Gross, Regional & Cross-Sectional CT/MRI Anatomy, Skull Foramina & Neurovascular Compartments

The posterior cranial fossa extends from the dorsum sellae and petrous temporal ridges to the internal occipital protuberance. On the posteromedial aspect of the petrous temporal bone, the internal acoustic meatus transmits the facial nerve (CN VII), vestibulocochlear nerve (CN VIII), and the labyrinthine artery (typically branching from the anterior inferior cerebellar artery). Vestibular schwannomas expanding within the internal acoustic meatus and cerebellopontine angle compress CN VIII early (causing sensorineural hearing loss, tinnitus, and disequilibrium), followed by CN VII (ipsilateral lower motor neuron facial palsy) and CN V (loss of the afferent corneal blink reflex). Inferior to the petrous ridge, the jugular foramen is formed between the temporal and occipital bones and is divided into an anteromedial pars nervosa (CN IX and inferior petrosal sinus) and a posterolateral pars vascularis (CN X, CN XI, and the sigmoid sinus transitioning into the internal jugular vein). Vernet syndrome (jugular foramen syndrome), classically triggered by a glomus jugulare tumor or basal skull fracture, produces ipsilateral loss of the gag reflex, dysphagia, hoarseness from vocal cord paralysis, soft palate drooping with uvular deviation to the contralateral side, and weakness of the sternocleidomastoid and trapezius muscles.

The cavernous sinuses are paired trabeculated dural venous spaces flanking the sella turcica and sphenoid air sinuses. Along the lateral dural wall of each cavernous sinus course four nerves arranged superior to inferior: the oculomotor nerve (CN III), the trochlear nerve (CN IV), the ophthalmic nerve (CN V1), and the maxillary nerve (CN V2). Travelling directly through the central venous blood space of the cavernous sinus are the cavernous segment of the internal carotid artery (wrapped by the postganglionic sympathetic carotid plexus) and, immediately lateral to the artery, the abducens nerve (CN VI). Because CN VI is suspended freely within the trabeculated lumen rather than sheltered within the lateral dural wall, septic cavernous sinus thrombosis (often secondary to facial skin infections in the danger triangle draining via valveless facial and ophthalmic veins) or internal carotid-cavernous aneurysms produce isolated lateral rectus palsy (CN VI) as the earliest ocular motor deficit, accompanied by chemosis, proptosis, periorbital edema, and forehead/corneal sensory loss (CN V1).

Cross-sectional thoracic computed tomography divides the mediastinum into three primary functional compartments below the sternal angle of Louis (T4-T5 junction). The anterior (prevascular) mediastinum lies posterior to the sternum and anterior to the pericardium and great vessels; its classic differential diagnosis comprises the four Ts: Thymoma (strongly associated with myasthenia gravis and pure red cell aplasia), Teratoma and germ cell tumors (showing fat, calcifications, or elevated beta-hCG/alpha-fetoprotein), Thyroid substernal goiter, and Terrible lymphoma (Hodgkin or non-Hodgkin lymphadenopathy). The middle (visceral) mediastinum contains the heart, pericardium, ascending aorta, aortic arch, superior and inferior venae cavae, main pulmonary arteries, trachea, carina, mainstem bronchi, and phrenic and vagus nerves; masses here are typically bronchogenic cysts, pericardial cysts, or sarcoid/metastatic hilar lymphadenopathy. The posterior (paravertebral) mediastinum lies posterior to the pericardium and trachea, housing the descending thoracic aorta, esophagus, thoracic duct, azygos and hemiazygos veins, and thoracic sympathetic chain; masses in this compartment are predominantly neurogenic tumors (schwannomas, neurofibromas, neuroblastomas, ganglioneuromas) arising from intercostal nerves or the sympathetic chain.

The thoracoabdominal diaphragm separates the thoracic and abdominal cavities and is innervated both motorically and sensorily (central tendon) by the phrenic nerve (C3, C4, C5). Irritation of the central diaphragmatic pleura or peritoneum (such as from a subphrenic abscess, ruptured spleen with hemoperitoneum, or gallbladder perforation) refers sharp somatic pain to the ipsilateral C3-C5 supraclavicular dermatome over the shoulder tip (Kehr sign). Three major apertures traverse the diaphragm at sequential thoracic vertebral levels. At T8, the caval opening passes through the central tendon to transmit the inferior vena cava and branches of the right phrenic nerve; contraction of the diaphragm during inspiration widens the tendinous caval ring, augmenting venous return to the right atrium. At T10, the esophageal hiatus passes through the muscular right crus of the diaphragm (forming a physiological lower esophageal sphincter sling) and transmits the esophagus, anterior (left) and posterior (right) vagal trunks, and esophageal branches of the left gastric artery and vein. At T12, the aortic hiatus passes posterior to the median arcuate ligament between the diaphragmatic crura, transmitting the aorta, thoracic duct, and azygos vein without compression during diaphragmatic contraction.

Three major arterial collateral networks interconnect the unpaired visceral trunks. First, between the celiac trunk and the SMA, the superior pancreaticoduodenal artery (from the gastroduodenal branch of the common hepatic artery) anastomoses with the inferior pancreaticoduodenal artery (the first branch of the SMA) around the head of the pancreas; this arcade spares the duodenum from ischemia during isolated proximal celiac or SMA stenosis. Second, between the SMA and the IMA, the marginal artery of Drummond and the central meandering artery of Riolan connect the middle colic artery (SMA) with the ascending branch of the left colic artery (IMA). However, this anastomosis is tenuous at the splenic flexure (Griffith point), making the splenic flexure the most common site of non-occlusive ischemic colitis during systemic hypotension or cardiogenic shock. Third, between the IMA and the internal iliac arterial system, the sigmoidal arteries anastomose with the superior rectal artery at the rectosigmoid junction (Sudeck point), while the superior rectal artery (IMA) anastomoses with the middle and inferior rectal arteries (internal iliac/pudendal) across the pectinate line.

The hepatic portal vein is formed posterior to the neck of the pancreas by the confluence of the splenic vein and the superior mesenteric vein (with the inferior mesenteric vein typically draining into the splenic vein). In portal hypertension (most commonly from hepatic cirrhosis), elevated portal venous pressure decompresses through four clinically critical portosystemic anastomoses: (1) at the distal esophagus, where the left gastric (coronary) vein (portal) anastomoses with esophageal tributaries of the azygos system (systemic), producing life-threatening gastroesophageal varices; (2) at the paraumbilical region, where paraumbilical veins in the ligamentum teres hepatis (portal) anastomose with superficial and inferior epigastric veins (systemic), producing caput medusae; (3) at the anorectal junction across the pectinate line, where the superior rectal vein (portal) anastomoses with middle and inferior rectal veins (systemic), producing internal rectal varices; and (4) in the retroperitoneum, where colic/splenic veins anastomose with renal and lumbar veins (shunts of Retzius). Retroperitoneal structures (mnemonic SAD PUCKER: Suprarenal glands, Aorta/IVC, Duodenum 2nd-4th parts, Pancreas head/body, Ureters, Colon ascending/descending, Kidneys, Esophagus, Rectum) lie posterior to the parietal peritoneum; hemorrhage or perforation of these structures tracks within the anterior pararenal, perirenal, or posterior pararenal spaces without initially causing generalized peritoneal rebound tenderness or free subdiaphragmatic air.

Distinguishing groin hernias relies on their anatomical neck relative to two invariant landmarks: the inferior epigastric vessels and the inguinal ligament. An indirect inguinal hernia—the most common hernia overall in both males and females—arises from failure of the embryonic processus vaginalis to obliterate. Abdominal viscera enter the deep inguinal ring lateral to the inferior epigastric vessels, traverse the entire length of the inguinal canal enclosed within all three fascial coverings of the spermatic cord (internal spermatic fascia from transversalis fascia, cremasteric fascia from internal oblique, and external spermatic fascia from external oblique aponeurosis), and can exit the superficial ring directly into the scrotum or labia majora. A direct inguinal hernia results from acquired weakness of the transversalis fascia in older men (chronic cough, benign prostatic hyperplasia straining, heavy lifting); it protrudes directly through Hesselbach triangle medial to the inferior epigastric vessels, is covered only by peritoneum, transversalis fascia, and external spermatic fascia, and rarely enters the scrotum. Both inguinal hernias present with a bulge superior and medial to the pubic tubercle. Conversely, a femoral hernia protrudes inferior to the inguinal ligament and inferolateral to the pubic tubercle through the femoral ring (medial to the femoral vein, lateral to the sharp, rigid lacunar ligament); although less common overall, femoral hernias occur predominantly in multiparous females and carry the highest incidence of incarceration and strangulation due to the unyielding bony-ligamentous boundaries of the femoral ring.

The pelvic floor is supported primarily by the pelvic diaphragm, composed of the levator ani muscle group (puborectalis, pubococcygeus, and iliococcygeus) and the coccygeus muscle, innervated by direct branches of S2-S4 and the perineal branch of the pudendal nerve. The puborectalis forms a muscular U-shaped sling around the anorectal junction that maintains the 90-degree anorectal angle for fecal continence. Inferior to the pelvic diaphragm lies the perineum, divided into an anterior urogenital triangle and a posterior anal triangle. The pudendal nerve (S2, S3, S4—keeps the pelvis off the floor) exits the pelvis through the greater sciatic foramen inferior to the piriformis muscle, hooks posteriorly around the ischial spine and sacrospinous ligament, and re-enters the perineum through the lesser sciatic foramen to travel within the pudendal (Alcock) canal on the lateral wall of the ischioanal fossa. During obstetric transvaginal pudendal nerve block, the needle is guided transvaginally just posteromedial to the palpable ischial spine through the sacrospinous ligament to anesthetize the perineum before operative vaginal delivery or episiotomy repair.

Three intermuscular apertures between the scapula, humerus, teres minor, teres major, and long head of the triceps brachii organize the posterior shoulder neurovasculature. The quadrangular space is bounded superiorly by the teres minor (and subscapularis anteriorly), inferiorly by the teres major, medially by the long head of the triceps brachii, and laterally by the surgical neck of the humerus; it transmits the axillary nerve (C5-C6, innervating deltoid and teres minor and supplying regimental badge lateral shoulder sensation) and the posterior circumflex humeral artery. Surgical neck fractures of the humerus or anterior glenohumeral dislocations injure these structures within the quadrangular space. Medial to the long head of the triceps lies the triangular space (bounded superiorly by teres minor, inferiorly by teres major, and laterally by the long head of triceps), which transmits the circumflex scapular artery (participating in the collateral anastomotic network around the scapula). Inferior to the teres major lies the triangular interval (bounded superiorly by teres major, medially by the long head of triceps, and laterally by the humeral shaft), which transmits the radial nerve (C5-T1) and the profunda brachii (deep brachial) artery as they enter the spiral (radial) groove of the posterior humerus.

At the volar wrist, the transverse carpal ligament (flexor retinaculum) bridges the pisiform and hook of the hamate medially to the scaphoid tubercle and trapezium laterally, converting the osseous carpal arch into the enclosed carpal tunnel. Ten structures traverse the carpal tunnel: four tendons of the flexor digitorum superficialis, four tendons of the flexor digitorum profundus, the tendon of the flexor pollicis longus, and the median nerve (C5-T1). Compression of the median nerve within the carpal tunnel causes nocturnal paresthesias in the palmar aspect of the lateral 3.5 digits and weakness/atrophy of the thenar eminence (abductor pollicis brevis, opponens pollicis, superficial head of flexor pollicis brevis—mnemonic LOAF includes the lateral two lumbricals), while sparing sensation over the central thenar palm because the palmar cutaneous branch of the median nerve arises 5 cm proximal to the wrist and travels superficial to the flexor retinaculum. The ulnar nerve and ulnar artery bypass the carpal tunnel entirely, coursing superficial to the flexor retinaculum and lateral to the pisiform through the ulnar canal (Guyon canal) beneath the volar carpal ligament. Compression within Guyon canal (handlebar palsy in cyclists or hook of hamate fracture) causes sensory loss in the medial 1.5 digits (both palmar and dorsal only if proximal, though dorsal ulnar cutaneous branch is usually spared at the wrist) and intrinsic hand weakness (interossei, hypothenar muscles, adductor pollicis, medial two lumbricals).

The popliteal fossa is a diamond-shaped depression posterior to the knee joint bounded superolaterally by the biceps femoris tendon, superomedially by the semimembranosus and semitendinosus tendons, and inferolaterally and inferomedially by the lateral and medial heads of the gastrocnemius muscle. Within the popliteal fossa, the major neurovascular structures are arranged from superficial (posterior) to deep (anterior, immediately against the posterior capsule of the knee joint and popliteal surface of the femur): the tibial nerve and common fibular (peroneal) nerve most superficially, the popliteal vein at the intermediate level, and the popliteal artery deepest against the bone. Because the popliteal artery is tethered proximally at the adductor hiatus and distally at the tendinous arch of the soleus muscle right against the posterior knee capsule, supracondylar fractures of the femur or posterior knee dislocations carry an extraordinarily high risk of popliteal artery intimal tear, thrombosis, or transection, requiring immediate ankle-brachial index assessment and CT angiography even if distal pulses initially appear palpable. Laterally, the common fibular nerve exits the fossa by wrapping superficially around the neck of the fibula, where fibular neck fractures or tight plaster casts compress it, causing foot drop (deep fibular nerve) and loss of foot eversion and dorsal foot sensation (superficial fibular nerve).

The lower leg is partitioned by the tibia, fibula, interosseous membrane, and crural intermuscular septa into four unyielding osteofascial compartments: (1) the Anterior compartment (tibialis anterior, extensor hallucis longus, extensor digitorum longus, fibularis tertius; supplied by the deep fibular nerve and anterior tibial artery; most frequently affected in acute compartment syndrome after tibial shaft fractures, presenting with pain on passive toe flexion and first webspace paresthesia); (2) the Lateral compartment (fibularis longus and brevis; supplied by the superficial fibular nerve and perforating branches of the fibular artery); (3) the Superficial posterior compartment (gastrocnemius, soleus, plantaris; supplied by the tibial nerve); and (4) the Deep posterior compartment (popliteus, tibialis posterior, flexor digitorum longus, flexor hallucis longus; supplied by the tibial nerve and posterior tibial/fibular arteries). As the deep posterior compartment tendons and neurovascular bundle pass posteroinferior to the medial malleolus beneath the flexor retinaculum (laciniate ligament), they traverse the tarsal tunnel in a strict anteromedial-to-posterolateral order (mnemonic: Tom, Dick, And Very Nervous Harry): Tibialis posterior tendon, flexor Digitorum longus tendon, posterior tibial Artery, posterior tibial Vein, tibial Nerve, and flexor Hallucis longus tendon. Entrapment of the tibial nerve in the tarsal tunnel produces burning paresthesias across the plantar aspect of the foot and toes (via the medial and lateral plantar nerves), exacerbated by prolonged standing or dorsiflexion-eversion of the ankle.