Peter Lee, MD Claudia Kirsch, MD Vinh Nguyen, MD ASNR 2016 The Problematic Perplexing Parathyroid: A Review of Imaging Pitfalls and Challenging Parathyroid Adenomas on CT Imaging Peter Lee, MD Claudia Kirsch, MD Vinh Nguyen, MD ASNR 2016

Objectives To discuss the role of parathyroid CT in the diagnosis of primary hyperparathyroidism. To review the typical imaging characteristics of parathyroid adenomas. To describe the imaging pitfalls and challenging cases of parathyroid adenoma localization on time resolved CT imaging.

Primary Hyperparathyroidism Autonomous overproduction of parathyroid hormone. Incidence: 25/100,000 in the U.S. F:M 3:1 Peak incidence: >50 years Sporadic > familial (MEN1, MEN2A)

Primary Hyperparathyroidism Symptoms: Commonly asymptomatic. Renal stones. Osteopenia. Bone/joint pain. PUD, GERD, pancreatitis. Depression, memory loss. Labs Hypercalcemia Elevated parathyroid hormone (PTH)

Spectrum of lesions in primary hyperparathyroidism Solitary parathyroid adenoma: 88% Double adenoma: 4% Multiple gland hyperplasia: 6% Parathyroid carcinoma: < 1%

Surgical Management Minimally invasive parathyroidectomy (MIP): Small unilateral incision in one quadrant. Indication: Solitary parathyroid adenoma. Requires accurate preoperative localization Fewer complications, better cosmesis, and shorter operative time. Four gland cervical exploration: Large transverse incision. Indications: Negative imaging studies or recurrent hyperparathyroidism. alexorl.edu.eg/alexorlfiles

Surgical Management Surgical success: Decline in intraoperative venous parathyroid hormone level. Potential complications: Recurrent laryngeal nerve injury, hypoparathyroidism. emedicine.medscape.com

Imaging Modalities Ultrasound and/or nuclear scintigraphy First line Ultrasound: Accurate for eutopic parathyroid adenomas. Caveat: Can miss ectopic adenomas. Nuclear scintigraphy: Detects eutopic and ectopic adenomas. Caveat: Low spatial resolution. Parathyroid CT First or second line Detects eutopic and ectopic adenomas. High spatial resolution. Excellent anatomic localization. Enhancement characteristics improve diagnostic accuracy.

Parathyroid CT Protocol No consensus. Protocol variations: Noncontrast and 1, 2, or 3 contrast-enhanced phases. Two contrast-enhanced phases only. Our protocol: Noncontrast and two contrast-enhanced phases (arterial and venous). Coronal and sagittal reformations of all phases.

What the surgeons want to know: Number of parathyroid lesions Location Size Diagnostic confidence

Typical Appearance of a Parathyroid Adenoma on CT Noncontrast: Low attenuation compared to thyroid. Arterial phase: Peak enhancement. Venous phase: Washout. Over 60 HU decrease compared to arterial phase or More washout compared to thyroid gland.

Typical Appearance of a Parathyroid Adenoma Figure 1a: A lesion posterior to the left upper pole of the thyroid demonstrates avid arterial enhancement (arrow). Figure 1b: During the venous phase, the lesion shows washout (arrow). Findings are consistent with a parathyroid adenoma.

Artifacts Beam hardening artifact Motion Washout seen only on reformats Adenoma seen only on reformats Optical Illusion due to high density of intra-arterial contrast

Beam Hardening Artifact Shoulders and clavicles can attenuate the X-ray beam and cause beam hardening artifact in the inferior neck.

Beam Hardening Artifact: Example 1 Figures 2a and 2b: Beam hardening artifact (blue arrow) obscures a lesion abutting the inferior pole of the right thyroid (orange arrow). There is a small cleft (red arrow) separating the lesion from the thyroid. The lesion shows arterial enhancement and venous washout, compatible with a parathyroid adenoma. Figure 2a: Arterial phase. Figure 2b: Venous phase.

Beam Hardening Artifact: Example 1 (continued) Figure 2c: The adenoma is better seen on a sagittal reformation, but is still difficult to resolve. Figure 2d: Color Doppler ultrasound confirms the parathyroid adenoma by its echogenicity and vascularity. Parathyroid CT and ultrasound can be complementary.

Beam Hardening Artifact: Example 2 Figure 3: A lesion in the right tracheoesophageal groove (orange arrow) is obscured by beam hardening artifact (blue arrows). Strategies to avoid beam hardening artifact: Ask patient to depress shoulders, place a towel between shoulders, or use a shoulder strap.

Motion Artifact Figure 4a: A lesion in the left tracheoesophageal groove shows avid arterial enhancement, compatible with a parathyroid adenoma (orange arrow). Figure 4b: However, respiratory motion artifact more superiorly limits evaluation for additional adenomas. Breathing instructions are important for reducing motion artifact.

Washout seen only on reformats Figures 5a and 5b: A lesion posterior to the left hemithyroid midpole does not appear to wash out on the axial venous phase. Figure 5a: Arterial phase Figure 5b: Venous phase Figures 5c and 5d: The lesion does wash out on the coronal venous phase. In some cases, venous washout is seen only on reformats. Figure 5c: Arterial phase Figure 5d: Venous phase

Adenoma seen only on reformats Figure 6a: Arterial phase. Figure 6b: Venous phase. An adenoma abutting the inferior pole of the right hemithyroid is seen only on the sagittal reformation (arrow). This lesion is not seen on the axial images (not shown).

Optical Illusion Figure 7a: Noncontrast. Note radiolucent thyroid (blue arrow). This entity will be discussed on a later slide. Figure 7b: Arterial phase. Figure 7c: Venous phase. A parathyroid adenoma abutting the right lower pole (orange arrow) appears to increase in enhancement between the arterial and venous phases. However, there is actually a 60 HU washout. This optical illusion is due to the high density of intra-arterial contrast.

Variant Anatomy and Parathyroid Mimics Cystic parathyroid Radiolucent thyroid Parathyroid calcifications Parathyroid mimics

Cystic Parathyroid Adenoma: Example 1 Figures 8a and 8b: Coronal and axial images demonstrate a partially cystic mass at the level of the right hemithyroid lower pole. Figure 8b Figure 8a Figures 8c and 8d: The solid component of the lesion demonstrates avid arterial enhancement (8c) and venous washout (8d). Findings are compatible with a partially cystic parathyroid adenoma. Figure 8c Figure 8d

Cystic Parathyroid Adenoma: Example 2 Figure 9a: Axial arterial phase image shows a large cystic mass deep to the left hemithyroid. The mass does not demonstrate arterial enhancement or venous washout. Figure 9b: Venous phase sagittal reformat better demonstrates septations within the mass. Pathology was consistent with cystic parathyroid adenoma.

Radiolucent Thyroid Figure 10a: A radiolucent thyroid gland demonstrates atypically low attenuation on noncontrast imaging (arrow).

Radiolucent Thyroid (continued) A lesion inferior to the left hemithyroid inferior pole (orange arrow) shows arterial enhancement and venous washout (10d, 10e), compatible with a parathyroid adenoma. Radiolucent thyroids demonstrate poor contrast enhancement. Because washout of parathyroid adenoma can be defined relative to washout of the thyroid, this can lead to misdiagnosis. Figure 10b: Arterial Figure 10c: Arterial Figure 10d: Venous Figure 10e: Venous

Parathyroid Calcification Figure 11a: Arterial phase. Figure 11b: Venous phase. There is a calcified lesion (arrow) inferior to the left hemithyroid. Calcifications are rarely seen in parathyroid adenomas and are more common in parathyroid carcinoma. In this case, pathology was consistent with adenoma.

Parathyroid Mimic: Concurrent Lymphoma Figure 12a and 12b: A lesion in the left tracheoesophageal groove (orange arrows) shows mildly increased enhancement on venous phase. This may represent an atypical parathyroid adenoma. The patient had concurrent lymphoma. Note bilateral cervical lymphadenopathy (blue arrows), left greater than right, which can mimic parathyroid adenomas. Figure 12a: Arterial phase. Figure 12b: Venous phase.

Conclusion Parathyroid CT is a first or second line imaging modality for primary hyperparathyroidism, offering accurate anatomic localization of eutopic and ectopic parathyroid adenomas. Though there is no consensus, most protocols include noncontrast and contrast-enhanced phases. Awareness of image artifacts and pitfalls on parathyroid CT can improve diagnostic accuracy.