Cross-sectional Anatomy - CT and MRI

Overview

• Accurate cross-sectional interpretation rests on four interdependent pillars: (1) baseline anatomical appearances at each modality and plane; (2) the wide spectrum of normal variants, many simulating disease; (3) expected evolution of anatomy with age and physiological state; and (4) characteristic morphological signatures of pathological processes.
• At least 20% of individuals carry a clinically relevant anatomical variant.
• Errors in any pillar propagate into misdiagnosis, unnecessary investigation, or missed disease.

Normal Imaging Appearances by Modality

CT: Hounsfield Unit Reference

• Normal appearances vary with acquisition phase.
• In the portal-venous phase the liver is slightly denser than the spleen.
• The spleen enhances in an arciform (heterogeneous) pattern during the arterial phase, a normal finding frequently mistaken for pathology.
• Diagnostic catheter angiography has a bleeding-detection threshold of approximately 0.5 mL/min; MDCTA is more sensitive at approximately 0.35 mL/min, with nuclear medicine techniques (sulfur colloid scintigraphy ~0.1 mL/min; RBC scintigraphy ~0.2-0.4 mL/min) the most sensitive non-invasive methods.

MRI Sequence Characteristics

• T1-weighted sequences are the "anatomy sequences", fat is bright, cortical architecture is well defined, and most pathologies appear as low-signal intrusions.
• T2/STIR sequences are highly sensitive to pathological water content, including oedema, inflammation, and early neoplastic infiltration.
• T1 fat-saturation with gadolinium (T1FS post-Gd) exquisitely highlights hypervascularity and BBB breakdown.
• STIR and T2FS are superior for detecting soft-tissue and bone-marrow oedema accompanying pathological states.
• Metallic artefact reduction sequences (MARS) are preferred for imaging periprosthetic soft tissues.
• MRI can be performed during pregnancy with no known adverse fetal effect after appropriate clinical consideration.
• Gadolinium-based contrast agents are generally avoided in pregnancy as fetal effects are unknown; when essential, macrocyclic agents are preferred over linear agents.
• Gadolinium results in significant T1 shortening (contrast mechanism) and a small T2 effect.

Normal Variants Mimicking Disease

Brain and Skull Base

• Arachnoid granulations vs sinus thrombosis: Granulations follow CSF signal on T1, T2, and FLAIR.
• Acute thrombus diverges, T1 hyperintense, T2 variable, and lacks the smooth margins typical of granulations.

Sellar Region Normal Variants (High-Yield)

• Empty sella: CSF fills the sella; pituitary gland flattened against the floor; infundibulum intact and midline. Distinguished from a cystic sellar mass by the intact stalk and absence of a discrete wall.
• "Kissing" carotid arteries: Medially deviated ICA segments meeting at the midline, important surgical pitfall; must not be mistaken for an intrasellar mass.
• Pituitary physiological hyperplasia: Convex superior contour; height up to 10-12 mm in late pregnancy or puberty; uniform enhancement; normal infundibular position. Distinguished from adenoma by clinical context (pubertal or pregnant female) and uniform enhancement pattern.
• Rathke cleft cyst: Midline intra/suprasellar cyst; typically T1 hyperintense (proteinaceous), T2 variable; no solid enhancement; no calcification (contrasting with craniopharyngioma).

Spine

• Transitional lumbosacral anatomy is a critical pitfall.
• Sacralization of L5 (elongated transverse processes, unilateral or bilateral fusion to sacrum) or lumbarization of S1 must be explicitly identified, as incorrect level-counting leads to wrong-level surgery.
• Count from the cervicothoracic junction or sacrum on whole-spine sagittal sequences, referencing rib anatomy at the thoracolumbar junction.

• Uncovertebral joints (C3-C7): Normal articulations formed between the uncinate processes of the lateral superior edge of each vertebral body and the vertebral body above.
• Degenerative hypertrophy may encroach on the neural foramen and can be mistaken for an osseous tumour or loose body.

Chest

Thymus (paediatric): In young children normal thymic tissue is soft, malleable, and compliant, moulding to adjacent structures. Recognise:

• Thymic wave sign: Lobulated undulation along rib margins
• Sail sign: Triangular right thymic lobe with sharp lateral margin

Thymic involution during illness or steroid exposure may be dramatic. Rebound thymic hyperplasia following chemotherapy or steroid cessation produces a mass-like anterior mediastinal soft-tissue enlargement, must not be reported as recurrent lymphoma without clinical context and follow-up.

• Chest wall soft-tissue assessment on CT/MRI should account for rib and intercostal space anatomy.
• The diaphragm is best assessed with multiplanar reformats (MPR), maximum intensity projections (MIP), and 3D volume rendering.

Abdomen and Vasculature

• Mesenteric arterial variants are common at the celiac axis and SMA.
• Key variants include: replaced right hepatic artery (from SMA), replaced left hepatic artery (from left gastric artery), and common hepatic artery arising directly from the SMA.
• Misidentifying a replaced right hepatic artery as an enlarged lymph node or tumour in the hepatoduodenal ligament is a classic pitfall in oncological staging and pre-surgical planning.
• Transient hepatic attenuation differences (THAD): Geographic, wedge-shaped, subsegmental arterial-phase hyperenhancement resolving on portal-venous phase.
• Caused by arterioportal shunting or focal perfusion alteration (e.g. portal cavernoma, adjacent inflammation).
• Distinguished from hypervascular HCC by: geographic non-mass shape, portal venous phase resolution, absence of washout, no capsule.
• In the context of portal vein thrombosis/cavernous transformation, peripheral THAD reflects preferential arterial perfusion due to portoportal collateral redistribution.
• External auditory canal (EAC): Approximately 25 mm in length; fibrocartilaginous lateral third, osseous medial two-thirds; S-shaped course.
• Deficiencies in the inferior fibrocartilaginous portion (fissures of Santorini) can act as conduits for infection and tumour spread, a normal anatomical variant with direct pathological consequence.

Changing Appearances with Age

Brain

Myelination follows a predictable posterior-to-anterior, inferior-to-superior sequence. Assessment:

• T1: Myelinated white matter is bright before approximately 6 months of age
• T2: Myelinated white matter becomes dark (hypointense) after approximately 6 months; adult pattern largely complete by 2 years
• Incomplete or delayed myelination must be interpreted against developmental (corrected) age, particularly in premature infants.
• Premature infants should be assessed against corrected gestational age, not chronological age.

• Alzheimer disease pattern recognition: In typical (late-onset) AD, hippocampal and entorhinal atrophy predominate.
• In younger-onset AD, hippocampal sparing may occur with predominant precuneus and posterior cingulate/parietal cortex involvement, avoid false-negative reporting by recognising this atypical pattern.
• Vascular white matter lesions on MRI are an independent risk factor for dementia and co-exist with AD in many elderly patients.

Normal age-related brain changes relevant to variant recognition include:

• Physiological iron deposition in globus pallidus, substantia nigra, red nucleus, and dentate nucleus, appears as T2/SWI hypointensity; should not be overcalled as haemosiderin
• A Fazekas score of ≥2 (confluent periventricular or extensive deep white matter lesions) is considered abnormal and warrants clinical correlation

Spine

Modic endplate changes represent a progressive degenerative spectrum:

• Discriminating Modic Type I from discitis-osteomyelitis: Both show T1 low / T2 high endplate signal.
• Infection additionally demonstrates: disc T2 hyperintensity, loss of intranuclear cleft, loss of the dark cortical endplate band on T1, and contrast enhancement of the disc itself.
• Modic I change does not enhance within the disc space.

Breast

Fibroglandular tissue volume and background parenchymal enhancement (BPE) change with age, menstrual cycle phase, pregnancy, lactation, and exogenous hormone use. ACR BI-RADS breast composition categories:

• Density typically decreases after menopause.
• Increased density reduces mammographic sensitivity.
• MRI is the most sensitive modality for cancer detection in dense breasts (high-risk screening).
• During pregnancy and lactation, fibroglandular tissue is engorged and BPE is markedly increased, reducing specificity of enhancing lesions on breast MRI.

Physiological States Altering Normal Appearances

Morphological Changes of Pathological Tissues

Pathological processes alter anatomy according to nine fundamental morphological categories (after the radiological pathology tutorial framework):

1. Added to: tumour, haematoma, abscess, calcification
2. Missing: resection, congenital absence, avascular necrosis with collapse
3. Enlarged: lymphadenopathy, organ hypertrophy, aneurysm
4. Shrunken: atrophy, cirrhosis, radiation fibrosis
5. Pushed / displaced: extrinsic mass compression
6. Pulled / tethered: fibrosis, desmoplasia, adhesion
7. Local structural abnormality: fracture, erosion, perforation, fistula
8. Diffuse structural abnormality: infiltrative malignancy, diffuse oedema, marrow replacement
9. Abnormal contour: spiculation, lobulation, irregular margin

CT and MRI Signal as Pathological Markers

Systematic Approach to Cross-Sectional Reporting

1. Technical adequacy: phase of acquisition, field-of-view, sequences obtained, contrast agent used, artefacts present
2. Dominant finding: location, morphology, size, margin, density/signal, enhancement pattern
3. Modality-specific characterisation: HU measurement on CT; signal on each MRI sequence with and without fat saturation
4. Secondary signs: mass effect, perilesional oedema, bone erosion, adenopathy, metastatic disease
5. Variant knowledge: is this finding in a known variant location? Does it follow a normal structure's signal completely?
6. Age and physiology context: expected for this patient's age, sex, hormonal, and clinical state?
7. Differential diagnosis: ranked by likelihood; identify discriminating features
8. Recommendation: further sequences, follow-up interval, or additional investigation if uncertainty remains

Differential Diagnosis Framework

Key Pitfalls and Errors

1. Overcalling normal variant as pathology: Prominent perivascular spaces, arachnoid granulations, splenic arciform arterial enhancement, THAD, and rebound thymic hyperplasia are among the most frequently overcalled entities.

2. Undercalling pathology as variant: Subtle Modic Type I misread as normal; early perineural spread along cranial nerve foramina; periosteal reaction dismissed as variant ossification.

3. Age-inappropriate interpretation: Applying adult myelination criteria to a neonate; over-diagnosing atrophy in a young brain; under-diagnosing hippocampal atrophy in an elderly patient because "atrophy is expected."

4. Physiological enhancement misread: Pregnant or pubertal pituitary convexity misreported as adenoma; secretory endometrium mistaken for endometrial lesion; post-exercise muscle T2 signal attributed to myositis.

5. Phase-of-contrast timing errors: Arterial-phase hepatic heterogeneity (arciform splenic enhancement, THAD) reported as mass; cortical-medullary differentiation on early nephrographic phase misattributed to pyelonephritis.

6. Incorrect spinal level counting: Transitional lumbosacral anatomy without explicit level-counting methodology leads to wrong-level surgery, always document counting method.

7. Susceptibility artefact overread: Normal iron deposition in the globus pallidus and substantia nigra on SWI/GRE mistaken for haemosiderin from a vascular malformation or prior haemorrhage.

8. DWI T2 shine-through: High DWI signal with correspondingly high ADC indicates T2 prolongation (e.g. chronic cyst, vasogenic oedema), not true diffusion restriction. Always correlate DWI with ADC map.

9. Gadolinium deposition: T1 hyperintensity in dentate nucleus and globus pallidus on unenhanced T1 in patients with prior gadolinium administrations. Linear agents carry greater deposition risk than macrocyclic agents. Note but do not pathologise in isolation.

10. Post-treatment morphology: Post-surgical, post-radiation, and post-ablation appearances of the sellar region, liver, spine, and soft tissues frequently mimic recurrence. Enhancement alone is insufficient, correlate with baseline imaging, clinical timeline, and functional sequences (DWI, perfusion, MR spectroscopy).

11. Head and neck pitfalls: Widening of cranial nerve foramina (perineural spread), middle ear ossicular erosion (cholesteatoma), and subtle laryngeal cartilage erosion in SCC require meticulous high-resolution CT/MRI review. Fissures of Santorini in the EAC are normal variant conduits for spread of infection and malignancy, their presence should be noted in relevant clinical contexts.

Summary Principles

• The expert radiologist operates as an applied anatomist, contextualising every finding against: the known spectrum of normal (recognising that "normal" is a spectrum, not a fixed point); the expected changes of biological age and physiological state; and the characteristic morphological signatures of disease.
• Nine morphological categories of pathological change, systematic region-by-region pattern recognition, modality-specific signal characterisation, and validated measurement thresholds form the substrate