Imaging Decisions, Protection, Positioning and Report Writing

Part III rarely asks you to name a bone; it asks whether this patient needs a film at all, which view will answer the question, how to keep the dose down, what the image shows, and what you should write and recommend next. Imaging decisions are clinical decisions: a film ordered without a reason exposes the patient and finds incidental noise, while a film not ordered in the patient with red flags misses a fracture, tumor or infection. This chapter teaches the reasoning that connects the history to the right study, the right projection and the right report.

Section 1Indications and contraindications for imaging

The question comes first

Every film should answer a question that will change management. Before ordering, state the question in one line: "Is there a fracture after this fall?", "Is there destructive bone disease in this 68 year old with night pain and weight loss?", "Is there instability after this flexion injury?" If you cannot write the question, or if no possible answer would alter what you do, the film is not indicated. Plain radiographs are excellent for fracture, gross alignment, advanced degeneration, many bone tumors and congenital anomalies. They are poor for disc herniation, nerve root compression, early infection, early metastasis (roughly 30 to 50 percent of trabecular bone must be lost before a lytic lesion shows), ligament injury and soft tissue disease. Knowing these limits is as important as knowing the indications.

Red flags that justify imaging

For spinal pain, current guidelines agree that routine imaging of acute low back pain without red flags is not helpful: findings such as disc degeneration and facet arthrosis are common in people without pain, they do not predict outcome, and early imaging is associated with more procedures, more time off work and no better recovery. Imaging becomes appropriate when the history or examination raises suspicion of serious pathology, or when a patient fails to improve after about 4 to 6 weeks of appropriate care.

Decision rules for cervical trauma

Two validated rules decide whether an alert, stable adult with neck trauma needs cervical imaging. The Canadian C spine rule asks three questions in sequence. First, is there any high risk factor that mandates imaging: age 65 or older, a dangerous mechanism (fall from 3 feet or 5 stairs or more, axial load to the head as in diving, high speed motor vehicle collision over 100 km/h or with rollover or ejection, motorized recreational vehicle, bicycle collision), or paresthesias in the extremities? If yes, image. Second, is there a low risk factor that allows safe assessment of range of motion: simple rear end collision, sitting position in the emergency department, ambulatory at any time, delayed onset of neck pain, or absence of midline cervical tenderness? If none is present, image. Third, can the patient actively rotate the neck 45 degrees left and right? If yes, no imaging is needed; if not, image. The rule has a sensitivity close to 100 percent for clinically important cervical injury. It does not apply to patients under 16, those with a Glasgow Coma Scale below 15, unstable vital signs, acute paralysis, known vertebral disease or prior cervical surgery.

The NEXUS criteria say imaging is not needed if all five are present: no posterior midline cervical tenderness, no focal neurological deficit, normal alertness, no intoxication, and no painful distracting injury. NEXUS is simpler but somewhat less specific than the Canadian rule. In an emergency setting, CT is now the preferred first study for adults who meet imaging criteria, because it is far more sensitive than plain films for fractures; in the chiropractic office, the decision that matters is that the patient needs imaging before any cervical manipulation, and that a high risk patient is immobilized and sent for it.

Extremity decision rules

The Ottawa ankle rules call for ankle radiographs when there is pain in the malleolar zone plus bone tenderness at the posterior edge or tip of either malleolus (distal 6 cm) or inability to bear weight for four steps both immediately and at examination. Foot films are indicated for midfoot pain plus tenderness at the base of the fifth metatarsal or the navicular, or the same inability to bear weight. Sensitivity approaches 100 percent, so a negative rule reliably excludes fracture. The Ottawa knee rule calls for films with age 55 or older, isolated patellar tenderness, fibular head tenderness, inability to flex to 90 degrees, or inability to bear weight for four steps.

When imaging is not indicated

• Acute nonspecific low back or neck pain under about 6 weeks with no red flags.
• Routine films to "find subluxations", to establish a baseline for every new patient, or to document progress in an improving patient. Repeat films need a new clinical question.
• Imaging for reassurance alone, or to satisfy an insurer, when the result would not change care.
• Radiographs when the question is a soft tissue one (disc herniation, cord, ligament, tendon) and the next step would be MRI or ultrasound anyway.

Pregnancy and children

Ask every patient of reproductive age about pregnancy and the date of the last menstrual period before any exposure that includes the abdomen or pelvis, and document the answer. Elective imaging is deferred during pregnancy. When imaging is truly needed, the dose of a single lumbar or pelvic series is well below the threshold associated with fetal malformation (commonly taught as about 50 to 100 mGy); a lumbar series delivers a fetal dose on the order of 1 to 4 mGy. The theoretical concern at low doses is a small increase in childhood cancer risk, so the rule is to avoid unnecessary exposure, prefer nonionizing studies (ultrasound, MRI without gadolinium), and never withhold a necessary study. The older "10 day rule" (elective pelvic exposure only in the first 10 days after menses onset) reflects the same principle.

Children are more radiosensitive than adults because their tissues are dividing and they have more years in which a cancer can develop. Their unfused growth plates, ossification centers and normal pseudosubluxation also make films easy to misread. Image children only for specific indications, use the fewest views, collimate tightly and use pediatric technique factors. Scoliosis follow up should use the lowest dose method available, PA projection and the minimum number of films needed to make treatment decisions.

Section 2Patient protection and positioning

ALARA and the three cardinal principles

ALARA means keeping exposure "as low as reasonably achievable" while still producing a diagnostic image. For the operator, the three cardinal principles are time (minimize it), distance (maximize it) and shielding (use it). Distance works through the inverse square law: intensity is proportional to 1 divided by the square of the distance, so doubling the distance from a source reduces exposure to one fourth. The operator stands behind the control booth barrier, never holds a patient during exposure, and wears a dosimeter badge. The main source of operator exposure is scatter from the patient, not the primary beam.

For the patient, dose is reduced mostly by avoiding unnecessary films, then by technique: tight collimation, adequate filtration, appropriate kVp and mAs, fast image receptors, correct positioning on the first attempt (repeats double the dose) and, where regulations require it, shielding.

Collimation, filtration and shielding

• Collimation restricts the beam to the area of interest. It is the single most effective technical way to reduce patient dose, and because less tissue is irradiated it also produces less scatter, which improves contrast. The collimated field must never be larger than the image receptor, and a visible collimation border on all sides is evidence of good practice.
• Filtration (aluminum in the tube housing, at least 2.5 mm aluminum equivalent total for units above 70 kVp) absorbs low energy photons that would be absorbed in skin without contributing to the image. Compensating (wedge or trough) filters even out density across body parts of unequal thickness, for example the cervicothoracic junction or a full spine film.
• Shielding of the gonads, thyroid and breast has traditionally been taught when these organs lie within or near (about 5 cm) the primary beam and the shield will not obscure the anatomy of interest. Current physics guidance has moved away from routine gonadal and fetal shielding, because modern equipment uses automatic exposure control that may increase dose when a shield enters the field, and shields frequently hide pathology and force repeat films. Follow the regulations that apply where you practice; for the exam, know both the traditional rule and the reason it is changing.

Technique factors

With digital radiography, an underexposed image looks noisy while an overexposed image still looks good because software rescales brightness. This allows dose creep, in which operators drift toward higher exposures without noticing. Watching the exposure index and staying within the manufacturer's target range is the digital version of ALARA.

Standard and supplemental views

Every region has a minimum standard series, and long bones and joints always need at least two views at 90 degrees to each other, because a fracture or dislocation can be invisible in a single plane. For a long bone, include the joints above and below. Supplemental views are added to answer a specific question raised by the history, examination or the standard films.

Section 3Normal radiographic anatomy and variants in cases

Why normal comes first

Most misreads on Part III come from two directions: calling a normal variant disease (and frightening or overtreating the patient) or accepting real disease as normal. The protection against both is a confident picture of normal on each standard view, including the lines and measurements that define alignment, and a short list of variants that are known to mimic pathology.

The lateral cervical film

The neutral lateral is the single most informative cervical view. On it, four smooth, roughly parallel curves should be traced: the anterior vertebral line along the front of the bodies, the posterior vertebral line (George's line) along the back of the bodies, which forms the anterior wall of the canal, the spinolaminar line joining the junctions of the laminae with the spinous processes, which forms the posterior wall of the canal, and the posterior spinous line through the spinous tips. A step in any line suggests translation (subluxation, dislocation, spondylolisthesis), and fanning of the spinous processes suggests posterior ligament disruption.

The other standard views

• APOM: the dens sits centered between the lateral masses of C1, the lateral masses of C1 line up with the articular pillars of C2, and the joint spaces are symmetric. Bilateral overhang of the C1 lateral masses beyond C2 totaling more than about 7 mm suggests a Jefferson burst fracture with transverse ligament rupture. In young children, physiological overhang (pseudospread of the atlas) is normal.
• AP lower cervical: spinous processes in a straight line (a sudden rotational offset suggests unilateral facet dislocation), uncinate processes and uncovertebral joints visible, pedicles and lateral masses symmetric, trachea midline.
• AP lumbar: paired oval pedicles ("owl's eyes") at every level; the interpediculate distance normally widens gradually from L1 to L5. A missing pedicle ("winking owl" sign) suggests lytic metastasis, while widening of the interpediculate distance suggests an intraspinal mass or burst fracture. Psoas shadows, renal outlines, sacroiliac joints and the hips are reviewed in the same film.
• Lateral lumbar: disc heights increase from L1 to L4 to L5; L5 to S1 is often slightly narrower and that alone is not abnormal. George's line should be continuous. The lumbosacral (Ferguson's sacral base) angle averages about 41 degrees (normal range roughly 26 to 57); the lumbar lordosis is measured from the top of L1 to the sacral base.
• Oblique lumbar: the "Scottie dog": nose (transverse process), eye (pedicle), ear (superior articular process), neck (pars interarticularis), body (lamina), front leg (inferior articular process). A collar on the neck is spondylolysis; a decapitated dog suggests spondylolisthesis.
• Thoracic AP and lateral: pedicles at every level, paraspinal lines close to the spine (a bulging paraspinal line suggests hematoma, abscess or tumor), smooth kyphosis about 20 to 40 degrees, and vertebral body heights equal anteriorly and posteriorly (an anterior height loss over about 20 percent suggests compression fracture).

Variants that mimic disease

Section 4Description and interpretation of images

A systematic search

Satisfaction of search, stopping after the first abnormality, is the classic interpretive error. A fixed sequence prevents it. Begin with the label (name, date, side marker, view), confirm the film is adequate (positioning, exposure, the whole region included), and then follow ABCS:

• A, alignment: the curves and lines described above, translation, rotation, scoliosis, listhesis, joint congruity.
• B, bone: each bone's size, shape, outline (cortex continuity) and density (generalized osteopenia or sclerosis, focal lytic or blastic lesions). Count the pedicles. Check the endplates, posterior elements and the edges of the film.
• C, cartilage (joint spaces): uniform versus nonuniform narrowing, subchondral sclerosis, cysts, osteophytes, erosions, calcification of cartilage (chondrocalcinosis), fusion.
• S, soft tissues: prevertebral swelling, paraspinal lines, psoas shadows, abdominal aorta calcification and diameter, renal and gallbladder stones, lung apices, foreign bodies, soft tissue calcification or gas.

Review "hot spots" deliberately: the dens and C1 to C2 on every cervical film, the lung apices, the abdominal aorta (an aortic wall more than 3 cm apart suggests aneurysm), the hips and sacroiliac joints on lumbar films, and the corners of the image where tumors hide.

Describing a bone lesion

Plain film features remain the best way to judge whether a bone lesion is slow growing (usually benign) or aggressive (malignancy or infection). Age is the most powerful clue: a solitary aggressive lesion under 30 suggests a primary tumor such as osteosarcoma or Ewing sarcoma, while over 40 the leading diagnoses become metastasis and multiple myeloma.

1. Location: which bone; axial or appendicular; epiphysis (chondroblastoma, giant cell tumor after physeal closure), metaphysis (osteosarcoma, osteochondroma, nonossifying fibroma), diaphysis (Ewing sarcoma, fibrous dysplasia, myeloma); central, eccentric or cortical; vertebral body versus posterior elements (metastasis destroys the pedicle and body, aneurysmal bone cyst and osteoblastoma favor the posterior elements).
2. Size and number: solitary or multiple; multiple lytic lesions over 40 suggest metastasis or myeloma.
3. Margin and zone of transition: graded with the Lodwick system (below). A narrow zone of transition means slow growth; a wide one means aggressive growth.
4. Matrix: what the lesion produces. Osteoid matrix is cloudlike or ivory dense (osteosarcoma, osteoblastic metastasis); chondroid matrix shows rings and arcs or popcorn calcification (enchondroma, chondrosarcoma); fibrous matrix gives a hazy ground glass density (fibrous dysplasia).
5. Periosteal reaction: solid, thick, uninterrupted new bone means a slow process; interrupted reactions (lamellated "onion skin", spiculated "sunburst" or "hair on end", and the Codman triangle where elevated periosteum is broken through) mean an aggressive process.
6. Cortical breakthrough and soft tissue mass: both indicate aggression.

Writing the report

A report is a legal and clinical document that another provider must be able to act on. A standard structure is:

1. Identifying data: patient name, date of birth, date of study, facility, referring clinician.
2. Clinical history: the question being asked ("fall, midline C6 tenderness, rule out fracture").
3. Views obtained and their quality, including limitations ("C7 obscured by shoulders").
4. Findings: organized by ABCS, describing what is seen in neutral terms (location, size, margin, matrix, periosteal reaction), including pertinent negatives ("no prevertebral soft tissue swelling") and incidental findings.
5. Impression: the conclusion in order of importance: the most significant or most likely diagnosis first, then a short differential, then minor or incidental findings. The impression is what busy readers read, so it must stand on its own.
6. Recommendations: specific and actionable: the next study (MRI, CT, bone scan, swimmer's view, follow up films in a stated interval), laboratory tests (CBC, ESR, CRP, SPEP, PSA), referral (orthopedic, oncology, emergency department) and urgency.

Findings that need action within hours, such as an unstable fracture, cord compression, aggressive lesion, aortic aneurysm or free intraperitoneal air, must be communicated directly (by phone or in person) to the responsible clinician or patient, and the communication (time, person, content) documented. Leaving a critical finding in a report that may not be read for days is a standard of care failure.

Section 5Choosing advanced imaging in cases

Match the study to the tissue and the question

Each advanced study sees a different property of tissue. CT measures X-ray attenuation in thin slices and shows cortical bone in fine detail. MRI maps the behavior of hydrogen protons in a magnetic field and shows water, fat and soft tissue contrast, including marrow, discs, nerves, cord and ligaments. A bone scan (technetium 99m methylene diphosphonate) shows osteoblastic activity and blood flow anywhere in the skeleton. Ultrasound shows tendons, muscles, fluid and vessels in real time without radiation. DEXA measures areal bone mineral density. Choosing well means knowing which property answers the question.

MRI signal basics for case questions

On T1 weighted images fat is bright and water is dark, so T1 is the anatomy and marrow sequence: normal adult marrow is bright from fat, and a dark marrow on T1 means replacement by tumor, infection or edema. On T2 weighted images water is bright, so cerebrospinal fluid, edema, cysts, a hydrated disc nucleus and inflammation are bright; a dark ("black") disc on T2 is a desiccated, degenerated disc. STIR and fat suppressed T2 sequences darken fat so that edema stands out, which is ideal for stress reactions, acute fractures and sacroiliitis. Gadolinium enhancement highlights vascular or inflamed tissue: it separates scar (enhances) from recurrent disc herniation (does not enhance centrally) after surgery, outlines abscesses and characterizes tumors.

A classic pattern contrast: in vertebral osteomyelitis and discitis the disc itself is destroyed and bright on T2, with adjacent endplates eroded and both vertebral bodies showing edema; in metastasis the disc is typically preserved while the body and pedicle are replaced. "The disc crosses the infection, the tumor respects the disc" is a useful board shortcut.

Timing: when each study becomes positive

Plain films lag behind biology. Osteomyelitis takes about 10 to 14 days to produce radiographic bone destruction; stress fractures may not show periosteal callus for 2 to 3 weeks, and some never do; a lytic metastasis must remove roughly a third to a half of trabecular bone before it is visible; scaphoid waist fractures are missed on initial films in a meaningful minority of patients. MRI and bone scan become positive within a day or two. This is why a normal radiograph does not exclude early infection, stress fracture or metastasis in a patient whose history suggests them.

Deciding the next study and how fast

DEXA interpretation

The T score compares the patient with a young adult of the same sex at peak bone mass: minus 1.0 or higher is normal, between minus 1.0 and minus 2.5 is low bone mass (osteopenia), and minus 2.5 or lower is osteoporosis. A fragility fracture of the hip or spine establishes osteoporosis regardless of the T score. The Z score compares the patient with people of the same age and sex and is used for premenopausal women, men under 50 and children; a Z score of minus 2.0 or lower is "below the expected range for age" and should prompt a search for a secondary cause (steroids, hyperparathyroidism, celiac disease, hypogonadism, myeloma). The FRAX tool combines bone density with clinical risk factors to estimate 10 year fracture probability.

Words worth knowing