Viral capsid assembly and stability in tailed, dsDNA phage and Herpesviridae are achieved by various means including chemical cross-links (unique to HK97), or auxiliary proteins (lambda, T4, ϕ29, and herpesviruses). All these viruses have coat proteins (CP) with a conserved, HK97-like core structure. We used a combination of trypsin digestion, gold-labeling, cryo-electron microscopy, 3D image reconstruction, and comparative modeling to derive two independent, pseudo-atomic models of bacteriophage P22 CP: before and after maturation. P22 capsid stabilization results from inter-subunit interactions among N-terminal helices and an extensive “P-loop”, which obviate the need for cross-links or auxiliary proteins. P22 CP also has a telokin-like, Ig domain that stabilizes the monomer fold so that assembly may proceed via individual subunit addition rather than via preformed capsomers as occurs in HK97. Hence, the P22 CP structure may be a paradigm for understanding how monomers assemble in viruses like ϕ29 and HSV-1.

Meckel’s cave is a dural recess in the posteromedial portion of the middle cranial fossa that acts as a conduit for the trigeminal nerve between the prepontine cistern and the cavernous sinus, and houses the Gasserian ganglion and proximal rootlets of the trigeminal nerve. It serves as a major pathway in perineural spread of pathologies such as head and neck neoplasms, automatically upstaging tumours, and is a key structure to assess in cases of trigeminal neuralgia. The purpose of this pictorial review is threefold: (1) to review the normal anatomy of Meckel’s cave; (2) to describe imaging findings that identify disease involving Meckel’s cave; (3) to present case examples of trigeminal and non-trigeminal processes affecting Meckel’s cave.Teaching points• Meckel’s cave contains the trigeminal nerve between prepontine cistern and cavernous sinus.• Assessment is essential for perineural spread of disease and trigeminal neuralgia.• Key imaging: neural enhancement, enlargement, perineural fat/CSF effacement, skull base foraminal changes.

Purpose Increasing use of advanced imaging in the emergency department (ED) has resulted in higher cost without better outcomes. Our goal was to evaluate the yield of CT head exams by scenario to guide efforts at improving patient selection. Methods We performed a retrospective study at an academic medical center over 4 years (1/1/2014-12/31/2017). The chief complaint, imaging order, and exam result text were obtained for all adult ED encounters. For the 50 most common chief complaints leading to CT head exams, the ratio of exams to total encounters and ratio of critical results to imaging studies were calculated. Significant difference in "yield" was assessed via binomial test. Results Over 708,145 adult ED encounters, 58,783 CT head exams were ordered, with an overall critical result yield of 8.0%. The three most common chief complaints had higher yield (p < 0.05): altered mental status (9.8%), fall (9.7%), and new headache (10.1%). Lower yield (p < 0.05) was found for 19 chief complaints: dizziness (6.2%), falls in patients > 65 years old (7.1%), syncope (5.3%), seizure with known epilepsy (4.8%), chest pain (3.7%), head injury (4.9%), headache re-evaluation (7.0%), alcohol intoxication (2.5%), fatigue (6.5%), headache-recurrent or in the setting of known migraines (5.2%), hypertension (4.4%), lethargy (5.8%), loss of consciousness (5.3%), migraine (3.2%), psychiatric evaluation (2.9%), near syncope (4.6%), drug problem (3.1%), symptomatically decreased blood sugar (3.2%), and suicidal (1.7%). ConclusionOur study provides a priority list of low yield scenarios of CT head use for improvement of patient selection.

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