Published online before print March 22, 2012, doi: 10.3174/ajnr.A3084
AJNR 2012 33: E66

A.R Sepahdari^a
aDepartment of Radiological Sciences

A. Harari^b
bDepartment of General Surgery
David Geffen School of Medicine
University of California Los Angeles
Los Angeles, California

We read with interest the recent case series by Gafton et al,¹ sharing their experience with dual-phase CT of the neck for detection of parathyroid hormone (PTH)–secreting lesions. To date, much of the literature on multiphasic CT for detection of parathyroid adenoma has focused on the overall diagnostic performance of the technique and less on the specific features of PTH-secreting lesions that differentiate them from normal tissue.^2⇓–4 In conjunction with recently published data by Beland et al,⁵ the bar graph depicting mean enhancement values for parathyroid hormone–secreting lesions, thyroid tissue, lymph nodes, and submandibular glands is a welcome step toward establishing criteria that are predictive of a PTH-secreting lesion.

We wish to make an observation regarding the data presented in this article, as to how they differ from our experience and how this difference may impact clinical scanning protocols.

Gafton et al¹ noted that all tissue other than PTH-secreting lesions accumulated contrast between the arterial and venous phases. Our experience has been different. We typically note a mild degree of contrast washout in thyroid tissue between the arterial and venous phases. This is likely related to slight differences in scan timing. The protocol described by Gafton et al begins scanning at 25 seconds after the initiation of a 120-mL contrast bolus delivered at 3 mL/s. Given the speed at which a 64-row multidetector scanner covers the imaged volume, it is possible that the thyroid gland had not yet reached peak enhancement during the arterial phase imaging in their series.

At our institution, the arterial phase scan is performed 30 seconds after the initiation of a 120-mL contrast bolus delivered at a rate of 4 mL/s, and the venous phase is performed at a 60-second delay. This slight difference in timing leads to a reversal of the arterial-to-venous attenuation relationship in thyroid tissue, which can obscure a PTH-secreting lesion adjacent to the thyroid gland. This issue is easily resolved by looking at the precontrast images, which we often find very useful.

While we agree wholeheartedly with efforts to reduce medical radiation to the minimum necessary levels, we believe that the optimal protocol for dynamic parathyroid CT remains an active question.

References

1. Gafton AR, Glastonbury CM, Eastwood JD, et al. Parathyroid lesions: characterization with dual-phase arterial and venous enhanced CT of the neck. AJNR Am J Neuroradiol 2012 Jan 12. [Epub ahead of print]
2. Rodgers SE, Hunter GJ, Hamberg LM, et al. Improved preoperative planning for directed parathyroidectomy with 4-dimensional computed tomography.Surgery 2006;140: 932–40discussion 940–41 » CrossRef » Medline
3. Harari A, Zarnegar R, Lee J, et al. Computed tomography can guide focused exploration in select patients with primary hyperparathyroidism and negative sestamibi scanning. Surgery 2008;144: 970–77 » CrossRef » Medline
4. Kutler DI, Moquete R, Kazam E, et al. Parathyroid localization with modified 4D-computed tomography and ultrasonography for patients with primary hyperparathyroidism. Laryngoscope 2011;121: 1219–24 » CrossRef » Medline
5. Beland MD, Mayo-Smith WW, Grand DJ, et al. Dynamic MDCT for localization of occult parathyroid adenomas in 26 patients with primary hyperparathyroidism. AJR Am J Roentgenol 2011;196: 61–65 » Abstract/FREE Full Text

Reply

Published online before print March 22, 2012, doi: 10.3174/ajnr.A3109
AJNR 2012 33: E67

J.K. Hoang^a
aDuke University Medical Center
Durham, North Carolina

A.R. Gafton^b
bYale University School of Medicine
Yale New Haven Hospital
New Haven, Connecticut

We thank Drs. Sepahdari and Harari for their interest in our article and for sharing their experience on thyroid parenchymal enhancement and the utility of noncontrast CT for parathyroid multiphase CT.¹

We agree that enhancement characteristics of parathyroid lesions and normal structures on multiphase CT scan could vary depending on the timing of the contrast phases. In particular, the thyroid gland is a structure that can give rise to false-positive and false-negative results because it can have nodular components and can be avidly enhancing.

In our case series, the mean thyroid gland attenuation was lower in the arterial phase than in the venous phase, but in 1 of our patients, we also found that the thyroid gland attenuation in the arterial phase was greater than that in the venous phase. For this reason, we attempted to refine the criteria with the use of the relative enhancement washout percentage and tissue-arterial ratios. These indices account for the magnitude of the attenuation change between arterial and venous phases and tissue attenuation relative to carotid artery attenuation, respectively, both of which are higher for parathyroid lesions than for the normal thyroid gland. However, our study is limited by a small number of patients, and future validation of these indices with more subjects will be important.

We agree that the optimal parathyroid multiphase CT protocol is yet to be determined. This will depend on CT technical factors that influence the speed of scanning as well as the patient’s cardiovascular status. We are interested in Drs. Sepahdari and Harari’s use of the noncontrast phase. If this phase has added value in certain cases, we offer a modified approach to our dual-phase protocol, in which the radiation dose could still be minimized. For patients with known multinodular thyroids, imaging could start with a noncontrast CT with the z-axis limited to coverage of the thyroid gland. In other patients, the radiologist could check the CT images while the patient is still in the CT scanner. If there is a multinodular thyroid or if enhancement of the thyroid is greater in the arterial than in the venous phase, the radiologist could request a very delayed-phase CT scan with the z-axis through the thyroid gland.