Teaching Case

A 61-year-old obese female patient with hypocalcaemia (1.34mmol/l: nl range 1.18-1.32) and a high PTH level (9.4pmol/l: nl range 1.18-8.43) was referred from our ENT clinic for CT evaluation. This patient had recently had a Tc-99m-MIBI-SPECT (MIBI-SPECT) and ultrasound examination (US), both resulting in negative or inconclusive reports.

CT perfusion examination of thyroid and parathyroid glands was performed with an Aquillion ONE Toshiba scanner (320 slices, 0.5 sec rotation) as a volumetric scan. Thirty ml contrast at 6ml/sec (Omnipaque 300mgI/ml) was injected into the right cubital vein (to avoid contrast artefacts from the left subclavian vein). Five seconds after injection start, a scan was obtained every 1.5 sec for 25 seconds and every 2 sec totalling 41.5 sec (25 low dose volumetric scans). Data was analyzed on a Vitrea workstation, every voxel´s Hounsfield unit was analyzed to perform colour image reconstruction where high perfusion was red and low perfusion blue and no perfusion presented as grey. The common carotid artery was used as an arterial reference and thyroid gland as tissue reference. Three enlarged parathyroid glands were visualized by CT perfusion scanning prior to surgical intervention and confirmed clinically during surgery and pathologically post surgery.

Surgical treatment of primary hyperparathyroidism has historically been performed with bilateral four-gland exploration. Today, unilateral minimally invasive parathyroidectomy is preferred to reduce operative complications [1, 2]. Precise preoperative imaging is essential to the successful localization of glands prior to this kind of surgery. In Denmark, Tc-99m-Sestamibi parathyroid scintigraphy and ultrasound scanning of the neck are the first choices as pre-operative imaging modalities but sensitivity can be equivocal, approx. 75-80% [4, 6]. 4D-CT of the neck is another imaging alternative with a relatively high sensitivity ranging between 85-90% [3-5, 9]. To our knowledge, CT perfusion scanning with a Toshiba Aquilion ONE 320 slice scanner has never before been used for the localization of parathyroid adenomas. This system is able to perform a volumetric 16 cm low dose scan (100kV, 40 mAs) in 0.5 sec, repetitively. Image interpretation was determined by the perfusion colour scale information visualizing local areas with hyperperfusion per individual voxel utilizing Hounsfield's units (HU/voxel). With this technique, parathyroid adenomas were identified by colour intensity (red), size (>4mm) and time to peak. In our previous experiences with 4D-CT, thyroid adenomas are seen slightly later (1-2 sec) than parathyroid adenomas. This is most likely due to the normal anatomical circulation from the thyroid artery's first side branch which perfuses the parathyroids first. This method of adenoma identification is supported by previous works with ^11C-Choline or Methionine tracers in PET/CT imaging [8, 9]. The total radiation burden to the patient with this modality was 6 mSv as compared to 6.5 mSv with a MIBI-SPECT/low dose CT scan. A 4D-CT of the neck at our department is on the average 3.9 mSv but can range from 3.5-6.5 mSV dependent on BMI. In this case, the patient had a high BMI, requiring a CT exposure in the upper limits regardless of CT modality. This case was confirmed both bio-chemically during surgery and pathologically post surgery. After utilizing this image modality, 3 adenomas were detected suggesting that this novel technique may be helpful in locating parathyroid adenomas in patients presenting with the clinical symptoms of primary hyperparathyroidism. The radiation burden from this technique in combination with other modalities can be significant. Therefore, this technique may be useful as a first-line diagnostic tool after negative US at sites where a 320 slice CT scanner is available. A study consisting of a statistically larger number of patients is required to corroborate this finding.