Reading Films Under Time: Approach and Normal Anatomy

An imaging station gives you a film, a short history and very little time, and it scores what you find, how you describe it and what you recommend. The candidates who do well are not the ones who know the rarest diagnoses; they are the ones who search every film the same way, know normal anatomy and its variants so well that abnormal stands out, and can say a clear one line impression and a safe recommendation before the signal. This chapter builds that engine; Chapters 3 and 4 load it with high yield pathology.

Section 1A fast systematic search pattern

Why a fixed pattern beats a good eye

Expert readers do not look at a film; they move through it. Eye tracking work on radiologists shows that experienced readers combine a quick global impression with a deliberate search, while novices fixate on the first abnormal or interesting spot and stop. On a timed exam the risk doubles: anxiety narrows attention, and the obvious finding becomes the only finding. A fixed search pattern costs perhaps 30 to 60 seconds per film once it is practiced, and it turns every film into the same task.

Before the search: the label check

Spend five seconds on the administrative data, and say it aloud if the station involves speaking. Confirm the patient name and age (age changes the differential more than any other single fact), the date, the view (AP, PA, lateral, oblique, open mouth, flexion, extension), the side marker, and whether the film is weight bearing. Check technical quality: is the area of interest included, is the exposure adequate, is the patient rotated? A rotated lateral cervical film doubles the posterior margins and can mimic a subluxation; an underpenetrated film hides lytic lesions. If the films provided are inadequate, recommending additional views is a legitimate answer.

ABCS

Different teachers expand the acronym (for example, ABCDS adds "D" for disc space, or ABCs plus "S" for soft tissue). The specific letters matter less than using one version every time. Begin with alignment because it is the fastest whole film overview, then go structure by structure.

Check every corner and the review areas

Lesions hide where the eye does not travel: at the edges of the film, behind dense structures, and in the parts of the film that are not the stated area of interest. A lumbar film includes the lower ribs, the kidneys, the abdominal aorta, the iliac wings, the sacroiliac joints and often the hips. A cervical film includes the skull base, the airway, the lung apices and the upper ribs. A thoracic film includes the heart, both lungs and the upper abdomen. The board often places a finding in one of these peripheral areas precisely to test whether the search was complete.

Satisfaction of search

Satisfaction of search is the tendency to stop searching after finding one abnormality. It is a classic source of missed second findings: the obvious spondylolisthesis that hides the renal stone, the obvious fracture that hides the second fracture, the degenerative changes that hide a lytic metastasis. Prevent it with three habits. First, finish the pattern even after you find something. Second, when you find one abnormality, ask what it is commonly associated with (a fracture of one forearm bone suggests looking for injury to the other bone or a dislocation at a joint; a lytic lesion in an older adult suggests looking for others). Third, look at the review areas last, deliberately, as the final step before you speak.

Global impression first, then detail

Before the formal search, take two seconds for a global look at the whole film from a distance: overall density, overall alignment, anything strikingly asymmetric. This gestalt catches large findings (a diffusely dense spine of metastatic prostate cancer or Paget disease, a scoliosis, a missing hemipelvis density) that a close up search can paradoxically miss. Then begin ABCS.

Section 2Normal cervical, thoracic and lumbar radiographs and their variants

The standard views and what each shows best

The lateral cervical film: lines and spaces

On the neutral lateral cervical film, trace four smooth, roughly parallel curves (Figure 2.2). The anterior vertebral line joins the anterior body margins. The posterior vertebral line (George's line) joins the posterior body margins and outlines the front of the spinal canal. The spinolaminar line joins the points where the laminae meet the spinous processes and outlines the back of the canal. The posterior spinous line joins the spinous tips. A step in any line suggests subluxation, dislocation or fracture; in children, C2 on C3 needs special interpretation (below).

The atlantodental interval (ADI) is measured from the posterior surface of the anterior arch of C1 to the anterior surface of the dens. It is 3 mm or less in adults and up to 5 mm in children; widening suggests transverse ligament insufficiency (trauma, rheumatoid arthritis, Down syndrome, inflammatory disease). The sagittal canal diameter, from the posterior body to the spinolaminar line, is normally more than about 12 to 13 mm in the lower cervical spine; values near 10 mm or less suggest stenosis, and the body to canal (Torg or Pavlov) ratio below about 0.8 suggests a developmentally narrow canal.

The prevertebral soft tissue (retropharyngeal space at C2, retrotracheal space at C6) is the soft tissue sign of the cervical spine. Widening suggests hematoma from a fracture, infection or a mass. Commonly taught adult upper limits are about 7 mm at C2 and about 22 mm at C6 (in children about 14 mm at C6). Crying in an infant and flexion of the neck can widen it spuriously.

The upper cervical spine and its normal mimics

On the open mouth view, the lateral masses of C1 should line up with the lateral margins of the C2 superior articular facets. Lateral overhang of the C1 masses suggests a Jefferson burst fracture; a combined overhang of more than about 7 mm suggests disruption of the transverse ligament (the rule of Spence). In children up to about 4 years old, the C1 ring grows faster than C2, so a pseudospread of the atlas is normal.

Several normal shadows mimic a dens fracture. The Mach band effect, a dark line created where the edge of the posterior arch of C1 or the incisors overlie the base of the dens, can look like a fracture; it extends beyond the bone margins and changes with a repeat view. Overlapping teeth, the tongue and the occiput can do the same. The subdental synchondrosis between the dens and the body of C2 fuses between about 3 and 6 years and is visible as a lucent line in young children; it is a growth center, not a fracture. The ossiculum terminale (a small ossicle at the tip of the dens that appears around age 3 and normally fuses by about 12) is a normal variant when it persists; os odontoideum, a larger ossicle separated from a short dens with a smooth corticated gap, is not a normal variant because it may be unstable and needs flexion and extension evaluation before any upper cervical manipulation.

A ponticulus posticus (posterior ponticle, arcuate foramen) is a bony bridge over the vertebral artery groove on the posterior arch of C1. It is common (seen in a sizable minority of adults) and benign, but many clinicians modify or avoid forceful rotatory upper cervical techniques in its presence because the artery runs through the ring. Mention it in a report.

Pediatric cervical variants

In children, ligamentous laxity and horizontal facets allow normal forward slip of C2 on C3 (and sometimes C3 on C4) of up to about 3 to 4 mm in flexion: pseudosubluxation. To tell it from true injury, draw the posterior cervical line (Swischuk line) from the anterior cortex of the C1 spinous process to the anterior cortex of the C3 spinous process. The anterior cortex of the C2 spinous process should lie within about 2 mm of this line; a greater deviation suggests a true injury such as a hangman fracture. Children may also show loss of lordosis, wedge shaped bodies that look like compression fractures (normal anterior wedging of young vertebrae) and unfused ring apophyses at the endplates that look like chip fractures.

Thoracic and lumbar variants that mimic disease

Section 3Normal pelvis, hip and extremity radiographs and variants

The AP pelvis and hip: lines that should be smooth

The AP pelvis is read with the same ABCS discipline, plus a few lines that reveal subtle hip disease (Figure 2.3). Shenton's line is a smooth arc formed by the inferior border of the superior pubic ramus continuing onto the medial (inferior) border of the femoral neck. A break suggests a femoral neck fracture, hip dislocation or developmental dysplasia. Klein's line runs along the superior border of the femoral neck on the AP view and should intersect a portion of the lateral femoral epiphysis; if it does not, think slipped capital femoral epiphysis (SCFE) in an adolescent. Kohler's line (the ilioischial line) runs from the medial border of the ilium to the medial border of the ischium; if the acetabular floor or femoral head crosses medial to it, the finding is protrusio acetabuli (seen with rheumatoid arthritis, Paget disease, osteomalacia and other conditions). The teardrop marks the acetabular floor and helps judge rotation and protrusion.

Growth plates versus fractures

Children's films are full of lucent lines that are normal. A physis (growth plate) is a lucent band with smooth, well defined, often slightly undulating and sclerotic margins, in a predictable location, symmetric with the other side. A fracture line is usually sharp, uncorticated, irregular, in an unexpected place or orientation, and associated with soft tissue swelling. When in doubt, comparison views of the opposite side resolve the question; they are a legitimate recommendation. Know the order of ossification centers at the elbow, which is commonly remembered as CRITOE: capitellum (about 1 year), radial head (about 3), internal or medial epicondyle (about 5), trochlea (about 7), olecranon (about 9) and external or lateral epicondyle (about 11). The ages vary and girls are generally earlier; the order is what matters. A trochlear center seen before a medial epicondyle center suggests the "trochlea" is actually a displaced medial epicondyle fragment trapped in the joint.

Two elbow lines catch subtle injury. The anterior humeral line, drawn along the anterior humeral cortex on a true lateral, should pass through the middle third of the capitellum; if it passes anterior to it, suspect a supracondylar fracture with posterior displacement. The radiocapitellar line, through the center of the radial neck, should pass through the capitellum on every view; if not, the radial head is dislocated. A visible posterior fat pad, or a raised anterior fat pad (the sail sign), indicates a joint effusion and, after trauma, an occult fracture.

Accessory ossicles and sesamoids

Accessory ossicles are separate bones from unfused ossification centers, and sesamoids are bones within tendons. Both are smooth, rounded, completely corticated, often bilateral, and lie in predictable places. Acute fracture fragments are sharp, uncorticated on at least one side and have a matching defect in the parent bone.

Nutrient canals, bone islands and other normal findings

Nutrient canals are thin, oblique lucent lines through the cortex of long bones carrying the nutrient artery. They are smooth, with sclerotic margins, and run obliquely. A traditional memory aid states their direction: the canals point toward the elbow and away from the knee ("to the elbow I go, from the knee I flee"). A fracture line is sharper, often crosses the whole cortex at a different angle, and has soft tissue change.

Section 4Normal chest and abdominal film anatomy for the chiropractor

Why a chiropractor reads chest and abdomen

Chiropractors order and read spinal films that always include part of the chest or abdomen, and they see patients whose back or shoulder pain comes from the viscera. A thoracic spine film is also a chest film; a lumbar film is also an abdominal film. You are responsible for recognizing a lung mass at the apex of a cervical film, an aneurysm in front of the lumbar spine or a large kidney stone on an AP lumbar view, and for referring appropriately. You need a working knowledge of normal anatomy so that abnormal stands out.

Is the chest film adequate?

Before reading anatomy, judge quality, because poor technique creates false disease. The checks can be remembered as rotation, inspiration and penetration.

Normal PA chest anatomy

Read the chest with the same ABCS idea adapted to the region, or use any fixed pattern that covers the airway, bones, cardiac silhouette, diaphragm, lung fields, hila and soft tissues (Figure 2.4). The trachea is midline (a slight rightward displacement at the aortic knob is normal). The right heart border is the right atrium, with the superior vena cava above it. The left heart border, from top to bottom, is formed by the aortic knob (arch), the main pulmonary artery, the left atrial appendage (normally flat or slightly concave) and the left ventricle. The cardiothoracic ratio (widest cardiac diameter divided by the widest internal thoracic diameter) is normally less than 0.5 on a PA film. The mediastinum on a PA film is normally less than about 8 cm wide at the level of the aortic knob.

The hila contain the pulmonary arteries and bronchi; the left hilum normally sits slightly higher than the right because the left pulmonary artery arches over the left main bronchus. The lung markings are vessels; they taper toward the periphery and are larger at the bases on an erect film because of gravity. The right hemidiaphragm is normally about 1.5 to 2 cm higher than the left because the liver lies beneath it, and the gastric air bubble lies under the left. Both costophrenic angles are sharp. Look for the horizontal fissure on the right (from the hilum laterally to about the fourth rib). Check the breast shadows, axillae and chest wall soft tissue.

On the lateral film, the retrosternal clear space sits behind the sternum and in front of the heart, and the vertebral bodies should become more lucent (darker) from top to bottom because less soft tissue overlies the lower thorax. If the lower thoracic vertebrae look whiter than the upper ones (the spine sign), suspect a lower lobe density.

The silhouette sign

Borders are seen only where tissues of different density meet. When lung next to a structure fills with fluid or tissue, the border disappears: the silhouette sign. Loss of the right heart border places the disease in the right middle lobe; loss of the left heart border places it in the lingula; loss of a hemidiaphragm places it in the lower lobe; loss of the aortic knob suggests the left upper lobe. This is how a chiropractor localizes a density on a single film and words a referral.

Normal abdominal film anatomy

The AP lumbar or KUB (kidneys, ureters, bladder) film shows the liver shadow in the right upper quadrant, the splenic tip on the left, the psoas margins as oblique lines diverging from the spine to the pelvis, and the kidneys, which lie at roughly T12 to L3, are about three to four vertebral body heights long, lie along the psoas, and usually sit with the left slightly higher than the right. The bowel gas pattern is normally a gastric bubble, gas scattered through the colon, and small amounts in nondilated small bowel. A common rule of thumb for upper limits of diameter is 3, 6 and 9 cm for the small bowel, colon and cecum. Small bowel has valvulae conniventes crossing the full width of the lumen; the colon has haustra that cross only part of it.

Section 5Describing and reporting under time

Describe before you diagnose

When you see something abnormal, resist naming it at once. A description built from standard features does three things: it shows the examiner a systematic mind, it narrows the differential logically, and it protects you when the diagnosis is uncertain, because a correct description with a safe recommendation earns credit even if the final label is not the one on the answer key. The description has a fixed order: location, morphology (size, shape, number, density), margins and zone of transition, matrix, periosteal reaction, cortex and soft tissue. Then comes the patient's age, which with location narrows most bone lesion differentials more than any other features.

Location

Name the bone, the side, the part of the bone and the position within it. In long bones the parts are the epiphysis (or apophysis), the metaphysis and the diaphysis; within the cross section a lesion is central (medullary), eccentric, cortical or juxtacortical (surface). In the spine, name the level and whether the lesion involves the body, the pedicle, the posterior elements or the disc. Location is powerful: epiphyseal lesions in a skeletally immature patient suggest chondroblastoma or infection; lesions of the posterior elements in young patients suggest osteoid osteoma, osteoblastoma or aneurysmal bone cyst; body and pedicle destruction in an older adult suggests metastasis.

Margins, zone of transition and the pattern of bone destruction

The zone of transition, the border between lesion and normal bone, is the single best indicator of biological activity. A narrow zone (sharp, geographic edge), especially with a sclerotic rim, means the bone has had time to wall off a slow lesion. A wide zone means the lesion is permeating faster than bone can respond. Moth eaten destruction is multiple small holes that coalesce; permeative destruction is innumerable tiny holes, often best seen in the cortex, typical of round cell tumors (Ewing sarcoma, lymphoma, myeloma) and of osteomyelitis. Remember that infection can look as aggressive as a malignancy.

Matrix

Matrix is the mineralized material a lesion produces. Osteoid matrix is cloud like, fluffy or ivory dense (osteosarcoma, osteoblastoma, bone island). Chondroid matrix forms rings and arcs or popcorn calcification (enchondroma, chondrosarcoma). Fibrous matrix gives a hazy ground glass appearance (fibrous dysplasia). A purely lytic lesion has no visible matrix. Naming the matrix narrows the differential to bone forming, cartilage forming or fibrous lesions.

Periosteal reaction

The periosteum responds to an underlying process by laying down new bone. A solid, thick, smooth reaction means slow, benign or chronic processes (healing fracture, osteoid osteoma, chronic infection, hypertrophic osteoarthropathy). Lamellated (onion skin) reaction means repeated episodes of periosteal lifting, classically Ewing sarcoma but also infection. Spiculated reaction, in sunburst or hair on end patterns, means a very rapid process, classically osteosarcoma. A Codman triangle is a cuff of periosteal new bone lifted at the edge of a rapidly growing lesion and broken through; it signals aggressiveness, usually malignancy, though infection and hemorrhage can produce it.

The one line impression

The impression distills everything into one sentence: the most likely diagnosis (or the leading differential), its location, and any qualifier that matters for care. Good impressions are short and specific: "Grade 1 isthmic spondylolisthesis of L5 on S1." "Aggressive lytic lesion of the L2 body and left pedicle in a 70 year old, suspicious for metastasis." "Normal lateral cervical spine for age." If you are unsure, give the leading diagnosis and one or two alternatives: "Lytic lesion of the T10 body, metastasis versus myeloma." Avoid hedging long lists; on a timed exam, three plausible items is enough.

The recommendation

The recommendation is where many points are won or lost, because it is the safety step. It answers three questions: does this need more imaging or tests, does it need referral and how urgently, and is manipulation of this region safe?

Putting it together in under a minute

A complete, timed oral report follows a template you can say almost without thinking: label, search summary, finding described by location and features, impression, recommendation. Practice the template on normal films so the structure is automatic, then the abnormal content drops into place. In a written answer, use the same order in short sentences.

Words worth knowing