Adrenal gland enlargement and nodules





Anatomy, embryology, pathophysiology





  • The adrenal glands are multifunctioning, inverted Y-shaped, retroperitoneal endocrine glands normally located superior to the kidneys in the perirenal space ( Fig. 28.1 ).




    • The adrenal glands mediate the stress response by releasing cortisol and are involved in secondary sex hormone synthesis and blood pressure regulation.



    • Absence of the Y-shape indicates development in the absence of an orthotopic kidney (i.e., agenesis or ectopia).




    Fig. 28.1


    Normal adrenal glands. The axial T2-weighted image (A) shows normal bilateral adrenal glands ( arrows ) as thin, linear, hypointense Y-shaped structures. The corresponding coronal T2-weighted image (B) shows the normal position of the adrenal glands surrounded by retroperitoneal fat and their relationship to the kidneys. The out-of-phase (C) and in-phase (D) images demonstrate some out-of-phase signal loss as a result of the presence of fat-containing enzymes and enzymatic precursors. The T1-weighted, fat-suppressed, postcontrast image (E) shows normal avid early adrenal enhancement.

    (From Roth C, Deshmukh S. Fundamentals of Body MRI , ed 2. Philadelphia: Elsevier; 2016.)



  • The histological zonal architecture of the adrenal gland is divided into two distinct components: the cortex, which is derived from mesothelium, and the medulla, which is derived from neural crest cells.




    • The adrenal cortex is composed of the zona glomerulosa, zona fasciculata, and zona reticularis.




      • The superficial zona glomerulosa is the primary site of aldosterone production.



      • The middle zona fasciculata synthesizes glucocorticoids (cortisol and corticosterone).



      • The inner zona reticularis produces androgens.



      • Adrenal cortical masses include adrenal adenoma, adrenal cortical carcinoma, adrenal myelolipoma, adrenal cysts, and metastasis.



      • Adrenal hemorrhage can involve the cortex.




    • The adrenal medulla is in the centermost portion of the gland and is responsible for catecholamine production.




      • Adrenal medullary masses include pheochromocytoma, neuroblastoma, ganglioneuroma, or ganglioneuroblastoma.





  • Arterial blood supply is via the superior adrenal artery (via phrenic artery), middle adrenal artery (via abdominal aorta), and inferior adrenal artery (via renal artery).



  • The left adrenal vein drains to the left renal vein and the right adrenal vein drains to the inferior vena cava (IVC).



Techniques


Computed tomography





  • Primary modality for evaluating adrenal adenoma



  • Adrenal glands enhance after iodinated contrast administration to approximately 50 to 60 Hounsfield units (HU).



  • Both unenhanced computed tomography (CT) and dynamic contrast-enhanced CT have a role.



  • Benign adenomas and malignant masses show rapid contrast enhancement, but adenomas demonstrate rapid washout of contrast.



Magnetic resonance imaging





  • Alternative to CT when contrast-enhanced CT is contraindicated.



  • Normal adrenal has low to intermediate signal on both T1- and T2-weighted imaging.



  • Chemical shift imaging with T1-weighted gradient echo (GRE) sequences can be used to evaluate for microscopic fat in nodules (see Physics Pearls box).



  • Frequency selective fat saturation can be used to determine presence of macroscopic fat in nodules.



Ultrasonography





  • Normal adult adrenal glands are not seen on ultrasound. Incidental masses near the superior renal poles may be adrenal in origin.



  • Can be used in pediatric evaluation.



Nuclear medicine





  • Malignant adrenal lesions typically demonstrate 18-Fluoride deoxyglucose (18-FDG) uptake exceeding that of normal liver.



  • Whole body 123-Iodine scintigraphy with metaiodobenzylguanidine (MIBG) compounds can detect functional lesions and is superior to other modalities in the detection of extraadrenal lesions.



  • For patients with suspected pheochromocytoma (i.e., elevated urine catecholamine breakdown products) and negative MIBG scintigraphy, FDG-positron emission tomography (PET) or PET/CT may be useful.



Protocols


Suggested computed tomography protocols





  • Unenhanced CT.




    • An adrenal nodule measuring less than 10 HU can be reliably diagnosed as a benign adenoma (98% specific), unless greater than 4 cm ( Fig. 28.2 ).




      Fig. 28.2


      An incidentally discovered left adrenal nodule has measured attenuation values of less than 10 Hounsfield units on unenhanced imaging. This is consistent with a lipid-rich adenoma.

      (From Zagoria RJ, Brady CM, Dyer RB. Genitourinary Imaging: the Requisites , ed 3. Philadelphia: Elsevier; 2016.)




  • Adrenal washout CT.




    • Absolute washout protocol: noncontrast, venous phase, and delayed phase (15 minutes) imaging.



    • Absolute Washout=HUvHUdHUvHUn
      Absolute Washout = HU v – HU d HU v – HU n
      , where HU n , HU v , and HU d are the HU of an region of interest (ROI) in the nodule on noncontrast, venous phase, and delayed phase imaging, respectively.



    • Absolute washout over 60% is diagnostic of benign adenoma, unless greater than 4 cm in size ( Fig. 28.3 ).




      Fig. 28.3


      Contrast material washout in an adrenal adenoma. A, Unenhanced computed tomography (CT) scan shows a homogeneous low-attenuation adrenal nodule with attenuation of 6 Hounsfield units (HU) (typical of a lipid-rich adenoma). B, On the portal venous phase of contrast enhancement, the nodule enhances with attenuation values of 86 HU. C, On 15-minute-delayed CT, the mass has decreased enhancement with attenuation of 32 HU. The contrast enhancement during the portal venous phase washed out by 68% by the time of the delayed scans, confirming that this is an adenoma. Absolute washout of 60% or greater is diagnostic of an adenoma.

      (From Zagoria RJ, Brady CM, Dyer RB. Genitourinary Imaging: the Requisites , ed 3. Philadelphia: Elsevier; 2016.)





    • Determination of benignity after noncontrast acquisition may negate need to complete postcontrast series.




  • Relative washout CT.




    • Relative Washout=HUvHUdHUv
      Relative Washout = HU v – HU d HU v
      .



    • Relative washout over 40% is diagnostic of benign adenoma, unless greater than 4 cm in size.



    • This protocol can be performed if there is concern for radiation exposure.




Suggested magnetic resonance imaging protocols





  • Chemical shift imaging protocol.




    • Uses in-phase and opposed-phase T1-weighted GRE.



    • Adenomas will demonstrate signal dropout from microscopic fat.



    • Other lesions containing microscopic fat (e.g., well-differentiated adrenocortical carcinoma, clear cell renal cell carcinoma, and hepatocellular carcinoma) may also demonstrate signal dropout on opposed phase imaging.




  • Fat suppression can be used to demonstrate macroscopic fat as seen in myelolipoma.



Specific disease processes


Patterns of adrenal gland enlargement ( fig. 28.4 ).



Jun 13, 2021 | Posted by in GASTROINTESTINAL IMAGING | Comments Off on Adrenal gland enlargement and nodules
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