Anatomy of Medical Imaging

Medical imaging has transformed clinical anatomy from a purely cadaveric discipline into a dynamic, in-vivo science. Every imaging modality — plain radiograph, computed tomography (CT), magnetic resonance imaging (MRI), or ultrasound — displays anatomy through a different physical principle, with different tissue contrasts, spatial resolutions, and clinical applications. Interpreting imaging requires knowing what normal anatomy looks like in each modality, recognising key landmarks, and understanding the relationship between the imaging plane and the underlying three-dimensional structure. This guide is for educational purposes only.

## Plain Radiograph (X-Ray): Principles and Reading Strategy

Conventional radiography records differential attenuation of X-ray photons by tissues. Dense tissues (bone, calcification, metal) attenuate many photons and appear white (radiopaque); air attenuates few photons and appears black (radiolucent); soft tissues and fluids appear in intermediate grey. The five radiographic densities from most to least dense are: metal > bone/calcium > soft tissue/water > fat > air.

A systematic approach to reading any X-ray prevents errors. For a chest radiograph: assess technical quality (rotation — the medial ends of the clavicles should be equidistant from the spinous processes — and inspiration — at least six anterior ribs should be visible above the diaphragm), then survey the structures in sequence. The mediastinum contains the trachea (midline, deviating away from tension, towards collapse or effusion), the carina (the bifurcation of the trachea at T4–T5, where the angle — normally less than 70 degrees — widens with left atrial enlargement), the aortic knuckle (the aortic arch visible in the left upper mediastinum), and the cardiac silhouette. The right heart border is formed by the right atrium; the left heart border by the left atrial appendage (just below the pulmonary trunk) and the left ventricle. The costophrenic angles are normally acute and clear; blunting indicates a pleural effusion of at least 300 mL. The lung fields should be assessed for increased density (consolidation, effusion, tumour) or increased lucency (pneumothorax, emphysema).

On an abdominal radiograph, the key landmarks are the psoas shadows (bilateral, linear soft-tissue densities along the lumbar spine — loss of the psoas shadow can indicate retroperitoneal pathology), the liver (right upper quadrant solid soft-tissue density), the gas pattern of the small bowel (central, with valvulae conniventes — thin circular folds spanning the full width of the bowel) versus the large bowel (peripheral, with haustral folds that only partially cross the bowel), and the pelvis (bladder, visible as a suprapubic soft-tissue density when full).

## CT: Cross-Sectional Anatomy

CT uses X-rays acquired in multiple detector rows while the gantry rotates around the patient, generating cross-sectional images reconstructed in axial, coronal, and sagittal planes (and obliques). CT values — Hounsfield units (HU) — quantify tissue density relative to water (water = 0 HU, air = –1000 HU, cortical bone = +400 to +1000 HU, fat = –100 to –50 HU, blood = 40–60 HU, enhanced vessels after contrast = 150–400 HU). Window width and level settings determine which density range is displayed optimally — a lung window (wide width, low centre) renders airways and vessels clearly; a bone window (wide width, high centre) reveals bony detail; a soft-tissue window (intermediate width and level) displays solid organs.

Axial CT sections of the thorax at the level of the aortic arch display: anteriorly, the ascending aorta (right) and the pulmonary trunk (left); the superior vena cava (right lateral); the trachea (midline posterior, oval air column); the oesophagus (posterior to the trachea, often collapsed and subtle); the descending thoracic aorta (left posterior); and the thoracic vertebral body and spinal canal. At the level of the heart, axial sections display the four chambers (right ventricle anterior, left ventricle posterior-left, right atrium right, left atrium posterior-right, the atria separated by the interatrial septum and the ventricles by the interventricular septum), the coronary arteries (visible only with cardiac-gated CT angiography), and the pericardium (a thin hyperdense line surrounding the heart).

Abdominal CT landmarks in axial section: at T12/L1, the coeliac trunk (the first major abdominal aortic branch) arises anteriorly and divides almost immediately into the common hepatic, left gastric, and splenic arteries. At L1, the superior mesenteric artery (SMA) arises; at L1–L2, the renal arteries arise laterally to reach the renal hila (the left renal artery passes posterior to the left renal vein). The aorta bifurcates at L4. CT is the gold standard for evaluating the abdominal aorta, mesenteric vasculature, solid organs, and retroperitoneum.

## MRI: Sequences and Anatomy

MRI exploits the magnetic properties of hydrogen nuclei (protons) in tissue water and fat. Different pulse sequences manipulate radiofrequency pulses and gradients to produce images with different tissue contrasts. In a T1-weighted sequence, fat is bright (white), fluid is dark, and anatomy is displayed with excellent soft-tissue contrast — T1 is ideal for anatomy, gadolinium-enhanced lesion detection, and marrow assessment. In a T2-weighted sequence, fluid is bright ("water is white on T2"), fat is bright but less so, and solid tissues are darker — T2 is ideal for demonstrating oedema, joint fluid, CSF, and pathological processes that increase tissue water content (tumour, infarction, inflammation). FLAIR (fluid-attenuated inversion recovery) is a T2 variant that nulls free fluid signal, allowing periventricular white matter lesions (e.g., in multiple sclerosis) to be seen that would otherwise be obscured by bright CSF.

MRI of the brain in axial T2 displays the bilateral cerebral hemispheres separated by the interhemispheric fissure (containing the falx cerebri), the basal ganglia (caudate nucleus medially, putamen laterally, separated by the anterior limb of the internal capsule; the posterior limb of the internal capsule separates the putamen and globus pallidus from the thalamus), the thalami flanking the third ventricle, the midbrain (with the characteristic "Mickey Mouse" appearance at the level of the superior colliculi — the cerebral peduncles forming the ears), the pons (the largest part of the brainstem on axial section, with the basilar artery anterior), and the cerebellum (posterior fossa, bilateral hemispheres separated by the vermis). White matter appears grey-white on T2; grey matter (cortex, basal ganglia, thalamus) appears slightly darker. Periventricular white matter hyperintensities on T2/FLAIR are common incidental findings in older adults and correlate with small vessel cerebrovascular disease.

## Ultrasound: Windows and Landmarks

Ultrasound uses high-frequency sound waves (2–18 MHz) reflected at tissue interfaces. It is real-time, portable, non-ionising, and excellent for soft-tissue, vascular, and abdominal imaging, but is limited by bone (which reflects all sound, creating acoustic shadowing) and air (which scatters sound, causing poor transmission). Tissue echogenicity is described relative to a reference: hyperechoic (brighter than reference — fat, fibrous tissue, calcification), isoechoic (similar), and hypoechoic (darker — muscle, fluid-containing structures). Anechoic describes a completely echo-free structure (simple fluid).

In the FAST (Focused Assessment with Sonography in Trauma) examination, four anatomical windows are interrogated for free fluid (haemoperitoneum appears anechoic). The right upper quadrant (RUQ) view displays the hepatorenal space (Morison's pouch) — the potential space between the liver and right kidney, the most dependent part of the peritoneal cavity in the supine position and therefore the first site to accumulate fluid. The left upper quadrant (LUQ) view displays the splenorenal space. The suprapubic view assesses the pelvis (pouch of Douglas in females, rectovesical pouch in males). The subxiphoid view displays the pericardial space. Paracentesis (peritoneal fluid aspiration) is performed under ultrasound guidance using the left iliac fossa (avoiding the inferior epigastric arteries — which run just lateral to the rectus abdominis — and the caecum on the right) or the flank approach.

## Normal vs Abnormal: Key Imaging Landmarks

Recognising normality is the foundation of detecting pathology. The cardiothoracic ratio (the width of the cardiac silhouette divided by the maximal internal thoracic diameter on a PA chest X-ray) should be less than 0.5; a ratio above 0.5 suggests cardiomegaly. The trachea should be midline (or very slightly right-deviated by the aortic arch); deviation suggests pneumothorax (away from the affected side — tension pneumothorax), large pleural effusion (away), or collapse/lobar atelectasis (towards). On MRI of the spine, the spinal cord occupies the centre of the spinal canal and ends as the conus medullaris at L1–L2 (identifiable on sagittal MRI); the cauda equina nerve roots below appear as linear low-signal structures in the high-signal CSF on T2. Disc herniations (prolapsed intervertebral disc) are identified on axial MRI as posterior disc material displacing or compressing the thecal sac and nerve roots — the anatomy of the disc and the direction of herniation (central, paracentral, foraminal, extraforaminal) determine which nerve root is affected and the corresponding clinical syndrome.