear medicine for this condition. Although preoperative nuclear medicine imaging of parathyroid lesions remains controversial for initial exploration, the surgery literature suggests that nuclear medicine localization has become routine, even in institutions where hyperparathyroidism was traditionally treated without such localization (8,9,11–13). Preoperative nuclear medicine localization enables a surgical success rate of more than 95%, unilateral exploration with low morbidity as regards recurrent laryngeal nerve damage and hypothyroidism, and a shorter duration of surgery. High accuracies for preoperative localization with radionuclide studies and savings of approximately $1,000 per patient have been reported (9–11).
In the preceding article , Nguyen presents a comprehensive pictorial essay on double-phase Tc-99m sestamibi scintigraphy of parathyroid disease. The posterior location of eutopic and ectopic parathyroid disease and the importance of oblique and lateral projection images as complements to routine anterior planar images are correctly emphasized. The current clinical practice is routine use of planar and SPECT imaging at 10 minutes and 2 hours with computer display of the reprojection images. In addition, good practice routinely requires axial and coronal reconstruction images for precise localization of abnormal accumulation of the radiopharmaceutical. Nguyen provides an excellent review of the anatomic and physiologic principles for localization of parathyroid disease and presents clinical cases that illustrate the logistics of detection and differential diagnosis and the pitfalls in interpretation of radionuclide images.
The following considerations are also important:
1. I-123 or Tc-99m pertechnetate and Tc-99m sestamibi subtraction images help localize parathyroid lesions in patients who require repeat surgery. Chen and coworkers (10) reported that, for the differential diagnosis of ectopic parathyroid adenomas, the sensitivity was increased from 59% for double-phase scintigraphy to 70% for the subtraction technique. This combined subtraction radiopharmaceutical approach is also useful in cases of asymmetry or anomaly of the salivary gland and increases the detection rate for parathyroid hyperplasia (14).
2. The addition of pinhole imaging in the delayed phase of Tc-99m sestamibi imaging in indeterminate or initially negative cases further increases the detection rate for parathyroid adenomas (15).
3. The sensitivity of Tc-99m sestamibi imaging in patients who require repeat surgery could be increased if the interval between previous surgery and imaging was extended to a few weeks to avoid delayed trapping of the radiopharmaceutical in the thyroid; postoperative inflammation is attributed to such trapping (16).
4. It is advisable, if not imperative, to use a computer screen for interactive viewing of the scintigraphic data. Adjusting the lower and upper thresholds and changing the negative and positive polarity of the displayed images can help detect small lesions (<300 mg) and subtle increased uptake in lesions with a smaller percentage of oxyphilic cells (ie, lesions made predominantly of clear cells).
5. MR imaging should play a complementary role to double-phase Tc-99m sestamibi scintigraphy because use of both tests improves the sensitivity and specificity of preoperative localization (17).
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