Absent nasal septum

Cocaine
Wegener's granulomatosis
Surgery
Trauma
Syphillis
Sarcoidosis

Retroperitoneal lesions

Fat containing:
Pelvic lipomatosis
Lipoma
Lipoblastoma (paediatric)
Liposarcoma
Hibernoma
Teratoma
Myelolipoma
Angiomyolipoma

Reference:
Craih WD et al. Fat-containing Lesions of the Retroperitoneum: Radiologic-Pathologic Correlation. RadioGraphics 2009;29:261-290. link: http://radiographics.rsnajnls.org/cgi/content/abstract/29/1/261

Tracheobraonchial tumours

Malignant:
SCC, adenoid cystic carcinoma, adenocarcinoma, neuroendocrinal (carcinoid), sarcoma, lymphoma

Invasion:
from thyroid, larynx, lung or oesophageal carcinoma

Metastasis:
melanoma, breast, renal cell carcinom, colonic carcinoma

Benign:
papillomatosis, mesenchymal, mucus gland, salivary gland, neurogenic tumors

Reference:
Park CM et al. Tumors in the Tracheobronchial Tree: CT and FDG PET Features. RadioGraphics 2009;29:55-71. Link: RadioGraphics 2009;29:55-71

Differentiating acute and chronic pulmonary embolism

Acute PE: diameter of pulmonary artery may be increased
Chronic PE: diameter of pulmonary artery may be decreased

Acute PE: may be central or eccentric; if eccentric, the filling defect shows acute angle with vessel wall; if central, defect may be surrounded by contrast
Chronic PE: peripheral crescent defect making obtuse angles with vessel wall

Acute PE atteniuation value (about 30 HU) is less than chronic PE (about 85 HU); this may be due to thrombus enhancement, iron and calcium.

Additional signs in acute PE: dilated right ventricle, deviation of interventricular septum towards left ventricle (right ventricular strain)

Additional signs in chronic PE: dilated bronchial arteries, dilated intercostal arteries, right ventricular hypertrophy.

Reference:
http://radiographics.rsnajnls.org/cgi/content/abstract/29/1/31
Castañer et al. CT Diagnosis of Chronic Pulmonary Thromboembolism. RadioGraphics 2009;29:31-50

Renal sinus

Sinus fat: Renal sinus lipomatosis, Replacement lipomatosis
Renal sinus cyst: Peripelvic cyst (often bilateral, multiple, small, confluent, non-interconnected cysts), Parapelvic cyst (usually single, large, mostly from adjacent parenchyma), Parapelvic lymphatic cyst, parapelvic lymphangiectasia
Vascular: renal artery aneurysm, AVM, AVF, renal varices
Inflammation: spread of infection, especially fungal
Benign: leiomyoma, hemangioma, hemangiopericytoma
Malignant: TCC, SCC, leiomyosarcoma, metastasis, lymphoma
Others: Urinoma, Haematoma, multilocular cystic nephroma

Reference:
http://radiographics.rsnajnls.org/cgi/content/full/24/suppl_1/S117

Leptomeningeal thickening

Leptomeningeal thickening may or may not enhance. Leptomengeal thickening secondary to , inflammation, tumour usually enhance. Leptomeningeal thickening secondary to SAH or craniotomy may not enhance. I have not included dural tail sign here.

Inflammation:
Histiocytosis X: paediatric
Sarcoidosis
Wegener's granulomatosis

Infection:
Meningitis - bacterial, TB, fungal, syphillis

Tumor:
Primary leptomeningeal glioblastomatosis
Primary leptomeningeal gliosarcomatosis
Sarcoma
Medulloblastoma - paediatric
Germinoma - paediatric
Pineoblastoma
Rupture of epidermoid - chemical meningitis
Meningeal carcinomatosis - breast, leukemia, lymphoma. lung, melanoma, GI, GU

Haemorrhage:
Old SAH

Post-Op:
Craniotomy scarring
Post LP

Mandibular lesions

Cystic lesions
radicular cyst = periapical cyst - commonest - around the apex
dentigenorus cyst = follicular cyst - very common - around unerupted tooth
odontogenic keratocyst - body or ramus of mandible, expand/erode cortex
Stafne's cyst = static bone cavity - medial surface of the posterior mandible, filled with salivary gland. not true cyst
residual cyst - post op
premordial cyst - cyst in place of tooth
medullary pseuso cyst
SBC, ABC - like in other bones

Osseous lesions:
Odontogenic:
Benign:
odontoma - most common, between teeth, may be a/w impacted tooth, simple or compund
amelablastoma - posterior mandible - 3rd molar, soap bubble, +/- cortical erosion,
odontogenic myxoma, cementoblastoma, etc
Malignant: odontogenic carcinoma, odontogenic sarcoma, etc
Non-odontogenic:
Benign: ossigying fibroma, cemento-osseous dysplasias, exostosis - torus mandibularis, osteoma, FD, Pagets
Malignant: OS, CS, mets, lymphoma, leukemia, myeloma/plasmacytoma (rare), SCC

Soft tissue lesions:
Benign: NF, schwannoma, AVM, gaint cell reparative cyst, hemangioma,

Metabolic diseases:
osteoporosis, osteomalacia, renal osteodystrophy, osteitis fibrosa cystica

References:
Dunfee BL et al. Radiologic and Pathologic Characteristics of Benign and Malignant Lesions of the Mandible. RadioGraphics 2006;26:1751-1768