Anatomy and Physiology / Cross-Sectional Anatomy
This domain tests anatomical relationships in three dimensions: which structure lies at which level, how images are oriented, and how organ systems function. Cross-sectional items usually ask you to identify a level from landmarks or to interpret a density.
Planes and orientation
- Axial (transverse) images are viewed as if from the patient's feet: the patient's right side appears on the viewer's left.
- Coronal and sagittal reformats follow the same convention; anterior is at the top of a sagittal image.
- Multiplanar reconstruction, maximum intensity projection, and volume rendering are post-processing of acquired data, not new acquisitions.
Cross-sectional landmarks
- Aortic hiatus T12; aortic bifurcation L4; inferior vena cava enters the right atrium at about T8-T9.
- Celiac trunk T12; superior mesenteric artery L1; renal arteries and renal hila L1-L2.
- Liver has eight Couinaud segments divided by hepatic and portal veins.
- Hounsfield units: water 0, air about -1000, fat -100 to -50, soft tissue 20-60, acute blood 50-80, bone 300 to over 1000, enhanced vessels 100-600.
Physiology commonly tested
- Cardiac output = heart rate x stroke volume; the SA node is the pacemaker.
- Respiratory: tidal volume, vital capacity, and alveolar gas exchange.
- Renal: glomerular filtration clears iodinated contrast; reduced filtration raises contrast-induced nephropathy risk.
- Endocrine: the thyroid concentrates iodine, which is why thyroid screening matters before contrast.
Worked example
An axial CT slice at a mid-abdominal vertebral body shows the superior mesenteric artery arising anteriorly and both renal veins draining into the inferior vena cava. The vertebra is L1. Reasoning: the SMA originates at L1 just below the celiac trunk, and the renal hila sit at L1-L2; two concordant landmarks establish the level more reliably than counting from the sacrum.
Traps
- Reversing left and right on axial images.
- Confusing T1 (fat bright, fluid dark) with T2 (fluid bright).
- Counting vertebral levels upward from an assumed sacrum instead of using landmarks.
- Mistaking a post-processing reconstruction for an additional acquisition.