Juxta-Articular Myxoma

Juxta-articular myxomas are uncommon benign lesions that usually (~90%) occur around the knee. Patients are typically middle-aged men who present with pain and complaints of a palpable mass. The lesions are typically between 2 cm - 7 cm in size.

These myxomas most frequently occur in the subcutaneous fat of knee, but can also arise anywhere in the intra- and extracapsular spaces. While benign, a recurrence rate of ~30% has been reported.

In contrast to the more common intramuscular myxomas, which have homogenous low signal intensity on Tl-weighted images and homogeneous high signal intensity on T2-weighted images, juxta-articular myxomas are more likely to be heterogeneous and have myxoid degeneration.

The case above is a juxta-articular myxoma arising in posterior suprapatellar fat pad. The lesion is cystic on ultrasound. T1-FS images reveals a lesion that is isointense to muscle and has mild peripheral enhancement. No other sequences were available.

The main differential consideration in this case is a ganglion cyst. Ganglion cysts have myxoid tissue that is much less developed than in juxta-articular myxomas and are typically smaller.

References

• Daluiski A, Seeger LL, Doberneck SA, Finerman GA, Eckardt JJ. A case of juxta-articular myxoma of the knee. Skeletal Radiol. 1995 Jul;24(5):389-91.
• Kosty JW, Moore JG. Juxta-articular myxoma within the suprapatellar pouch masquerading as a ganglion cyst. Orthopedics. 2009 Jul;32(7):527.

Facet Anatomy of the Greater Tuberosity

The superior (anterior) facet of the greater tuberosity is horizontal on coronal oblique MR images and is the site of attachment of the supraspinatus tendon.

The middle facet of the greater tuberosity is obliquely oriented and is the site of attachment of the supraspinatus and infraspinatus tendons. The supraspinatus tendon attaches to the superior half of the middle facet, while the infraspinatus tendon attaches to the entire middle facet, covering a portion of the supraspinatus tendon.

As we move posteriorly, we see both the middle and inferior (posterior) facets. The inferior facet is vertically oriented and is the site of attachment of the teres minor tendon.

References

• Minagawa H, Itoi E, Konno N, Kido T, Sano A, Urayama M, Sato K. Humeral attachment of the supraspinatus and infraspinatus tendons: an anatomic study. Arthroscopy. 1998 Apr;14(3):302-6.

Giant Cell-Rich Osteosarcoma.

Giant cell–rich osteosarcoma makes up about 3% of all osteosarcomas. Histologically, giant cell–rich osteosarcomas are characterized by an abundance of osteoclast-like giant cells and a paucity of tumor osteoid and bone. The histologic appearance can be similar to giant cell tumor.

This paucity of tumor osteoid results in imaging features unlike those of conventional osteosarcomas. In addition, differentiation from benign lesions can sometimes be difficult. Giant cell–rich osteosarcomas are usually lytic and have poorly defined borders. Periosteal reaction is scant or absent and a soft tissue mass is usually not present.

The biological behavior is not necessarily different from that of conventional osteosarcoma.

References

• Bathurst N, Sanerkin N, Watt I. Osteoclast-rich osteosarcoma. Br J Radiol. 1986 Jul;59(703):667-73.

Radiography and MRI for Assessment of Acromial Shape

Using 3-dimensional models constructed from MR images as their gold standard, Mayerhoefer et al compared outlet view radiographs and MRI for assessment of the shape of the acromion (types 1-3). They found that MRI had good correlation (K=0.66) to the 3-dimensional model when two oblique coronal slices were used in conjunction: A slice 4 mm medial to the lateral margin of the acromion (1) and a slice lateral to the acromioclavicular joint (2). Outlet view radiographs had moderate correlation to the shape of the acromion (K=0.55), which was still better than any single MRI slice.

References

Mayerhoefer ME, Breitenseher MJ, Roposch A, Treitl C, Wurnig C. Comparison of MRI and conventional radiography for assessment of acromial shape. AJR Am J Roentgenol. 2005 Feb;184(2):671-5.

The "50% Rule"

The "50% rule" in arthroscopic and orthopaedic surgery states that tendon and ligament injuries with structural involvement of less than 50% are best treated by observation, while those with involvement of greater than 50% are best treated by surgical repair or reconstruction.

The rule appears to have come out of studies on the management of hand flexor tendon injuries and transferred over to arthroscopy in the shoulder and knee. It should be remembered that the rule is derived from expert opinion and has never been subjected to a randomized controlled clinical trial nor validated for accuracy, reliability, or reproducibility. The role of imaging in all this is as murky if not more so.

References

Pedowitz RA, Higashigawa K, Nguyen V. The "50% rule" in arthroscopic and orthopaedic surgery. Arthroscopy. 2011 Nov;27(11):1584-7.

Drugs and Cardiac FDG Uptake

Certain drugs and patient factors can affect cardiac uptake of FDG.

Drug	Effect	Comment
Insulin	↑	Similar to the effect of eating close to FDG administration. Will also cause diffuse increase in muscle uptake and a decrease in liver uptake.
Bezafibrate	↑	Fibrate drug used for hyperlipidemia.
Benzodiazepines	↑
Levothryoxine	↓
Metformin	↓	Shown above. Increases colonic FDG uptake and can result in a competitive decrease in cardiac FDG uptake.

In addition to the above, certain patient factors can increase cardiac FDG uptake: Male gender, younger patients (< 30 years), fasting duration of <5 hours, and patients with heart failure tend to have higher cardiac FDG uptake.

References

• Israel O, Weiler-Sagie M, Rispler S, Bar-Shalom R, Frenkel A, Keidar Z, Bar-Shalev A, Strauss HW. PET/CT quantitation of the effect of patient-related factors on cardiac 18F-FDG uptake. J Nucl Med. 2007 Feb;48(2):234-9.
• Maurer AH, Burshteyn M, Adler LP, Steiner RM. How to differentiate benign versus malignant cardiac and paracardiac 18F FDG uptake at oncologic PET/CT. Radiographics. 2011 Sep-Oct;31(5):1287-305.

F18-FDG PET in Neuroendocrine Tumors

Somatostatin receptor scintigraphy (e.g., octreoscan) is more sensitive than both ¹²³I-MIBG scintigraphy and ¹⁸F-FDG PET for neuroendocrine tumors. However, ¹⁸F-FDG PET is more sensitive for detection of aggressive tumors, with a sensitivity of around 90%, compared to ~70% and ~45% for somatostatin receptor and ¹²³I-MIBG scintigraphy for tumors with proliferation index above 15%.

In these aggressive tumors, somatostatin receptor and ¹²³I-MIBG scintigraphy can underestimate the extent of disease dissemination and lead to suboptimal treatment for these patients (aggressive disease is treated with systemic chemotherapy, while less aggressive disease is treated with somatostatin analogs or α-interferon).

In the example above, FDG-PET shows uptake in several areas not seen on octreoscan. One example is an enlarged retroperitoneal lymph node that is not hot on octreoscan, but light s up on FDG-PET (black arrows).

References

Binderup T, Knigge U, Loft A, Mortensen J, Pfeifer A, Federspiel B, Hansen CP, Højgaard L, Kjaer A. Functional imaging of neuroendocrine tumors: a head-to-head comparison of somatostatin receptor scintigraphy, 123I-MIBG scintigraphy, and 18F-FDG PET. J Nucl Med. 2010 May;51(5):704-12.