Imaging Technology, Positioning, Normal Anatomy and Special Imaging

Diagnostic imaging carries about one sixth of the Part II exam, and almost every question about a fracture, tumor or arthritis on the next chapter assumes you already know how the film was made, which view shows what, and what normal looks like. This chapter builds that foundation: the physics you need to choose a technique and protect a patient, the standard views of the spine, extremities, chest and abdomen, the lines and angles the board loves to ask about, and the judgment to know when plain film is not enough and an MRI, CT, bone scan, ultrasound or DEXA is the right next step.

Section 1Imaging technology and radiation protection

How the x-ray tube makes a beam

An x-ray tube is an evacuated envelope containing a negative cathode and a positive anode. Tube current (mA) heats the tungsten filament of the cathode, which releases electrons by thermionic emission; a focusing cup keeps the cloud narrow. The high voltage (kVp) hurls the electrons onto the focal spot of a tungsten rotating anode, chosen for its high atomic number (74) and melting point. About 99 percent of the energy becomes heat and only about 1 percent becomes x-rays, which is why anodes rotate and tubes have heat ratings.

Two processes create the photons:

• Bremsstrahlung (braking radiation) occurs when an incoming electron is slowed and deflected by the positive nucleus of a tungsten atom and gives off the lost energy as a photon. Because the electron can lose any fraction of its energy, bremsstrahlung produces a continuous spectrum from near zero up to the kVp. It makes up most of the diagnostic beam (roughly 80 to 90 percent at typical spinal techniques).
• Characteristic radiation occurs when an incoming electron ejects an inner shell (usually K shell) electron; an outer shell electron drops in to fill the hole and emits a photon equal to the difference in binding energies. These photons have discrete energies characteristic of the target (about 69 keV for the tungsten K shell), so characteristic radiation only contributes once the kVp exceeds about 70.

The line focus principle angles the anode face so a large actual focal spot looks small to the patient, preserving detail while spreading heat. Its cost is the anode heel effect: photons leaving toward the anode pass through more target, so intensity is lower on the anode side and higher on the cathode side. Practical rule: put the thicker part under the cathode. For an AP thoracic film, the cathode goes toward the feet (the lower thoracic spine is thicker); for a full spine film, the cathode goes toward the thicker lumbopelvic end.

Filtration removes low energy photons that would be absorbed in the skin and never reach the receptor, which reduces patient dose without useful image loss. Inherent filtration (glass envelope, oil, window) plus added aluminum filtration must total at least 2.5 mm aluminum equivalent for tubes operating above 70 kVp. Compensating filters (wedge, trough or graduated filters) even out density across parts of unequal thickness: the cervicothoracic junction on a lateral cervical film, the upper thoracic spine on an AP, and the cervical and thoracic regions on a full spine film.

Exposure factors: kVp, mAs, distance and time

Milliampere seconds (mAs) is tube current times time and controls photon quantity: doubling mAs doubles photons, density and patient dose without changing energy. High mA with short time reduces motion blur.

Kilovoltage peak (kVp) controls photon energy and also raises quantity. Higher kVp penetrates better, lowers contrast (long gray scale) and increases scatter; lower kVp gives short scale contrast but needs more mAs and more skin dose. Extremities are shot at low kVp (about 50 to 70) for high contrast; the lumbar spine and pelvis need higher kVp (about 80 to 95) to penetrate.

The 15 percent rule: increasing kVp by 15 percent has about the same effect on receptor exposure as doubling mAs; decreasing kVp by 15 percent is like halving mAs. Using the higher kVp and lower mAs combination lowers patient dose at the cost of some contrast, a trade that digital processing often tolerates.

The inverse square law: intensity is inversely proportional to the square of the distance from the source. Doubling the source to image distance (SID) cuts intensity to one quarter, so mAs must be multiplied by four to keep the same exposure (the density maintenance formula: new mAs / old mAs = new distance squared / old distance squared). The same law protects staff: stepping from 1 meter to 2 meters from a scatter source cuts exposure to one quarter.

Photon interactions in the patient

In the diagnostic range two interactions matter. In the photoelectric effect, the photon is completely absorbed by ejecting an inner shell electron. Its probability rises steeply with atomic number (roughly Z cubed) and falls steeply with energy, so it dominates at low kVp and in bone, iodine and barium. Photoelectric absorption creates image contrast (bone looks white because it absorbs) and is the main source of patient dose. In Compton scatter, the photon ejects an outer shell electron and continues in a new direction with less energy. Compton scatter dominates in soft tissue at higher kVp, adds uniform gray fog that lowers contrast, and is the main source of occupational exposure, since scattered photons travel toward the operator. Pair production needs over 1.02 MeV and does not occur at diagnostic energies.

Grids and scatter control

Scatter rises with field size, part thickness and kVp. The three tools that fight it are collimation (smaller field), grids and the air gap. A grid is a plate of thin lead strips separated by radiolucent interspace material placed between patient and receptor. Primary photons travel parallel to the strips and pass; scattered photons arrive at an angle and are absorbed.

• Grid ratio is strip height divided by interspace width (h/D). Common ratios are 8:1 to 12:1. Higher ratios clean up more scatter but need more exposure and are less forgiving of tube angulation or off centering.
• Focused grids must be used within their focal range; wrong distance, upside down use or off centering causes grid cutoff. A Bucky moves the grid so the lead lines blur out.
• Rule of thumb: use a grid when the part is thicker than about 10 to 12 cm or the technique exceeds about 60 to 70 kVp. Most adult spinal views qualify; small extremities do not.
• The air gap technique lets scatter spread out and miss the receptor, as on the 72 inch lateral cervical film.

Film, screens and digital receptors

Traditional film screen systems use intensifying screens (rare earth phosphors in modern systems) that convert x-rays to light and cut dose dramatically; faster screens cut dose further at some cost in detail. In processing, developer reduces exposed silver halide to black metallic silver and fixer clears unexposed crystals.

Most offices now use digital receptors:

• Computed radiography (CR) uses a cassette with a photostimulable phosphor plate (usually barium fluorohalide) that stores a latent image; a laser in the reader releases the stored energy as light, which is digitized.
• Direct digital radiography (DR) uses a flat panel. In indirect DR, a scintillator (cesium iodide or gadolinium oxysulfide) converts x-rays to light, and amorphous silicon photodiodes with a thin film transistor array read it. In direct DR, amorphous selenium converts x-rays straight to electrical charge.

Digital systems have wide dynamic range and automatic rescaling, so an overexposed image still looks good. The danger is dose creep, upward drift in technique that the image no longer punishes; watch the exposure index. Underexposure shows as quantum mottle (grainy noise).

Image quality: the four qualities

Magnification factor equals SID divided by source to object distance (SOD). A long SID and short OID minimize magnification. Shape distortion occurs when the part, the central ray and the receptor are not aligned: tube angulation elongates, and a part tilted away from the receptor plane foreshortens. Motion is the most common cause of unsharpness and is controlled by short exposure times, clear breathing instructions and immobilization.

Biologic effects and radiation units

Ionizing radiation damages DNA mainly by indirect action through free radicals formed from water. The Law of Bergonié and Tribondeau states that radiosensitivity is greatest in cells that are rapidly dividing, have a long dividing future and are poorly differentiated. Lymphocytes (the most sensitive cell in the body), spermatogonia, erythroblasts, intestinal crypt cells and the embryo are highly sensitive; mature nerve and muscle cells are the least sensitive.

• Deterministic (non stochastic) effects have a threshold, and severity rises with dose: skin erythema, epilation, cataract, sterility, acute radiation syndrome. Diagnostic plain films are far below these thresholds.
• Stochastic effects have no known threshold, and the probability (not the severity) rises with dose: cancer induction and heritable genetic effects. Radiation protection assumes a linear no threshold model, which is the reason for ALARA.

Natural background is about 3 mSv per year. Approximate adult effective doses: PA and lateral chest 0.1 mSv, cervical series 0.2 mSv, lumbar series 1.5 mSv, CT abdomen and pelvis 8 to 10 mSv. A lumbar series is roughly six months of background, one reason imaging must be ordered for a reason.

ALARA and practical protection

ALARA (as low as reasonably achievable) is the guiding principle: every exposure must be justified (the result will change management), and every justified exposure must be optimized. For the patient this means:

• Order films only when history and examination justify them.
• Collimate tightly, filter properly and use high kVp, low mAs technique where appropriate.
• Position carefully to avoid retakes, which double the dose.
• Shield radiosensitive organs when the shield will not obscure the anatomy of interest. Traditional teaching calls for gonadal shielding when the gonads lie within about 5 cm of the primary beam in a patient with reproductive potential. Current medical physics guidance has moved away from routine gonadal and fetal shielding, because a shield can hide pathology or make automatic exposure control raise output, and favors collimation and technique instead.
• Use PA rather than AP projections where possible for the full spine and chest to lower breast and thyroid dose.

For staff, the three cardinal principles are time (minimize), distance (maximize; inverse square law) and shielding. The operator stands behind a protective barrier and never holds a patient or a receptor; if someone must hold a patient, it should be a non pregnant adult family member wearing a lead apron and gloves, never the same staff member repeatedly. Protective aprons are at least 0.5 mm lead equivalent. Primary barriers (struck by the primary beam) are typically 1/16 inch (1.6 mm) of lead; secondary barriers (scatter and leakage only) about 1/32 inch. Staff wear a personal dosimeter (optically stimulated luminescence or thermoluminescent badge) at collar level, outside the apron.

Pregnancy and the patient of childbearing age

The embryo is most vulnerable to lethality in the first two weeks after conception (an all or none effect), to malformations during organogenesis (about weeks 2 to 8), and to neurologic effects such as reduced IQ and microcephaly from about weeks 8 to 15. Fetal doses below about 50 mGy (5 rad) are not associated with an increase in malformations or fetal loss, and no single diagnostic plain film of the spine approaches that dose; a lumbar series delivers a fetal dose on the order of a few mGy. Even so, elective imaging of the abdomen and pelvis should be avoided in pregnancy.

• Ask every woman of childbearing age (roughly 12 to 50) about the possibility of pregnancy and the last menstrual period, and document the answer. Post a sign in the dressing area.
• The 10 day rule schedules elective abdominal and pelvic films in the first 10 days after the start of menses, when pregnancy is unlikely.
• If pregnancy is possible and the study is elective, defer it or obtain a pregnancy test. If imaging is truly needed, use nonionizing alternatives (ultrasound, MRI without gadolinium) or collimate tightly and image away from the abdomen.

Section 2Radiographic positioning and normal anatomy of the spine and pelvis

General conventions

Projections are named by the path of the central ray (AP enters the front), and the part closest to the receptor is sharpest. Most spinal views use a 40 inch (100 cm) SID; the lateral cervical, the PA chest and the full spine film use 72 inches (180 cm) to reduce magnification caused by the large object to image distance across the shoulders or chest. Films are viewed as if facing the patient, and every film needs a side marker and identification.

The cervical spine

The standard trauma and post traumatic series in chiropractic texts is the Davis series: AP lower cervical, APOM, neutral lateral, both obliques, flexion and extension (seven views). Flexion extension studies should be avoided in the acute trauma patient until fracture and gross instability are excluded, and should never be forced.

The oblique rule deserves memorizing. In an AP (posterior) oblique, the patient's back is toward the receptor and the foramina shown are those farther from the receptor (the side the patient is rotated away from). In a PA (anterior) oblique, the foramina shown are those closest to the receptor. So a right posterior oblique (right back against the receptor) shows the left cervical foramina, and a right anterior oblique shows the right foramina. The lumbar spine is the opposite: posterior obliques show the facets and pars closest to the receptor.

Normal lateral cervical anatomy and values

On the neutral lateral, trace four smooth lordotic curves: the anterior vertebral line (anterior body margins), the posterior vertebral line or George line (posterior body margins), the spinolaminar line (anterior margins of the spinous processes where the laminae meet, ending at the posterior arch of C1 and the opisthion region), and the posterior spinous line. A step in any of them means displacement. Normal values worth knowing:

Normal variants of the cervical spine

A variant is a deviation from typical anatomy that has no clinical significance in itself. The board tests them because mistaking one for disease leads to wrong diagnoses and needless referrals.

• Posterior ponticle (ponticulus posticus, Kimmerle anomaly): a bony arch over the vertebral artery groove on the posterior arch of C1, forming an arcuate foramen. Present in roughly 10 to 15 percent. Some authors advise caution with upper cervical rotation, but evidence of risk is limited.
• Os terminale: an unfused secondary ossification center at the tip of the dens; stable.
• Mach band: a dark (lucent) line across the base of the dens produced by the overlapping edge of the posterior arch of C1, the occiput or the teeth on the APOM. It extends beyond the bone margins and can mimic a type II odontoid fracture.
• Cervical rib: an extra rib from the C7 transverse process (about 0.5 to 1 percent), occasionally causing thoracic outlet syndrome. C7 transverse processes slope downward, whereas T1 transverse processes slope upward, which helps you count.
• Bifid spinous processes C2 to C6, the nuchal bone and calcified laryngeal cartilages.
• Pseudosubluxation of C2 on C3 in children (and sometimes C3 on C4) up to about 3 to 4 mm on flexion, recognized as normal when the posterior cervical line of Swischuk (drawn along the spinolaminar points of C1 to C3) passes within about 1.5 to 2 mm of the C2 spinolaminar point.

The thoracic spine

Standard views are the AP thoracic (central ray at T7, cathode toward the feet) and the lateral thoracic (arms forward; a breathing technique blurs ribs and lung markings), with a swimmer's view for the upper levels. On the AP, check the paraspinal lines (the left paraspinal line is normally a thin stripe; a bulge suggests hematoma, abscess or mass), the pedicles at each level, the costovertebral joints and the ribs. Thoracic kyphosis normally measures about 20 to 45 degrees.

Thoracic variants include bifid (Luschka) ribs, rib fusions, an azygos fissure and calcified costal cartilages; Schmorl nodes (corticated endplate depressions from vertical disc herniation) are usually incidental.

The lumbar spine

The Scotty dog on the lumbar oblique is the classic way to inspect the pars interarticularis. Its parts map onto the posterior elements as shown in Figure 7.2. A lucent band across the neck of the dog is a pars defect (spondylolysis), often described as a collar on the dog.

Normal lumbar anatomy and variants

On the AP, each pedicle is an oval cortical ring, and the paired pedicles look like an owl's eyes; the spinous process forms the beak. The interpediculate distance normally widens gradually from L1 to L5; abrupt widening suggests a burst fracture or an intraspinal mass. Disc heights normally increase from L1 to L4 to L5, with L5 to S1 often slightly narrower than L4 to L5. The psoas shadows should be symmetric; loss of one may mean retroperitoneal hemorrhage, abscess or mass, though it is often positional.

Common lumbar variants and incidental findings:

• Transitional lumbosacral segments (sacralization of L5 or lumbarization of S1), seen in perhaps 10 to 15 percent of people. Count carefully from T12 (last rib bearing vertebra) or from C2 on a full spine film; mislabeling levels causes errors when correlating with MRI or surgery.
• Spina bifida occulta of L5 or S1 (failure of fusion of the laminae), usually insignificant.
• Facet tropism (asymmetric facet orientation, one sagittal and one coronal) at L4 to L5 or L5 to S1.
• Limbus bone: a small triangular ossicle at the anterosuperior corner of a vertebral body, caused by disc herniation through the ring apophysis in adolescence; it must not be mistaken for a fracture fragment (its edges are corticated and rounded).
• Nuclear impressions, Hahn clefts, ununited apophyses and the knife clasp deformity (long L5 spinous process with S1 spina bifida, painful in extension).

The pelvis, sacrum and sacroiliac joints

The AP pelvis (central ray midway between the iliac crests and the pubic symphysis, feet internally rotated about 15 degrees to show the femoral necks) shows the hips, SI joints, ilia, pubic rami and symphysis. SI joints are best seen with a cephalad angled AP (Ferguson view) or obliques because their surfaces curve obliquely. The normal SI joint is 2 to 5 mm wide, with thin, crisp subchondral cortex on both sides; the lower two thirds is the synovial portion.

Pelvic variants and incidental findings include the paraglenoid (preauricular) sulcus (a notch below the SI joint on the ilium, more often in women who have borne children), os acetabuli at the acetabular rim, pubic ear, phleboliths (round calcifications with a lucent center in pelvic veins), vacuum phenomenon in the symphysis or SI joint, and osteitis condensans ilii (triangular sclerosis on the iliac side of the lower SI joint in young multiparous women, with a normal joint space and no erosion, unlike sacroiliitis).

The full spine film

A full spine radiograph (14 by 36 inch receptor or stitched digital images, 72 inch SID, upright) is used in some chiropractic techniques to assess posture, scoliosis and pelvic unleveling. It must be justified, use compensating filtration and tight collimation, and preferably a PA projection to cut breast and thyroid dose. Its detail is poorer than regional views, so it never replaces a regional series when pathology is suspected.

Section 3Radiographic positioning and normal anatomy of the extremities, thorax and abdomen

Extremity views

The rule for extremities is two views at 90 degrees at minimum (three for joints), and include the joint above and below for long bone shaft injuries. Extremities are thin, so they are shot without a grid, at low kVp, with a small focal spot for detail.

Extremity variants include the bipartite patella (superolateral, corticated), os trigonum and accessory navicular. Know the order of elbow ossification centers summarized by CRITOE (capitellum, radial head, internal or medial epicondyle, trochlea, olecranon, external or lateral epicondyle, appearing roughly at 1, 3, 5, 7, 9 and 11 years). On a lateral elbow, a visible posterior fat pad or a raised (sail shaped) anterior fat pad means joint effusion, and after trauma implies an occult fracture (radial head in adults, supracondylar in children). A fat fluid level (lipohemarthrosis) on a cross table lateral knee means an intra articular fracture has released marrow fat.

The chest film

The standard chest study is the upright PA at 72 inches plus a left lateral, both keeping the heart near the receptor; an AP film magnifies the heart and cannot judge cardiomegaly. Check technical adequacy before anatomy:

• Inspiration: about 10 posterior ribs (or 6 anterior ribs) above the right hemidiaphragm.
• Rotation: the medial ends of the clavicles are equidistant from the spinous processes.
• Penetration: the thoracic spine is faintly visible through the heart shadow.

The cardiothoracic ratio (widest cardiac diameter divided by widest internal thoracic diameter) should be under 0.5 on a PA film. The right heart border is the right atrium, with the superior vena cava above it. The left border, from top to bottom, is the aortic knob, the pulmonary trunk, the left atrial appendage (normally flat) and the left ventricle. The trachea is midline; the left hilum is usually slightly higher than the right; the right hemidiaphragm is usually 1 to 2 cm higher than the left because of the liver. Costophrenic angles are sharp. On the lateral, the retrosternal clear space and the retrocardiac space should be lucent, and lucency should increase as you go down the thoracic spine (the spine sign; increasing density lower down suggests lower lobe disease).

The silhouette sign is the key to localizing disease: a normal border disappears when water density lies against it. Loss of the right heart border means right middle lobe disease; loss of the left heart border means lingular disease; loss of a hemidiaphragm means lower lobe disease. The minor (horizontal) fissure is seen on the right PA film; the major (oblique) fissures are seen on the lateral.

The abdominal film

The plain abdomen, or KUB (kidneys, ureters, bladder), is a supine AP film from the diaphragm to the symphysis. An acute abdominal series adds an upright abdomen (or left lateral decubitus) and an upright PA chest, which is the most sensitive plain film for free intraperitoneal air under the diaphragm. On the KUB, inspect gas, masses, bones and stones:

• Gas pattern: a little gas in the stomach and colon is normal, with scattered small bowel gas. The 3, 6, 9 rule gives upper normal diameters: small bowel 3 cm, colon 6 cm, cecum 9 cm. Small bowel has valvulae conniventes that cross the full width; colon has haustra that cross only part way. Multiple air fluid levels and dilated small bowel suggest obstruction or ileus.
• Soft tissue outlines: psoas margins, renal shadows, liver edge, spleen tip and bladder.
• Calcifications: phleboliths, vascular calcification (including the abdominal aorta), renal and ureteral stones, gallstones, pancreatic and costal cartilage calcification.
• Bones: lower ribs, lumbar spine, pelvis and hips are on every KUB, and the spinal film is also an abdominal film. Every lumbar series must be inspected for aortic calcification and aneurysm, stones and masses.

Section 4Methods of interpretation and mensuration

A systematic search pattern

The most common reading error is satisfaction of search: finding one abnormality and stopping. A fixed order prevents it. The classic chiropractic mnemonic is ABCS:

• A, alignment: overall curves, the four lateral cervical lines, George line, scoliosis, listhesis, rotation, joint relationships.
• B, bone: general density (osteopenia or sclerosis), then each bone's size, shape, cortex and trabecular pattern; count pedicles on every AP spine film.
• C, cartilage: joint spaces and disc heights, whether narrowing is uniform or not, subchondral bone, erosions, calcification.
• S, soft tissues: prevertebral spaces, paraspinal lines, psoas, lung apices and bases, aortic calcification, stones and masses.

Check the corners, where incidental lesions hide, and finish by asking whether the film answered the clinical question.

Mensuration: the lines, angles and grades the board tests

Measurements are only as good as the positioning, so no single number makes a diagnosis.

Cobb angle and Meyerding grade in practice

To measure a Cobb angle, find the end vertebrae: the most tilted vertebrae at the top and bottom of the curve, whose endplates converge toward the concavity. Draw a line along the superior endplate of the upper end vertebra and the inferior endplate of the lower one, then draw perpendiculars and measure where they meet (or measure the angle between the endplate lines directly). Measurement error is about 3 to 5 degrees, so a change of 5 degrees or more between films is needed to call progression. Rotation is graded by the position of the pedicle on the convex side (the Nash Moe method), and skeletal maturity by the Risser sign, the ossification of the iliac crest apophysis from lateral to medial (grade 0 to 5); low Risser grades mean more growth ahead and more risk of progression.

The Meyerding grade is measured on the lateral by dividing the top of the lower vertebra (usually the sacral base) into quarters and noting which quarter the posteroinferior corner of the upper body has reached. The percent slip method (displacement divided by the width of the sacral base times 100) gives a continuous number for follow up.

Section 5Clinical applications of special imaging

Magnetic resonance imaging

MRI places the patient in a strong magnetic field, tips hydrogen protons with radiofrequency pulses and records the signal as they relax. Repetition time (TR) and echo time (TE) set the weighting. MRI uses no ionizing radiation and is the best test for soft tissue: discs, cord, nerve roots, marrow, ligaments, menisci and tendons.

Disc terminology follows a standard lexicon: a bulge is generalized extension of disc tissue beyond the endplate margins over more than 25 percent (90 degrees) of the circumference; a protrusion is focal, with a base wider than the displaced material; an extrusion has displaced material wider than its base or migrated above or below the endplate; a sequestration is a free fragment. Modic changes describe endplate marrow: type 1 is edema (T1 dark, T2 bright, associated with active pain), type 2 is fatty replacement (T1 and T2 bright), type 3 is sclerosis (dark on both).

MRI contraindications include non MRI conditional pacemakers and defibrillators, cochlear implants, certain ferromagnetic aneurysm clips and metallic orbital foreign bodies (screen metal workers with orbital films). Gadolinium is avoided in severe renal impairment (risk of nephrogenic systemic fibrosis) and in pregnancy unless essential.

CT, bone scan, ultrasound and DEXA

DEXA results are reported as a T score, the number of standard deviations from the young adult mean: 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. The Z score compares with age matched peers and is used in premenopausal women, men under 50 and children; a Z score of minus 2.0 or lower is below the expected range and calls for a search for secondary causes. Screening is recommended for women 65 and older, younger postmenopausal women at increased risk, and many guidelines include men 70 and older.

When advanced imaging is indicated

Appropriateness criteria published by the radiology profession rate each test for each clinical scenario. Their message for spinal pain is consistent: uncomplicated acute low back pain or neck pain, with or without radiculopathy, does not need imaging in the first 6 weeks, because findings rarely change care and incidental findings (disc bulges, degeneration) are common in people without pain. Imaging is indicated when red flags are present or conservative care has failed.

Words worth knowing