Parathyroid 4D CT

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Parathyroid four-dimensional (4D) CT refers to multiphase computed tomography of the neck used to localize abnormal parathyroid glands (i.e., involved with adenoma, hyperplasia, or, rarely, carcinoma). The "4D" indicates that imaging is performed in multiple phases of contrast, with time being the fourth dimension in addition to the multiplanar format of CT; the number of phases is not necessarily four.

Indications

Parathyroid 4D CT is performed in patients who have a biochemical diagnosis of primary hyperparathyroidism. The purpose of imaging is to localize the abnormal parathyroid gland(s) in surgical candidates. Some studies have shown equivalent or superior performance of 4D CT compared to ultrasound or sestamibi SPECT, but current preferences for one modality or a combination vary by institution depending on expertise and experience .

Technique

A typical protocol consists of scanning in three phases :

  • non-contrast phase
  • arterial phase: 25-30 seconds after start of contrast injection
  • delayed phase: 60-80 seconds after start of contrast injection

    • The axial coverage, at least for the postcontrast phases, includes the entire neck (up to the angle of mandible or maxilla) and the mediastinum (down to the carina) .

    Approach

    A systematic approach is suggested based on anatomic and attenuation characteristics :

    • typical morphology
      • parathyroid lesions are usually oval or round, smooth or slightly lobulated
      • abnormal parathyroid glands are usually supplied or drained by a prominent polar (as opposed to hilar) vessel
    • anatomic search pattern
      • search eutopic locations around the thyroid gland
        • ~85% of parathyroid glands are eutopic, more frequent for superior glands and less frequent for inferior glands
        • superior glands are located posterior to the tracheoesophageal groove and may fall caudally when enlarged, while inferior glands are anterior to the tracheoesophageal groove
      • search ectopic locations in the parapharyngeal space including carotid sheath, retropharyngeal/retroesophageal space, thyrothymic ligament continuing into the anterior mediastinum, and tracheoesophageal groove continuing into the posterior mediastinum
        • 3% of superior parathyroid glands are retropharyngeal/retroesophageal
        • 28% of inferior parathyroid glands are located along the thyrothymic ligament and anterior mediastinum
      • evaluate the thyroid gland for intrathyroid parathyroid glands or thyroid nodules, which may appear similar on CT although the latter are much more common
    • contrast phase search pattern
      • review the arterial phase images for lesions that avidly enhance, greater than thyroid
        • 20% of parathyroid lesions are distinct on arterial phase (known as type A pattern) 
      • review the delayed phase for lesions that have washed out contrast more than thyroid (whereas thyroid tissue and lymph nodes progressively enhance)
        • 57% of parathyroid lesions are distinct on delayed phase but not arterial phase (known as type B pattern) 
      • review the noncontrast phase for lesions that are lower attenuation than thyroid (whereas thyroid tissue has high attenuation)
        • 22% of parathyroid lesions are distinct on noncontrast phase but not arterial or delayed phases (known as type C pattern) 
      • recognize variants such as glands with areas of low attenuation due to cystic or fat components, or areas of high attenuation due to calcification

    Radiology report

    Imaging findings important for operative planning should be reported :

    • number of candidate lesions: single gland disease may be treated less invasively than multiglandular disease
    • size of candidate lesions: large size (>13 mm) favors single adenoma while small size (<7 mm) favors multiglandular disease
    • lesion location: the site should be described with respect to surgically visible landmarks such as the cricoid cartilage, tracheoesophageal groove, thyroid gland, or suprasternal notch, but not the cervical vertebrae

    If multiple candidates are identified, prioritizing them by diagnostic confidence can also influence the surgical approach.

    Certain incidental findings should be highlighted :

    • vascular anomalies that predict the presence of a nonrecurrent laryngeal nerve, usually aberrant right subclavian artery
    • thyroid nodules that require further preoperative evaluation to determine whether concurrent resection is warranted at the time of parathyroid surgery