Diagnostic Ultrasound - Abdomen and Pelvis

Myelolipoma
TERMINOLOGY
Definitions
• Rare benign tumor composed of mature fat tissue and
hematopoietic elements (myeloid and erythroid cells)
IMAGING
General Features
• Best diagnostic clue
○ Heterogeneous fat-containing adrenal mass
• Location
○ Adrenal gland (85%): Thought to arise in zona fasciculata
of adrenal cortex
○ Typically unilateral, only very rarely bilateral: 10:1
○ Extraadrenal (15%): Retroperitoneal (12%) and
intrathoracic (3%)
○ Possible right-sided predilection
• Size
○ Usually 2-10 cm, rarely 10-20 cm
Ultrasonographic Findings
• Grayscale ultrasound
○ Well-defined, homogeneous, hyperechoic suprarenal
mass (when predominantly composed of fatty tissue)
○ When small, difficult to distinguish from echogenic
retroperitoneal fat
○ Apparent diaphragm disruption: Propagation speed
artifact; decreased sound velocity through fatty mass
leads to this appearance, usually seen when tumor > 4
cm
○ Heterogeneous mass (when myeloid cells predominate),
may be isoechoic or hypoechoic
○ Heterogeneous echo pattern may also be due to internal
hemorrhage (common), ± calcification
• Color Doppler
○ Avascular to hypovascular adrenal mass
Radiographic Findings
• Radiography
○ Lucent mass with rim of residual normal adrenal cortex
○ May see calcification due to previous hemorrhage
CT Findings
• CT appearance usually characteristic
○ Typically appears as heterogeneous fat-containing
adrenal mass
– Macroscopic fat within tumor is diagnostic
– Low attenuation of fat density (-30 to -90 HU)
– Amount of fat is widely variable: Completely fat, to >
1/2 fat (50%), to only a few tiny foci of fat in soft tissue
mass (10%)
• Usually well-defined mass with recognizable capsule
• Interspersed "smoky" areas of higher CT values than those
of retroperitoneal fat because of presence of
hematopoietic tissue in myelolipoma
• Mass may have attenuation values between fat and water
due to diffusely mixed fat and myeloid elements
• Higher density areas may be seen with hemorrhage
• Punctate calcification in 25-30% of cases
• Occasionally the mass may appear to extend into
retroperitoneum
• Thin sections are recommended (to avoid volume
averaging) if fatty tissue is not predominant
○ Or consider pixel mapping
MR Findings
• Varied MR appearance depending on mixture of elements
○ Fat within mass is hyperintense on T1- and T2WI
○ Hematopoietic elements are T1 hypointense and
moderately hyperintense on T2WI
• Fat-suppressed sequences best demonstrate intratumoral
fat
• Opposed-phase sequences can be helpful for characterizing
presence of both fat and water
○ India ink artifact at boundary of fat and water confirms
diagnosis
○ If mass is predominantly composed of mature fat cells
with little intracellular water from soft tissue, little to no
loss of signal seen on opposed phase sequence
• Soft tissue elements enhance avidly after intravenous
administration of gadolinium-based contrast
Angiographic Findings
• Conventional
○ Differentiate myelolipoma from retroperitoneal
liposarcoma by determining origin of blood supply and
vascularity of tumors
Imaging Recommendations
• Best imaging tool
○ Optimally imaged with NECT or MR with fat-suppression
sequence
• Protocol advice
○ Ultrasonography may be useful in diagnosis of large
tumors; however, CT or MR are better in detecting
smaller masses
DIFFERENTIAL DIAGNOSIS
Adrenal Hemorrhage
• Usually well defined and round in shape
• Acute: Hyperechoic or heterogeneous in echogenicity
• Chronic: Well defined, hypoechoic, cystic, or calcified
• Often in setting of blunt abdominal trauma, bleeding
disorder, stress, or underlying tumor
• Best characterized with CT or MR
Pheochromocytoma
• Variable appearance: Purely solid (68%), complex (16%),
and cystic tumor (16%)
• Small tumor: Round, solid, well-circumscribed mass with
uniform echogenicity
• Large tumor may appear as purely solid mass of
homogeneous (46%) or heterogeneous (54%) echo pattern
• Highly vascular; prone to hemorrhage and necrosis
Adjacent Neoplasm
• Renal angiomyolipoma
○ Well-defined, fat-containing, hyperechoic lesion arising
exophytically from upper pole of kidney
○ Multiplanar CT or MR best demonstrates claw of renal
parenchyma around part of lesion
• Renal cell carcinoma (RCC)
○ Well-defined mass of variable echogenicity
Diagnoses: Adrenal Gland
589