Radiologic Anatomy: Chest X-ray & Cross-Sectional Imaging
Turn shadows and slices back into anatomy
Reading the frontal and lateral chest film
Projection. The standard film is a PA view: anterior chest against the detector, beam entering from behind, full inspiration. Portable films are AP, often supine; the heart lies farther from the detector and closer to the tube, so it is magnified, and in the supine patient effusions layer posteriorly as a haze while a pneumothorax collects anteriorly.
Landmarks. A straight film has the medial clavicular heads equidistant from the spinous processes. The right hemidiaphragm sits slightly higher than the left (liver), and the left hilum sits higher than the right because the left pulmonary artery arches over the left main bronchus. A plain film separates only five densities — air, fat, soft tissue/fluid, bone, metal — and an edge shows only where two different densities meet.
| View / border | Structures | Enlargement clue |
|---|---|---|
| PA, right border | SVC above, right atrium below | RA enlargement bulges the right border |
| PA, left border (top to bottom) | Aortic knob (arch), pulmonary trunk, left atrial appendage (normally flat or slightly concave), left ventricle (apex) | Straightened border from a bulging LA appendage (mitral stenosis); prominent trunk in pulmonary hypertension |
| Lateral, anterior | Right ventricle against the lower sternum | Retrosternal clear space fills in |
| Lateral, posterior | Left atrium (upper), left ventricle (lower) | LA bulges backward toward the esophagus |
Heart size and chamber enlargement
Cardiothoracic ratio. Widest transverse cardiac diameter ÷ widest internal thoracic diameter. Because an AP film magnifies the heart, an apparently large heart on a portable film should be confirmed on a PA film or echocardiogram.
Left atrial enlargement. The LA is the most posterior chamber, just beneath the carina and directly in front of the esophagus. When it enlarges — classically in mitral stenosis or chronic mitral regurgitation — it widens the carinal angle and lifts the left main bronchus, casts a double density behind the right heart border, straightens the left heart border, and displaces the barium-filled esophagus posteriorly.
A giant left atrium can compress the esophagus (dysphagia) and the left recurrent laryngeal nerve (hoarseness, Ortner syndrome); its posterior position is why transesophageal echo is best for LA appendage thrombus.
Right ventricle. It forms no frontal border; enlarged, it fills the retrosternal clear space on the lateral film. As the most anterior chamber, it is the one most often injured by a stab wound just left of the sternum.
The silhouette sign and lobar localization
Principle. When lung next to a soft-tissue structure fills with fluid, pus, or cells, or collapses, the density difference vanishes and that border is lost. An opacity that erases a border touches that structure; one that overlaps a border but leaves it crisp lies in front of or behind it. Branching air bronchograms inside an opacity mark airspace filling such as pneumonia.
| Border lost | Lobe | Why |
|---|---|---|
| Right heart border | Right middle lobe (medial segment) | Abuts the right atrium |
| Left heart border | Lingula of the left upper lobe | Abuts the left ventricle |
| Hemidiaphragm | Lower lobe on that side | Basal segments rest on the dome |
| Aortic knob | Left upper lobe | Contacts the arch |
| Descending aorta | Left lower lobe | Lies against it posteriorly |
RML versus RLL. RML disease erases the right heart border and spares the diaphragm; RLL disease erases the hemidiaphragm and spares the heart border. Collapse also pulls the fissures, trachea, and mediastinum toward the opacity, whereas a large effusion or tension pneumothorax pushes them away.
Checking tubes and lines
- Endotracheal tubeTip 3–5 cm above the carina (which lies at about the sternal angle, T4–T5) with the neck neutral; the tip moves with the chin — flexion pushes it deeper, extension pulls it up.
- Too deep?It enters the wider, shorter, more vertical right main bronchus: the left lung undergoes absorption atelectasis (opacified, volume loss, mediastinum shifted left), left breath sounds vanish, and hypoxemia follows. Withdraw the tube.
- Central venous catheterTip in the lower SVC at the cavoatrial junction; deep in the right atrium it risks arrhythmia and perforation. After a subclavian or internal jugular line, look for a pneumothorax.
- Nasogastric tubeMidline down the esophagus, across the diaphragm, tip in the stomach under the left hemidiaphragm.
An NG tube that tracks down a bronchus is in the airway; feeding through it causes aspiration pneumonitis. Reposition and reimage before use.
Axial CT and MRI anatomy
Orientation. Axial slices are viewed as if from the supine patient's feet: anterior at the top, the patient's right on the viewer's left — the liver on the left of the screen, the spleen on the right. Coronal images are displayed as if facing the patient.
- Great vesselsBrachiocephalic trunk, left common carotid, and left subclavian from right to left, with the left brachiocephalic vein crossing in front.
- Aortic arch (~T4)A curved tube with the SVC on its right and the trachea medially; just below is the aortopulmonary window, where the left recurrent laryngeal nerve hooks under the arch.
- Pulmonary trunk bifurcationThe right pulmonary artery passes behind the ascending aorta and SVC; the left arches over the left main bronchus.
- Four-chamber heartRV most anterior behind the sternum, LA most posterior against the esophagus and descending aorta.
- Upper abdomenLiver and IVC on the right, spleen and stomach on the left; thin Y-shaped adrenals above the kidneys, the right directly behind the IVC.
- Pancreas level (~L1–L2)Splenic vein behind the pancreas; the left renal vein crosses between the aorta (behind) and SMA (in front) to reach the IVC.
The aorta–SMA angle is a pinch point: left renal vein compression is nutcracker syndrome (hematuria, left varicocele); compression of the third part of the duodenum lower down is SMA syndrome.
Modality basics: CT, MRI, and ultrasound
CT. Shades reflect attenuation in Hounsfield units (table); soft tissue is slightly positive, about +20 to +80. Acute blood is denser than brain, so noncontrast head CT detects fresh hemorrhage; window settings change only the display. The arterial phase (~20–40 s after contrast) shows arteries and hypervascular lesions — hepatocellular carcinoma enhances then washes out, a hemangioma fills in centripetally, a simple cyst never enhances. The portal venous phase (~60–70 s) is routine: liver parenchyma and portal veins enhance.
MRI. T1 shows fat bright and fluid dark — the anatomy sequence, read again after gadolinium. T2 shows fluid, edema, and CSF bright (T2 goes with H2O); edema is dark on T1 and hypodense on CT. FLAIR is T2 with CSF suppressed.
Gadolinium in advanced kidney disease (eGFR <30 or dialysis) risks nephrogenic systemic fibrosis; ferromagnetic foreign bodies and non-MRI-conditional devices preclude scanning.
Ultrasound. Portable and free of ionizing radiation, so first-line in pregnancy and children; gas scatters the beam. CT, by contrast, carries far more radiation than a plain film.
| Tissue | CT | T1 MRI | T2 MRI | Ultrasound |
|---|---|---|---|---|
| Simple fluid (bile, urine, cysts, ascites) | ~0 HU, gray | Dark | Bright | Anechoic with posterior enhancement |
| Fat | ~−100 HU, dark gray | Bright | Fairly bright | Variable |
| Air | ~−1000 HU, black | Dark | Dark | Scatters the beam |
| Cortical bone | Up to ~+1000 HU, white | Dark | Dark | Echogenic with acoustic shadowing (as are gallstones and kidney stones) |