Modification of 4-dimensional computed tomography (4D-CT) technique with volume rendering reconstructions and significant dose reduction is a safe and accurate method of pre-operative localization for primary hyperparathyroidism. Modified low dose 4D-CT with volume rendering reconstructions provides precise preoperative localization and is associated with a significant reduction in radiation exposure compared to classic preoperative localizing techniques. It should be considered the preoperative localization study of choice for primary hyperparathyroidism.

Improvements in imaging techniques for primary hyperparathyroidism have been critical in the ability to transition from formal cervical four-gland exploration to minimally invasive/directed parathyroidectomy. A precise anatomic localization study is the key to the success of minimally invasive parathyroidectomy. Traditionally, sestamibi single photon emission computed tomography (SPECT) and ultrasound (US) have been used with varying success rates from 29%-79%[1-10]. With advent of 4-dimensional computed tomography (4D-CT) technology, there is improved sensitivity and a higher intraoperative correlation rate ranging from 70%-89% demonstrated by multiple institutions[1-5,11-15]. Despite this clear advantage, the use of 4D-CT has been limited. Numerous factors including the concern of higher cost, increased radiation exposure and a lack of expertise/knowledge have been proposed. At our institution we have modified our technique to address some of these concerns. In this manuscript, we detail our modified 4D-CT technique providing an in-depth review of technical aspects, image processing and adaptations to decrease the effective radiation exposure. The term 4D is used to describe the combination of cross-sectional imaging and basic functional analysis through perfusion information of parathyroid adenomas. The first three dimensions refer to multiplanar CT: axial acquisitions, sagittal and coronal reformatted images. The fourth dimension is the change in enhancement overtime from non-contrast images to arterial and delayed phase imaging.

Multiple studies including our institute’s experience have shown excellent diagnostic accuracy of 4D-CT as a preoperative localization study for primary hyperparathyroidism ranging from 70%-89%[1-5,11-13]. Despite these promising results, many institutions continue to utilize sestamibi SPECT and US as the primary modalities for preoperative localization for directed parathyroidectomy. Despite superiority, 4D-CT has not gained widespread acceptance for reasons unclear to us. Plausible explanations include technical challenges, fear of increased radiation exposure and added costs.

We have utilized this technique in 150 consecutive patients undergoing parathyroidectomy for primary hyperparathyroidism. The true positive rate for modified 4D-CT with volume rendering for this cohort was 133 (89%) of 150 with a false negative (FN) of 17 (11%) of 150. In addition, utilizing our technique, the effective radiation exposure dose is 11-13 mSv. This essentially is an equivalent radiation exposure dose to that of sestamibi SPECT (9-11 mSv) but lower than that previously published for 4D-CT (27 mSv)[17]. To put this in perspective, radiation exposure from some commonly performed procedures such as CT scan of abdomen and pelvis is 14 mSv or CT angiogram is 15 mSv. This reduced effective radiation exposure of our technique is most likely due to the fact that we utilize a lower tube current for non-contrast and delayed images. The most recent implementation of ASIR technology has further reduced the radiation exposure from 4D-CT to essentially equivalent levels to that of sestamibi SPECT.

4D CT is especially useful to identify parathyroid glands in ectopic locations as the axial imaging extends from the hard palate to the level of the pulmonary artery. It is also very useful in recurrent/persistent hyperparathyroidism and improves the success rate of minimally invasive parathyroidectomy in the reoperative setting. Despite these advantages it has some limitations especially in patients with short obese necks, multinodular goiters and multiple exophytic nodules. Another limitation is the observer experience with post image processing and reconstructing 3D images.

To our knowledge, this is the first paper detailing the technical aspects of a low dose 4D-CT with volume rendering reconstruction. It is our aim to share our institute’s technique and experience in the hope of improved utilization of this modality. With this technique, our results are comparable to those published in the literature for diagnostic accuracy regarding correlation to intraoperative pathology. The 3D volume rendering reconstruction of the parathyroid pathology shown in relation to the clavicle, thyroid gland, and skin provide superior surgical guidance and an essentially “cut here” approach for directed parathyroidectomy.

In conclusion, low dose 4D-CT with volume rendering reconstruction provides superior quality images while minimizing radiation exposure. The technique is easily reproducible and in our opinion should be the diagnostic modality of choice in patients with primary hyperparathyroidism.