Multimodality imaging of adrenal gland pathologies: A comprehensive pictorial review

Trauma

Patients that experience blunt abdominal trauma are at risk for adrenal hemorrhage. About 80% of adrenal hemorrhages have a traumatic etiology.^[27] Adrenal hemorrhage in the setting of trauma can occur bilaterally but tends to be more common in the right gland.^[15,28] Located deep within the retroperitoneum, the gland is relatively protected from traumatic injury. The adrenal glands are injured in up to 4% of severe blunt abdominal traumas.^[29] Patients with trauma and adrenal hemorrhage have higher mortality than those with uninvolved glands.^[30] Isolated adrenal gland trauma is rare and is typically associated with pulmonary contusions, solid abdominal organ lacerations, axial skeleton fractures, and head injury. Associated findings include posterior pararenal space hemorrhage, IVC compression, psoas muscle hematoma, and thickening of the diaphragmatic crus.^[22,23]

CT is currently the gold standard in the assessment of trauma patients. MRI has not been routinely used for emergent adrenal gland imaging in the setting of trauma due to its cost and time requirement, but its added benefit includes the ability to use gadolinium when iodinated contrast with CT may be contraindicated. It also provides the ability to use imaging in populations that cannot be exposed to radiation.^[31]

Stress

Adrenal hemorrhage may be associated with stress secondary to surgery, hypotension, pregnancy, sepsis, or burns. Massive bilateral hemorrhage may be evident in critically ill patients. Risk factors increase due to anticoagulant therapy or bleeding diathesis. The proposed mechanism involves increased adrenocorticotropic hormone release, resulting in increased adrenal blood flow.^[19,32] More so, catecholamine-induced vasoconstriction reduces outflow, which results in worsening of the intraglandular hemorrhage.^[19] On CT, bilateral adrenal hyperenhancement has been suggested to occur in >60% of patients with hypotension. During the arterial phase, there is increased peripheral zone enhancement giving a mosaic appearance [Figure 7]. Complete enhancement may be seen in the venous phase.^[33] Non-enhancing foci may also be evident, particularly if associated with necrosis or infarction [Figure 8]. Additional findings may include unilateral or bilateral hemorrhage of varying severity.

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Figure 7:

A 28-year-old man presented with adrenal hyperenhancement and presenting with hypotension. Axial (a) and coronal (b) arterial phase post-contrast computed tomography (CT) abdomen images of a patient with acute shock demonstrate enhancement of the adrenal gland cortex (arrow) and are consistent with the shock stress state and form part of the CT hypoperfusion complex.

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Figure 8:

A 67-year-old female with adrenal infarcts presented with severe Gram-negative sepsis and tachycardia. (a and b) Coronal contrast-enhanced computed tomography images demonstrate non-enhancing foci in the adrenal glands compatible with adrenal infarctions.

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Neoplasm

Adrenal intratumoral bleeding is a cause for concern in non-traumatic patients. Primary adrenal tumors and cysts have been reported to be the fourth most common cause of retroperitoneal hemorrhage.^[19] Acute hemorrhage in an adrenal tumor most commonly occurs with pheochromocytomas, but this type of bleeding can also occur in other primary and metastatic adrenal tumors.^[19,21] A series of 204 patients with spontaneous adrenal hemorrhage demonstrated that underlying adrenal pathology was not discovered in most of these patients until they exhibited shock-like symptoms or upon autopsy. Adrenal neoplasms were discovered in 20% of these patients.^[34,35] Although there is limited research on the pathophysiology of adrenal hemorrhage due to neoplasm, possible mechanisms of intratumoral bleeding include vascular fragility, blood vessel invasion, and tumor size outgrowing blood supply.^[34,35] CT imaging criteria and characterization of adrenal masses in the presence of hemorrhage are highly variable.^[5,36] Differentiating between tumoral and non-tumoral hemorrhage may be difficult. The presence of intralesional calcifications, enhancement, and hypermetabolic activity on PET may be indicative of an underlying tumor.^[21] MRI can be useful in detecting the presence of an underlying mass in patients who have exhibited acute or subacute adrenal bleeding. The presence of blood products obscures the details and may result in an overall heterogeneous, hyperdense appearance of the mass.^[35] However, MRI is overall the most sensitive and specific modality for diagnosing adrenal hemorrhage and is especially useful in determining the age of a hematoma.^[37]

Adenoma

Adrenal adenomas are the most common adrenal tumor. It is a benign tumor arising from the cortex, typically found incidentally. Due to its benign and hypovascular nature, massive hemorrhage is extremely rare.^[19] Adenomas are typically homogenous and <4 cm in diameter. Approximately 70% of tumors are lipid rich, demonstrating hypoattenuation (<10 HU) on non-enhanced CT. CT washout can be used to diagnose higher attenuating masses: An absolute washout of >60% and a relative washout of >40% are generally diagnostic. In patients with relative contraindications to ionizing radiation or CT contrast, chemical shift MRI may be used as an alternative in diagnosing higher attenuating masses. On MRI, lipid-rich adenomas show signal drop on out-of-phase sequences.^[26] In one study, it was found that a signal intensity drop of 20% correlated with 67% sensitivity for hyperattenuating adenomas (>10 HU) on non-enhanced CT.^[38] In other words, adenomas appear darker in out-of-phase images due to intravoxel signal cancellation of lipid and water protons. In non-lipid containing masses such as metastasis, there should be no significant signal loss.^[27] Adenomas may have a variable degree of hypermetabolism on F-18 FDG PET/CT; however, activity is usually less intense than adjacent liver background activity.^[26,39]

Myelolipoma

Adrenal myelolipomas are relatively uncommon tumors and are composed of mature fatty tissue and bone marrow. It has been suggested that at least 50% of the mass must have macroscopic fat for the definitive diagnosis of myelolipoma. The exact prevalence of the tumor is difficult to estimate given the high number of asymptomatic cases and its benign nature.^[40] Size ranges are typically from 2 cm to 13 cm and symptomatic presentation is typically due to hemorrhage or mass effect.^[26] Hemorrhage is more common in tumors with a diameter >10 cm with spontaneous rupture being reported in tumors as small as 7 cm.^[41] Although the literature on pathogenesis is limited, myelolipomas may be more prone to hemorrhage due to intratumoral aneurysms and vascular ectasia. Due to the fatty components, they appear hyperechoic on ultrasound imaging. On CT, the mass shows low attenuation (<−30 HU) due to macroscopic fat with interspersed hyperattenuating bone marrow.^[19,26] On MRI, areas of fat are hyperintense on T1 images with loss of signal on fat-suppressed sequences [Figure 9]. Adrenal myelolipomas typically demonstrate lower uptake than adjacent liver, although rare cases have been noted in the literature.^[1,42]

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Figure 9:

A 45-year-old male presented with adrenal myelolipoma and presenting with the left flank pain. (a) Axial non-fat saturating T2-weighted and (b) pre-contrast T1-weighted images demonstrated a large fatty mass (arrow) arising from the superior pole of the left kidney. Biopsy was consistent with adrenal myelolipoma.

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Adrenocortical carcinoma (ACC)

ACC is the most common primary malignant adrenal tumor with frequent metastasis to the liver, lung, or bones. These tumors are highly aggressive and may invade local vasculature, resulting in hemorrhage and distant metastasis.^[19] Poor prognostic factors at presentation include sharp back pain, shock, and retroperitoneal hemorrhage.^[19,26] On imaging, they are typically large (>6 cm) heterogeneous masses with areas of central hemorrhage or necrosis [Figure 10].^[19,26] Calcifications and the presence of venous infiltration are characteristic findings. The absolute percentage washout (APW) and relative percent washout (RPW) are <60 and 40%, respectively; however, this finding may also be seen in metastasis.^[1,26] ACC is typically intensely FDG avid due to elevated mitotic rate. The degree and volume of tracer uptake have been reported to be representative of tumor burden and are a useful tool in identifying primary tumors, recurrence, and metastasis, and determining prognosis.^[1,24] In one study of 28 patients, an FDG PET uptake intensity of >10 SUV or uptake volume of >150 mL was associated with 6-month mortality in 54 and 55% of patients, respectively.^[24]

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Figure 10:

A 52-year-old female presented with adrenal carcinoma and presenting with abdominal distension. Axial (a) pre-contrast T1, (b) post-contrast T1, and (c) T2-weighted images show a large heterogeneous mass arising from the left adrenal gland (arrow). Notice the intrinsic T1 signal on the pre-contrast image showing hemorrhage (star). Metastasis is also noted in the posterior segment of the right hepatic lobe (curved arrow). Pathology demonstrated adrenal carcinoma.

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Pheochromocytoma

Pheochromocytomas are adrenal medullary tumors arising from chromaffin cells. They are the most common bilateral primary adrenal gland tumor with an incidence of 10%.^[26,43] Pheochromocytomas have been reported to be the most common cause of massive hemorrhage from a primary adrenal tumor.^[19] Clinical presentation is typical of catecholamine excess including refractory hypertension, flushing, headache, and palpitations; however, approximately 10% are clinically silent.^[26] On CT, they appear as enhancing masses and may even show APW and RPW >60% and 40%, respectively. An absolute washout of >110–120 HU is highly suggestive of pheochromocytoma. Large tumors are heterogeneous, demonstrating central necrosis, hemorrhage, and cystic components. MRI typically demonstrates T1 hypointensity, T2 hyperintensity, and contrast enhancement. Hemorrhage or necrosis may demonstrate altered signal intensity [Figure 11].^[19,26] Overall, functional imaging alone is limited due to poor specificity. When used with CT or MRI, octreotide and I-123 metaiodobenzylguanidine (MIBG) can aid in whole-body scans and staging. Any focal uptake of I-123 MIBG has been reported to have a sensitivity of approximately 80–90% while uptake of In-111 octreotide has a reported sensitivity of 75–90%. When used complementarily, nearly all pheochromocytomas demonstrate uptake either one or both agents.^[27] Increased 18F-FDG uptake on PET is typically present and is a useful alternative to tumors that are not MIBG avid.^[26,39]

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Figure 11:

A 41-year-old male presented with pheochromocytoma and presenting with uncontrolled hypertension. Axial (a) T1-weighted and (b) T2-weighted MRI images demonstrate a large mass in the left adrenal gland (arrow) with an intrinsic high signal (star) on T1-weighted images secondary to hemorrhage in a pheochromocytoma.

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Neuroblastic tumors

Neuroblastic tumors may arise anywhere along the sympathetic chain; however, the majority of tumors arise in the adrenal medulla. Neuroblastomas represent the most common extracranial pediatric malignancies.^[26,44] Clinical presentation includes pain, abdominal distension, or neurologic deficits. Initial presentation in neonates includes massive intra-abdominal hemorrhage, resulting in life-threatening abdominal compartment syndrome.^[44] On plain films, neuroblastic tumors may present as retroperitoneal mass with calcifications in 30% of cases. Ultrasonography may reveal a heterogeneous mass with anechoic areas corresponding to hemorrhage or necrosis. CT is the gold standard in assessing neuroblastic tumors. Large tumors are typically heterogeneous demonstrating calcifications in 80–90% of cases.^[44] Areas of necrosis or hemorrhage may be noted as areas of low attenuation within the mass. Characteristic features include vascular encasement, midline crossing, and extension into the spinal canal. On MRI, tumors are typically variably enhancing hypointense of T1 and hyperintense on T2.^[44] Hemorrhages will present as hyperintensities on T1W images. MIBG is typically used for primary tumor identification and whole-body staging. FDG PET may be performed when the tumor is not MIBG avid and technetium-99 MDP can be used for the evaluation of bone disease.^[26,44]

Metastasis

Adrenals are a common site for metastasis, typically arising from primary lung, breast, gastrointestinal, prostate, melanoma, or kidney cancer.^[19,26] Although hemorrhagic adrenal metastasis is rare, bronchogenic carcinoma has been reported to be the most common cause.^[19] On CT, the typical presentation includes attenuation of >10 HU with relative and absolute post-contrast washouts <40 and 60%, respectively. Poorly defined contours due to periadrenal infiltration may be evident.^[19] MRI demonstrates contrast enhancement with no signal drop on out-of-phase images, helping differentiate from benign adenomas [Figure 12].^[26] Increased uptake on FDG PET relative to the adjacent liver may help to differentiate from a benign lesion, but one should be cautious as adrenal adenomas may also be sometimes hypermetabolic.^[26,39] In such cases, rapid interval growth is the most helpful feature to differentiate metastasis. It has been reported that up to 90% of adrenal metastasis from lung cancer demonstrate a significant FDG uptake.^[1]

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Figure 12:

A 75-year-old male presented with bilateral adrenal metastases from lung adenocarcinoma and presenting with shortness of breath. (a) Axial T1-weighted, (b) axial T2-weighted, and (c) post-contrast T1-weighted images show bilateral large heterogeneous masses arising from bilateral adrenal glands (arrows) compatible with bilateral adrenal metastasis.

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Granulomatous adrenalitis

The adrenal gland is commonly involved in granulomatous infections such as disseminated tuberculosis, histoplasmosis, or blastomycosis. Adrenal tuberculosis remains to be one of the most common causes of Addison’s disease in the developing countries. Symptoms typically occur when 90% of tissues are destroyed.^[43] Disseminated infection typically affects the bilateral adrenal glands due to an equal chance of hematogenous or lymphatic spread. Early CT findings include mass-like enlargement of the adrenals with peripheral enhancement and low central attenuation.^[24] At the later stage (>1 year), the adrenals may be smaller or normal in size due to fibrosis and calcifications within the lesions.^[24,43] Additional late findings may include bilateral soft-tissue masses, cystic changes, or calcifications.^[26] On FDG PET, tuberculosis may mimic malignancy due to increased tracer uptake secondary to granuloma formation.^[24]

Adrenal abscess

At times, particularly with disseminated infections, microorganisms in the adrenals lead to abscess formation. Patients with adrenal abscess typically present with longstanding fevers, rigors, anorexia, and asthenia.^[62,63] Commonly implicated organisms include Streptococcus pneumonia and Nocardia. The classic presentation of abscesses on CT includes low central attenuation, a well-defined enhancing fibrous capsule, and surrounding inflammatory changes. On MRI, adrenal abscesses typically present as T2Whyperintense and T1W-hypointense cystic lesion with wall enhancement [Figure 14]. MRI has the additional benefit of better characterizing internal contents and identifying periadrenal edema, which would present as a bright signal on T2W images.^[7,64]

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Figure 14:

A 58-year-old male presented with a right adrenal abscess and presenting with abdominal pain and fever. (a) Axial fat saturation T2-weighted and (b) coronal post-contrast T1 weighted image show a large cystic mass arising from the right adrenal gland (arrow) with wall enhancement compatible with an abscess. Notice there is no nodularity in the wall.

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Some adrenal tumors such as myelolipoma have been shown to develop superimposed hemorrhage and abscess formation and require urgent surgical intervention.^[65] Complex adrenal abscesses may be commonly misdiagnosed as pseudocysts or necrotic adrenal tumors. Characteristics that favor abscess over pseudocyst include round shape, homogenous density, diameter <4 cm, and absence of calcifications. The presence of rapid mass growth and liquefaction has also been described as a useful criterion in differentiating from the malignancy.^[64,66]