Round Atelectasis

Round atelectasis occurs adjacent to the pleural surface and is almost always asymptomatic. In about 70% of cases, there is a history of asbestos exposure, but any form of pleural thickening can be associated with round atelectasis.

Imaging Findings

The appearance is that of a rounded subpleural opacity that forms acute angles with the pleura. It is usually seen in the lower lobes posteriorly, and there is almost always adjacent pleural thickening. As in other forms of atelectasis, there is volume loss in the affected lobe. There may be air bronchograms within the mass, giving the appearance of cavitation (pseudocavitation).

The aunt Minnie is the comet tail sign, which is formed by vessels and bronchi of the affected collapsed lung that are pulled in as the lung collapses.

Theories

Two main theories attempt to explain how round atelectasis happens after a pleural effusion. One posits that round atelectasis is caused by infolding the visceral pleura into and surrounding an area of atelectasis. This somehow causes the lung to curl on itself. Then fibrous adhesions form that suspend the atelectatic segment and usually tilt the lung cranially. When the effusion resolves, aerated lung fills in the space between the area of round atelectasis.

Another theory proposes that local pleural irritation (e.g., by asbestos) cause contraction and thickening of the pleura in the setting of a pleural effusion. The underlying lung then shrinks, and a round area of atelectasis develops.

The main differential diagnosis is bronchogenic carcinoma.

Reference

Partap VA. The Comet Tail Sign. Radiology. 1999;213:553-554.

Embryology of Corpus Callosum

The corpus callosum forms between the 11th and 20th weeks. The anterior genu is the first part to form. Development then proceeds posteriorly: anterior body, followed by the posterior body and splenium. After all of this is done, development pays attention to the front again, forming the rostrum.

This is important in differentiating partial dysgenesis of the corpus callosum from a developmental injury when the splenium is not seen. Since the rostrum is the last to form, if you see a rostrum in the absence of a splenium, you know that the problem is injury to the splenium. If the rostrum is not seen, then the absent splenium is likely due to partial dysgenesis.

References

Loevner, LA. Case Review Series: Brain Imaging, 2nd edition.

Artery of Percheron

The artery of Percheron is a rare variant of the posterior aspect of thalamic blood supply. The thalami and the midbrain are supplied by both the anterior and posterior circulations.

Anterior Circulation

The anteroinferior aspects of the thalami and midbrain are supplied by the thalamoperforator arteries arising from the posterior communicating arteries.

Posterior circulation

The medial aspects of the thalami and midbrain are supplied via branches arising from P1 segments of the posterior cerebral arteries.

The lateral and superior aspects of the thalami are supplied by branches arising from the P2 segments of the posterior cerebral arteries.

The artery of Percheron refers to a single trunk that arises from one of the P1 segments and provides bilateral distribution.

Occlusion of the artery of Percheron causes bilateral infarctions in the medial aspects of thalami and brainstem.

References

Matheusa MG and Castilloa M. Imaging of Acute Bilateral Paramedian Thalamic and Mesencephalic Infarcts. AJNR Am J Neuroradiol. 2003 Nov-Dec;24(10):2005-8.

Recurrent Artery of Huebner

The recurrent artery of Huebner is the largest and longest of the penetrating branches of the anterior cerebral artery. It arises from either the proximal A2 segment, the A1 segment, or, less commonly, the anterior communicating artery. It terminates dorsal and slightly lateral to the carotid bifurcation.

The recurrent artery of Huebner and medial lenticulostriate arteries supply the caudate head, anterior limb of the internal capsule, and part of the basal ganglia.

Reference

Osborn AG, Chapters 5 and 6 in Diagnostic Cerebral Angiography (2nd ed). Lippincott, Williams, & Wilkins (1999).

Periapical Cyst

Periapical Cysts are the most common cyst of the jaw and are usually asymptomatic (as was this patient who presented for evaluation for trauma).

Periapical cysts are the result of periapical inflammation that occurs due to pulpal necrosis in a tooth (the pulp of a tooth is the central part of a tooth that contains odontoblasts).

Periapical cysts are usually less than 1 cm in diameter and are bordered by a thin rim of cortical bone. The radiographic appearance of periapical cysts and granulomas is similar. Periapical cysts are less common and often larger. Periapical cysts are treated by enucleation of the cyst lining and root canal.

In the case shown here, the periapical cyst has eroded into the maxillary sinus.

References

Scholl RJ, et al. Cysts and Cystic Lesions of the Mandible: Clinical and Radiologic-Histopathologic Review. Radiographics. 1999;19:1107-1124.

Differentiating Primary CNS Lymphoma from Toxoplasmosis

Cerebral toxoplasmosis and primary CNS lymphoma can present similarly both clinically and radiologically. PET and Thallium-201 brain SPECT can be used to help differentiate them.

Thallium-201 is sensitive for lesions larger than 2 cm. Initial and delayed (~4 hours) images are obtained. Lymphoma demonstrates increased activity on delayed images, while Toxoplasma demonstrates decreased activity over time.

With the advent of highly active antiretroviral therapy (HAART), however, new studies have found decreased specificity of Thallium-201 brain SPECT, presumably because of the improved immune response to Toxoplasmosis, resulting in increased uptake due to inflammatory cells. The increased numbers of patients with toxoplasmosis who demonstrate elevated uptake on Thallium-201 brain SPECT may make differentiation of toxoplasmosis and primary CNS lymphoma more difficult.

On PET, lymphoma shows high standardized uptake values (usually more than 3.5), while toxoplasmosis usually does not. PET has higher resolution than SPECT and can be used for smaller lesions. Of note: corticosteroid treatment may decrease F-18 FDG activity on PET.

References

• Giancola ML, Rizzi EB, Schiavo R, Lorenzini P, Schininà V, Alba L, Del Grosso B, Gigli B, Rosati S, Mango L, Bibbolino C, Antinori A. Reduced value of thallium-201 single-photon emission computed tomography in the management of HIV-related focal brain lesions in the era of highly active antiretroviral therapy. AIDS Res Hum Retroviruses. 2004 Jun;20(6):584-8.
• Lorberboym M, et al. Rapid Differential Diagnosis of Cerebral Toxoplasmosis and Primary Central Nervous System Lymphoma by Thallium-201 SPECT. The Journal of Nuclear Medicine. 1996 37 (7);1150-1154.

Tolosa-Hunt Syndrome and Idiopathic Orbital Inflammatory Pseudotumor

Tolosa-Hunt syndrome and idiopathic orbital inflammatory pseudotumor are both chronic granulomatous diseases with common clinical characteristics (painful ophthalmoplegia) and response to corticosteroid therapy. Tolosa-Hunt is characterized by inflammation of the cavernous sinus and/or superior orbital fissure, while idiopathic orbital inflammatory pseudotumor involves the orbit. There may be territorial overlap of the two entities.

Contrast-enhanced, thin-slice (axial and coronal) MRI of the cavernous sinus and fat-suppressed pre- and post-contrast images of the orbits are recommended for evaluation.

Tolosa-Hunt syndrome

Imaging may be normal or demonstrate inflammatory changes in the cavernous sinus, superior orbital fissure, and/or orbit. Suprasellar and parasellar tumor invasion of the cavernous sinus may have a similar appearance. Enlargement of the optic nerve or extraocular muscles may also bee seen. MRA may show narrowing of the cavernous portion of the internal carotid artery.

Inflammatory orbital pseudotumor

Inflammatory orbital pseudotumor most commonly involves the extraocular muscles and/or lacrimal gland. The margins are irregular and the lesion may look infiltrative, mimicking neoplasm or infection.

The most common pattern is termed myositic and involves the extraocular muscles (superior complex and medial rectus most common). The tendinous insertions are also involved, differentiating this entity from thyroid-associated orbitopathy (tendons are spared).

Isolated lacrimal gland involvement (case shown here) is the second most common pattern, with diffuse enlargement of gland in the anteroposterior dimension. Differential considerations include sarcoidosis and lymphoproliferative disorders.

Another pattern of involvement is isolated to the globe and retrobulbar orbit. There is thickened sclera with heterogeneous enhancement and variable involvement of the retrobulbar fat and optic nerve.

The least common pattern involves irregular thickening and enhancement of the optic nerve sheath. Differential considerations include optic neuritis.

A diffuse pattern may also be observed, involving several of the above patterns.

Reference

• Taylor DC. Tolosa-Hunt Syndrome. eMedicine, Mar 12, 2009.
• Wasmeier C, Pfadenhauer K, Rösler A. Idiopathic inflammatory pseudotumor of the orbit and Tolosa-Hunt syndrome--are they the same disease? J Neurol. 2002 Sep;249(9):1237-41.