Principles of Radiologic Interpretation

Lindsay J. Rowe

Terry R. Yochum

In radiography, as in photography, microscopy, and many other observations, the appearance which seems the most obvious does not always correspond with the real condition. Anonymous

GENERAL CONSIDERATIONS

Conventional radiographic procedures ( plain film) are the most frequently used imaging modality in the evaluation of the skeletal system. (1) Contemporary technologic advancements have led to the use of additional modalities, and each method provides its own specific area of pre-eminence. (2) Skeletal radiology is frequently the cornerstone of the diagnosis and appropriate management of a broad spectrum of conditions treated by numerous members of the health professions, including medicine, chiropractic, osteopathy, physiotherapy, and podiatry. (3,4,5,6) This chapter outlines the essentials of skeletal imaging, anatomy, and physiology and provides a working framework to enhance interpretive skills.

TECHNICAL CONSIDERATIONS

Numerous methods of investigating the skeletal system are available. (Table 7-1) Each modality has its own area of specific use and specialized information. Usually a number of these are used in combination to provide the necessary information for arriving at an accurate diagnosis.

Plain Film Radiography

Applications.

Plain film radiography is the most widely used skeletal imaging method. No contrast media is used to enhance various body structures. The visualization of these components depends on the natural contrast between the five radiographic densities—air, fat, water, bone, and metal. (Figs. 7-1 and 7-2) A basic premise in plain film evaluation is the absolute necessity of having a minimum of two views, preferably perpendicular to each other. (Fig. 7-3) These should be supplemented with additional projections, such as oblique, angulated, or stress studies, as clinically indicated. (1) (Figs. 7-4 and 7-5)

Considerable conflict has arisen in more recent times over the indications for skeletal radiography—particularly in the spine—as concerns for radiation safety, public health, and rising healthcare costs have gained momentum. Guidelines have been developed to assist in the imaging decision-making process. (2,3,4,5,6) (Table 7-2)

Applications of skeletal radiography are numerous. It is often the initial investigation of skeletal abnormalities and provides pivotal information regarding any additional imaging modality that may be indicated or contraindicated. Recognition of a definite diagnosis is frequently possible or at least a short list of differential
diagnoses can be determined. In addition, plain films are important for comparison of changes in the disease process over time or with therapy and provide incidental detection of unsuspected bony and soft tissue abnormalities. Biomechanical information can be gleaned from functional stress radiographs obtained at extremes of motion, on weight bearing, or with the application of compression-distraction forces. (7)

Table 7-1 Skeletal Imaging Modalities

Modality	Biomechanics	Pathology
Conventional Imaging
Plain film radiography	+	+++
Stress radiography	+++	+++
Tomography	+	+++
Xeroradiography	−	+++
Contrast Imaging
Arthrography	+	+++
Angiography	−	+++
Lymphangiography	−	+++
Myelography	+	+++
Discography	+	+++
Sinography	−	+++
Radioisotopic Imaging
Bone scan	−	+++
Advanced Imaging
Ultrasound	+	++
Cineroentgenology	+++	+
Computed tomography	+	+++
Magnetic resonance imaging	+	+++
+++, high sensitivity; ++, sensitivity; +, low sensitivity; -, no application.

Figure 7-1 NORMAL RADIOGRAPHIC DENSITIES. The four naturally occurring radiodensities are shown in this x-ray of a test tube: air (a), fat (f), water (w), and bone (b). Note the distinctive difference in density of each component.

Advantages.

Conventional radiographic studies are readily available, relatively inexpensive, and non-invasive. They depict anatomic details in a format that is readily understandable and requires relatively few anatomic computations for the understanding of three-dimensional relationships. Key benefits are the demonstration of bony landmarks and the ability to assess contiguous structures over considerable length.

Disadvantages.

The major drawbacks consist of the lack of soft tissue discrimination, diminished sensitivity in detecting bone density changes, resolution of small lesions, radiographic latent period, variability and effect of exposure differences, technical artifacts, and exposure to ionizing radiation.

Soft tissue depiction is limited to definition of anatomic planes determined largely by surrounding fat. Diagnostic sensitivity can be limited; up to 30-50% loss of bone density and a lesion size of at least 1-5 cm are often necessary before a structure is visible on a radiograph. (8,9,10) In this context the patient may have extensive histologic disease and have a normal-appearing radiograph. (Fig. 7-6) Similarly, the time interval from when the disease process manifests clinically until it becomes visible radiographically can be quite long. This is referred to as the radiographic latent period. Osteomyelitis in a peripheral bone takes a minimum of 10-14 days before it can be seen on the plain film, whereas in the spine this can take 21 days. Tumors may be expected to be slower than infections to manifest, though aggressive lesions can manifest within 4-6 weeks.

Tomography

The word tomography is derived from the Greek tomos, which means “a cutting.” Tomography involves visualizing a selected anatomic layer and has been called laminography and zonography. Essentially it permits the detailed examination of anatomy and abnormalities that are inaccessible by conventional radiography, allowing selective imaging at any selected level in the body. The level to be imaged is set at the fulcrum between the x-ray tube and film cassette, which move in opposite directions during the exposure. (Fig. 7-7) The movement may be linear or hypocycloidal. The effect is to produce a sharp, in-focus image at the level of the fulcrum while blurring out those structures above and below. Sequential films are taken at varying fulcrum levels to totally evaluate the structure in question.

Applications.

Tomography is especially useful in the evaluation of such complex structures as the skull and spine; in addition, it delineates the site, characteristics, and extent of a bony abnormality. (Fig. 7-8)

Advantages.

Tomography is beneficial when overlying structures obscure a lesion. Subtle fractures and early bone destruction can be identified when plain films are normal. Complex anatomic deformities of the spine and pelvis can be readily analyzed with tomography.

Disadvantages.

High radiation dose is the single greatest handicap because multiple exposures are required. Furthermore, long exposure times are required to allow for tube-cassette motion, which allows for patient motion artifacts. Enough slices must be obtained at thin intervals, otherwise lesions will be overlooked. Soft tissue planes are more readily identifiable, but internal structure is not visible as a rule.

Figure 7-2 EXAMPLES OF RADIOGRAPHIC DENSITIES. A. Air Density. Observe the radiolucent densities within the soft tissues of the thigh in this patient with gas gangrene (arrows). B. Fat Density. Note the well-demarcated radiolucent mass within the soft tissues of the anterior arm (arrows). This appearance is characteristic of the benign fatty tumor lipoma. C. Water Density. Posterior to the distal femoral shaft, note the round, well-circumscribed mass of the same density as the surrounding muscle (arrows). The diagnosis was malignant synovial sarcoma. D. Bone Density. Note the smooth bony growth extending from the anterior arm (arrows). Observe the smooth cortical margin and internal trabecular pattern characteristic of post-traumatic myositis ossificans. E. Metal Density. Note the numerous round metallic densities, aligned in an S shape, in the right lower quadrant of the abdomen. These represent ingested shot pellets lodged in the appendix. (Panel C courtesy of C. H. Quay, MD, Melbourne, Australia.)

Figure 7-3 MINIMUM RADIOGRAPHIC PROCEDURE. A. AP Tibia and Fibula. Note that no abnormality is evident. B. Lateral Tibia and Fibula. Observe the oblique fracture line in the tibia, which was not visible on the AP view. COMMENT: No radiographic study is complete without a minimum of two views perpendicular to each other.

Figure 7-4 SUPPLEMENTARY SPINAL RADIOGRAPHS. A. AP Lower Cervical Spine. Observe the subtle destructive lesion in the C6 pedicle and articular pillar (arrows). B. Oblique Lower Cervical Spine. Note that the destruction is now clearly depicted (arrow). This is a characteristic lesion of osteolytic metastatic carcinoma. COMMENT: Cervical pedicles, foramina, and articular pillars require oblique films for proper visualization and evaluation. Lumbar obliques allow demonstration of abnormalities of the pedicle, facets, and pars region.

Figure 7-5 ANGULATED LUMBOSACRAL VIEW. Matching the sacral base angle has greatly improved the depiction of the sacrum, sacroiliac joint, and the L5 body and disc space. Observe the osteolytic destruction of the sacral ala (arrows) from metastatic carcinoma and the large degenerative osteophytes on the contralateral side (arrowheads). COMMENT: This is an excellent projection for evaluating pathology in any patient with low back pain.

Table 7-2 Guidelines for Obtaining Skeletal Radiographs^a

Probable Indicators		Non-Indicators
	Trauma (recent, old)		Patient education
	Unexplained weight loss		Routine screening
	Night pain		Habit
	Neuromotor deficit		Discharge status assessment
	Inflammatory arthritis		Routine biomechanical analysis
	History of malignancy		Pre-employment status
	Fever of unknown origin (> 100°F)		Physical limitations of patient
	Abnormal blood finding		Inadequate equipment
	Deformity (scoliosis)		Non-trained personnel
	Failure to respond to therapy		Financial gain
	Medicolegal implications		Recent high-level radiation exposure
			Pregnancy
Possible Indicators
	> 50 years of age
	Drug or alcohol abuse
	Corticosteroid use
	Unavailability of alternate imaging
	Unavailable, lost, or technically inadequate previous studies
	Dated previous studies
	Research
	Constitutional or systemic disease
	Recent immigration
	Therapeutic risk assessment (contraindications)
	Therapeutic response
^a Most of these guidelines were based on literature relating to assessment of spinal disorders.

Figure 7-6 LIMITATIONS OF PLAIN FILM EXAMINATIONS. A. Initial View, AP Wrist. Although the patient presented with swelling, pain, and a slight fever, note that the radius shows no radiographic abnormality. B. Initial Presentation, Radioisotope Bone Scan. Note the increased area of blackness (hot spot) in the corresponding symptomatic radius (arrows), compared with the normal side (arrowhead). C. 10-Day Follow-Up, AP Wrist. Observe the moth-eaten-type radiolucencies within the radial metaphysis (arrows). These are early plain film findings in osteomyelitis. COMMENT: This study demonstrates the sensitivity of the bone scan to increased bone activity, even when the plain film is normal. At least 30-50% destruction of bone is needed before the lesion is demonstrated on plain films; bone scans are positive with 3-5% destruction. In addition, the need for follow-up x-rays when signs and symptoms persist is demonstrated. (Courtesy of David P. Thomas, MD, Melbourne, Australia.)

Figure 7-7 PRINCIPLES OF TOMOGRAPHY. The film and x-ray tube move synchronously in opposite directions. Note that during the motion, the fulcrum (red circle) remains in clear focus, while areas above and below will be blurred and rendered almost invisible.

Xeroradiography

Xeroradiography is different from conventional radiography in that it uses an electrically charged selenium plate as the “cassette.” When x-rays strike the plate, the charge is lost. A fine blue powder is blown onto the plate, which adheres to the charged regions. The powder is then fused by heat onto paper to produce the image. Finally, the selenium plate is washed and recharged, ready for the next use.

Figure 7-8 TOMOGRAPHY. A. AP Sacrum. Observe that close scrutiny of the lower sacrum reveals destruction of the sacral foramina, cortices, and trabecular patterns (arrows). Note the obscuring gas shadow (arrowheads). B. Tomography, AP Sacrum. Note that the degree of sacral destruction is now more clearly depicted. The overlying confusing gas shadows are eliminated by this form of study. COMMENT: The diagnosis is chordoma. (Courtesy of David P. Thomas, MD, Melbourne, Australia.)

Applications.

Xeroradiography has been used mainly for examination of the breast (mammography), though some applications in skeletal imaging have been found. (Fig. 7-9) It is especially useful in the detection of foreign bodies and soft tissue problems.

Advantages.

The major advantage of the xeroradiography is the exquisite edge enhancement between different densities of bone and soft tissue, allowing far more detail than conventional films. (11) For bone lesions, the procedure is useful in differentiating cartilaginous and osteogenic bone tumors as well as clearly demonstrating changes, such as calcification, in soft tissues. (11)

Disadvantages.

Specific x-ray apparatus, cassettes, film, and processor are required. Of greatest concern are the high levels of radiation exposure required. The use of xeroradiography in contemporary skeletal imaging and mammography has been largely overtaken by other modalities.

Contrast Examinations

The major contrast examinations that may be used in the evaluation of skeletal disease are arthrography, angiography, lymphangiography, myelography, discography, and sinography.

Arthrography

Contrast opacification of joint cavities is a useful procedure in the evaluation of joint disease. Air is often combined with the injected
water-soluble media to provide a double-contrast arthrogram that provides better evaluation of the articular surfaces, menisci, and synovium. (Fig. 7-10) The most common joint examined by this procedure is the knee. Other joints less frequently injected include the shoulder, hip, and wrist. In addition, tendon sheaths (tenography) and bursae (bursography) are examined by the same method. Injection into the spinal facet joints is often done before the injection of anti-inflammatory or analgesic agents. (12)

Figure 7-9 XERORADIOGRAPHY. Observe the exquisite bone and soft tissue detail as well as the foreign body glass artifact (arrow). COMMENT: Although seen here in a black and white reproduction, the image is light blue and is mounted on paper.

Applications.

Clearly the major benefit of arthrography is in identifying intra-articular derangements of menisci or cartilage, ligamentous, and synovial abnormalities. Loose bodies, tears in cartilages, ligament tears, arthritic processes, and extension into adjacent tissues can all be clarified. MRI has reduced its usage.

Advantages.

Arthrography is a readily available technique that allows assessment of joint integrity and intra-articular components. Radiolucent structures such as cartilage are identifiable.

Disadvantages.

Drawbacks include the invasive nature of the procedure, operator and interpreter dependency, demonstration of only the surfaces of structures, and possible oversight of small lesions.

Angiography

The introduction of a water-soluble contrast agent into an artery (arteriogram) or vein (venogram) can add vital information to the understanding of the disease process and its subsequent management.

Applications.

Angiography is used mainly in the evaluation of bone tumors, vascular disease, traumatic musculoskeletal injuries, and avascular necrosis. In bone tumors, the size and aggressiveness of the lesion can be assessed by the degree of neovascularity, speed of media removal, and local structure invasion. Traumatic injuries may tear, distort, obstruct, dilate, or produce anomalous connections in vessels, which will be demonstrated only by angiography.

Figure 7-10 ARTHROGRAM, SHOULDER. Contrast media was placed into the glenohumeral joint and adjacent bursa. Note the lucent zone paralleling the surface of the humeral head (arrows), representing the articular cartilage.

Advantages.

The isolation of arterial supply, the extent of the lesion, and its relationship to adjacent structures aid greatly in the surgical management.

Disadvantages.

Drawbacks to angiography include the requirement of specialized equipment in a specialized facility, the invasiveness of the procedure, potential complications (hematoma, infection, embolization), and relatively high radiation doses.

Lymphangiography

Evaluation of the lymphatic system can be achieved by the injection of an oil-based medium. Lymphangiography requires the injection of a blue-green dye between one or more webs of the toes, followed 1 hr later by an incision on the dorsum of the foot to identify a large lymphatic channel. Once identified, the channel is cannulated and an oil-based medium is injected slowly over 1-3 hr. Approximately 24 hr later, the contrast medium will be identifiable within the lower extremity, inguinal, iliac, and para-aortic nodes. The media may remain visible for many months.

Applications.

The major disorders evaluated with lymphangiography include lymphoma, metastasis, and lymphatic obstruction.

Advantages.

The lymph nodes and lymphatic channels are assessed according to location, size, shape, and configuration for evidence of abnormality. In addition, flow characteristics can be identified.

Disadvantages.

The main deterrents to the use of lymphangiography is the incision on the dorsum of the foot, length of time for the procedure, and the frequent difficulties in interpreting findings.

Myelography

Contrast examination of the spine and spinal cord is achieved by injection into the subarachnoid space, usually in the lumbar spine at the L2 or L3 level. The contrast media is a non-ionic water-soluble compound.

Applications.

Myelography aids in the diagnosis of disc, vertebral canal, spinal cord, and nerve root disease. (Fig. 7-11)

Advantages.

It is especially useful in determining multilevel spinal stenosis and injury at the junction of the nerve root and spinal cord.

Disadvantages.

This invasive procedure carries the risk of infection and iatrogenic complications, such as damage to nerve roots and dural tears. If bleeding occurs upon injection, the risk for arachnoiditis increases. Postmyelogram headache is encountered in about 40% of cases; those who experience headaches before myelography will develop a postmyelographic headache 60% of the time, whereas other patients will develop postmyelographic headache only 30% of the time. (13) Anaphylaxis, while uncommon, can be a life-threatening complication. In view of these problems and limitations, MRI is considered the technique of choice in the investigation of the spinal cord and its surrounding intraspinal structures. (14) A more complete discussion of myelography is offered in Chapter 6.

Figure 7-11 MYELOGRAM. A. Neoplasm, L4 Pedicle. Note the destruction of the pedicle and adjacent vertebral body (arrowheads), with adjacent displacement of the contrast column by the soft tissue mass (arrow). These are characteristic findings of osteolytic metastatic carcinoma. B and C. Disc Herniation: L4. Observe the smooth indentation in the myelographic column (arrows).

Discography

Placement of water-soluble contrast media into the intervertebral disc under fluoroscopic control allows assessment of each disc injected.

Applications.

Injection of an opaque medium into the intervertebral nucleus pulposus (discography) has been used to evaluate discal pathology. Four aspects of the injected disc are assessed: the volume of contrast that can be injected (degenerative discs accept > 2 mL of contrast), the pain response of the patient on injection (provocational study), the pattern of contrast distribution (fissuring, extravasation, pooling), and the amount of resistance to injection. (14)

Advantages.

Discography allows not only a morphologic analysis but also the functional aspect of pain provocation. The reproduction of symptoms on injection is interpreted as a positive isolating factor for identifying discogenic pain. (15) When combined with CT (CT discography), the extent and orientation of internal annular tears (internal disc disruption) can be determined; such lesions are not routinely definable by any other means, including MRI. (16)

Disadvantages.

Specialized equipment and a specialized facility are required to carry out discography. Use of fluoroscopy increases the radiation dose for the procedure. As an invasive procedure the usual risks for infection (0.1-0.2%), nerve root damage, dural trauma and tears, chemical meningitis, exacerbation or worsening pain, and allergic reactions do exist. The issue of whether discography can precipitate or accelerate disc degeneration is unclear at this stage, though most reports do not support the hypothesis that the procedure is a deleterious factor. (17) A detailed discussion of this procedure is offered in Chapter 6.

Sinography

Sinography is a selective technique applied in the evaluation of bone infections that are associated with a draining external sinus. The sinus is catheterized, and the contrast media is injected to show the origin and extent of the infection.

Radionuclide Imaging

The principle of nuclear imaging hinges on the selective uptake of certain compounds by different organs of the body. Tissues that can be imaged include the brain, heart, lung, kidneys, and skeleton. In bone imaging (bone scan) the most common isotope used is technetium-99m-methylene diphosphate (^99mTc-MDP).

The procedure is performed by injecting 15-20 mCi of ^99mTc intravenously and waiting various time periods—from 30 min to 2 hr—for concentration in bony areas of increased blood flow and osteoblastic activity. This is the typical triphasic (three-phase) study: an initial flow study, a slightly delayed blood pool study, and some hours later a delayed study. An imaging device—the γ-camera—is placed over the patient to record the bone-emitted γ-rays as the isotope undergoes decay to a lower energy level. These rays are filtered by a pinhole collimator and transformed into light, and an image is transposed onto a Polaroid or radiographic film.

Projections of the entire body in the posteroanterior (PA) and anteroposterior (AP) planes are performed. In addition, selected spot projections over questionable locations are also obtained. Regions of increased uptake will show as dark spots (hot spots). Normally the isotope will collect more in the long bone metaphyses, thyroid, bladder, kidneys, sternum, costochondral junctions, spine, sacroiliac joints, and at the injection site. (18) (Fig. 7-12) The isotope half-life is 6 hr and is readily excreted by the kidneys. The combination of tomography and isotopic bone scans (single-photon emission computed tomography, SPECT) allows accurate assessment of regions within bone or where bones overlap, such as the pars interarticularis in the lumbar spine.

Applications.

The indications for a bone scan include the detection of skeletal metastasis and other tumors, infection, arthritis, fracture, and avascular necrosis. This is especially useful in diagnosing occult, stress, and recent fractures; in addition it is used for patients whose plain films are normal but who have pain of undetermined origin. (Fig. 7-13) Bone scans of the spine can be helpful in both the diagnosis and the therapeutic decision-making process, helping identify degeneration, pseudo-arthrosis in spinal fusions, active spondylolysis, spondylolisthesis, active fracture, infection, tumor, and metabolic disorders. It is not useful for evaluating spinal stenosis, disc herniation, or postlaminectomy status. (19)

Advantages.

Only 3-5% bone destruction is necessary for pathologic bone destruction to be detected on a bone scan, representing up to 10 times more sensitivity than plain film radiography. Early, clinically unsuspected bone disease, such as metastasis, can be readily identified.

Disadvantages.

Diagnostically, the image obtained is quantitative only—it is a measure of activity. Therefore, the scan of a neoplasm may appear identical to one of an infection, with further correlation providing the definitive diagnosis. Some skeletal lesions remain inactive or show areas of no uptake on bone scans. This may occur in rapidly destructive processes and in multiple myeloma. A more complete discussion of radionuclide imaging is offered in Chapter 6.

Ultrasound

Ultrasonography is frequently used in diagnosing abdominal, pelvic, and vascular disease. In problems of the musculoskeletal system, ultrasound is playing an increasingly greater role.

Applications.

Soft tissue masses—defining whether the mass is cystic or solid—cellulitis, abscess, and hematoma are amenable lesions for ultrasonographic investigation. Integrity of peripherally located tendons, especially the rotator cuff and Achilles tendon, can also be assessed. Neonatal and pediatric hip evaluation can be extremely rewarding in the early detection of congenital hip dysplasia, congenital hip dislocation, and intra-articular effusion. In the spine, ultrasound has limited use in the evaluation of spinal stenosis, disc herniation, and paraspinal soft tissue abnormalities. (20,21) Ultrasound provides a method of evaluating spondylolisthesis without the use of ionizing radiation. (22)

Advantages.

Visualization of soft tissues adjacent to the peripheral articulations (e.g., shoulder, knee) is where ultrasound becomes increasingly useful. It can be rapidly performed, is non-invasive, does not involve ionizing radiation, and can provide a dynamic evaluation of the soft tissue. It can be useful in guiding biopsy or joint aspiration techniques. Another valuable application is the display of vascular integrity and flow velocity.

Disadvantages.

Although the periosteum is visualized, the medullary cavity of bone cannot be resolved on ultrasound. Similarly,
tissues close to bone may be obscured. Metal implants greatly inhibit detailed visualization of the adjacent soft tissues. The accuracy of the examination is extremely operator dependent and carries a long learning curve. (See Chapter 6.)

Figure 7-12 NORMAL RADIOISOTOPIC BONE SCAN. Note the normal increased uptake in the metaphysis of the humerus, sternum, costochondral junctions, lumbar spine, ilium, and sacroiliac joints. The dense region in the elbow is the site of the intravenous injection of the isotope (arrow). COMMENT: Bone scans are evaluated for asymmetry in uptake because, normally, metabolically active regions will concentrate the isotope. (Courtesy of Thomas E. Hyde, DC, DACBSP, North Miami, Florida.)

Figure 7-13 ABNORMAL RADIOISOTOPIC BONE SCAN. A. AP Sacroiliac Joints. Note that unilateral sacroiliitis (arrows) is evidenced by loss of the sacral and iliac articular margins, with prominent reactive sclerosis. B. Bone Scan. Observe the increased isotopic uptake (hot spot) over the corresponding sacroiliac joint (arrows). COMMENT: Although the diagnosis is not definitive on these findings, the scan confirms the presence of increased bone activity. The diagnosis after aspiration of the joint was infection caused by Staphylococcus aureus. (Courtesy of Gerald A. Fitzgerald, MD, Sydney, Australia.)

Videofluoroscopy

Visualization of dynamic motion of body structures through fluoroscopy with recorded images has been a source of fascination for decades. Until the 1980s the method of recording was with 16- or 35-mm film and was referred to as cineradiography. With the advent of videorecording the transcription process became greatly simplified. In the spine, motion patterns have been evaluated in normal and abnormal conditions, though the technique’s clinical efficacy has not been fully elucidated. (5,23) No consensus on normal standards of motion patterns exists, and a wide range of normal variations have been described.

Applications.

Motion of internal organs, passage of introduced contrast agents, and joint function have been the major outpatient applications of videofluoroscopy. Assessment of joint instability, such as at the atlantoaxial joint in the presence of odontoid anomalies, has been shown to be useful in preoperative evaluations. Effects of fusion (arthrodesis) on joint motion can be appreciated as can failure of the fusion procedure. Accurate assessment of joint function requires digitization. (5,23)

Advantages.

Dynamic characteristics of joint mechanics are visualized throughout the entire range of motion. Recording of the images allows the movement sequence to be slowed for frame-byframe analysis and digitization for accurate appraisal.

Disadvantages.

Radiation dose can be high if multiple planes are imaged. (24,25) The procedure itself is not lengthy, but the subsequent analysis requires considerable time. There is a paucity of literature on which to base meaningful estimations of motion magnitude and patterns. (26) The expense and training necessary to do good fluoroscopic studies renders this modality most suited to the laboratory and not to routine clinical use. (5,23)

The essential components of a CT system include a circular scanning gantry housing the x-ray tube and image sensors, a table for the patient, an x-ray generator, and a computerized data-processing unit. The patient lies on the table and is placed inside the gantry. The x-ray tube is rotated 360° around the patient while the computer collects the data and formulates a transverse image, or slice. Each cross-sectional slice represents a thickness of between 0.5 and 1.3 cm of body tissue. To produce an optimal examination, multiple sequential slices are obtained. In the lumbar spine 20-25 sequential slices produce an average total study dose of 1-3 rads. (27) Contemporary computers are capable of producing multiplanar reconstructions (sagittal, coronal, oblique, and three-dimensional). (28)

Applications.

Although CT has had its greatest impact in the evaluation of the central nervous system, various uses have been found in skeletal imaging. (29) Following its introduction in 1973 by Hounsfield, many technical advances have occurred to improve resolution and diagnostic capability. (30,31) Skeletal applications include assessment of neoplasms, trauma, infections, metabolic disease, and spinal syndromes. (29) (Fig. 7-14) Bone mass determinations
for osteoporosis can be readily achieved (quantitative bone mineral analysis). The analysis of spinal syndromes is greatly improved when CT is combined with other modalities such as myelography (CT myelography), discography (CT discography), and angiography (CT-enhanced scan).

Figure 7-14 CT. A. Neutral Lateral Cervical Spine. Observe the lytic destruction of the C4 spinous, lamina, and articular pillar (arrows). The diagnosis was aneurysmal bone cyst. B. CT. Note that a selected image of the lesion confirms the anatomic location of the tumor in the posterior arch (arrows). Additional information is gained about its size, internal matrix, and soft tissue extension.

Advantages.

Exquisite depiction of bony and soft tissue anatomy is possible. Manipulation of the acquired data allows alterations in relative density (windows), measurements of density or distances (cursor), and display in multiple planes (multiplanar imaging).

Disadvantages.

Large patients and motion produce technically degraded images. Metal implants produce star-shaped artifacts (blooming), and penetration of thick bones adjacent to lowattenuation soft tissues produces beam hardening. The averaging of densities throughout the thickness of the slice may result in inaccurate representation of tissue densities (volume averaging). The radiation dose can be relatively high and scanning times lengthy. (See Chapter 6.)

MRI

MRI is also referred to as MR and nuclear magnetic resonance (NMR). The patient is placed in a magnetic field that displaces mobile hydrogen atoms, which can be measured and formed into an image. Those structures that have a high water content—such as the liver, spleen, muscles, and heart—produce the highest image details. The nucleus pulposus of the disc is readily identified in normal and degenerative conditions. When bone is examined, it is actually the marrow that forms the image, owing to the low levels of osseous water and high levels of fat.

Applications.

The use of MRI is widening rapidly. (32) (Fig. 7-15) Soft tissues are exquisitely captured and easily identifiable. Shoulder rotator cuff and Achilles tendon integrity are key areas of investigation. Because the bone marrow is imaged directly, MRI is extremely sensitive in detecting marrow disease. (33) The internal architecture of the joints is seen with dramatic detail. In the spine, the structure and hydration of the disc are represented clearly, as are the cord, nerve roots, and intrinsic canal contents (such as the epidural fat, ligamentum flavum, and facet joints). (34) (Fig. 7-15, B and C) The use of intravenous contrast (gadolinium) shows areas of increased vascularity, including regions of neoplasm and inflammation.

Advantages.

Complications with MRI are few and can be avoided. Soft tissue contrast resolution is excellent; direct multiplanar imaging without reconstruction, use of non-ionizing radiation, and direct accurate characterization of pathologic tissue are its strengths.

Disadvantages.

Claustrophobia is a common problem because the patient is situated inside the noisy small tunnel for protracted times. The presence of some metallic clips, especially intracranially, may prevent use of MRI. Joint replacements are not a contraindication; however, pacemaker and cochlear implant patients should not be scanned. The expense of the study, availability, and extensive equipment and housing requirements do inhibit its use. (See Chapter 6.)

Figure 7-15 MRI. A. Lumbar Spine, AP View. Note the exquisite detail of the liver (L), kidneys (K), and psoas muscle (P) obtained. The surrounding white (high signal) tissue is fat. B. Lumbar Spine, Lateral View. Note the intense white appearance of the normally hydrated nucleus pulposus and straight contour of the posterior annulus fibers (arrowhead). In comparison the dehydrated degenerated discs show a blackened internal structure (low signal intensity) and a bulging convex posterior annulus margin (arrows). The vertical white structure posteriorly represents the cerebrospinal fluid surrounding the thecal sac (T). C. Lumbar Spine, Axial View. Observe that the cross-sectional view through the mid-L5 vertebral body and intervertebral foramen demonstrates the L5 vertebral body (V), psoas muscle (P), thecal sac (T), and spinous process (S). The high signal intense epidural fat is seen symmetrically surrounding the thecal sac and exiting nerve roots (arrowheads). COMMENT: MRI depends on the presence of the hydrogen proton to produce an image. The bone marrow is visible because of its high fat content.

SKELETAL ANATOMY AND PHYSIOLOGY

Skeletal Development

Bone is derived from mesodermal tissue. The first bone to ossify in the body is the clavicle. Two processes of bone formation occur: intramembranous and enchondral.

Intramembranous Ossification

Initially, a model is formed from condensed mesenchymal cells. These cells then differentiate into two forms: (a) fibroblasts, producing collagen fiber membranes, and (b) osteoblasts, producing osteoid. Subsequently, bone is formed in this fibrous membrane. There is no preformed stage of cartilage. Bones formed by this process include the parietal and temporal bones, squama, and tympanic parts of the temporal bone, upper occipital squamosa, vomer, medial pterygoid, and upper face. The clavicles and mandible are also membranous bones but later develop secondary cartilaginous centers. In normal bones the width of bone is largely controlled by this method, owing to the activity of the periosteum (appositional bone growth).

Figure 7-16 GROWING BONE, ANATOMIC DIVISIONS. A. Representative Long Bone. B and C. Radiographs. Observe the epiphysis (e), physis (p), zone of provisional calcification (z), metaphysis (m), diaphysis (d), and apophysis (a).

Enchondral Ossification

From the condensed mesenchymal model cartilage cells (chondroblasts and chondrocytes) form and produce a cartilage cast of the definitive bone. Subsequently, this cartilage template is transformed to bone as peripheral capillaries penetrate and induce the formation of osteoblasts. This peripheral collar of new bone then extends bidirectionally along the long axis of the bone. A similar but separate process occurs within epiphyseal centers. This process is responsible for the formation of all tubular bones, vertebrae, ethmoids, and inferior conchae. This method of ossification is primarily used to lengthen long bones after birth, until skeletal maturity.

Bone Structure

The anatomic structure of bone includes various divisions—epiphysis, physis, zone of provisional calcification, metaphysis, and diaphysis. (Fig. 7-16) In addition, two types of bone are identifiable: cortical and medullary. These are encased by an outer covering of invisible periosteum.

Epiphysis

The end of a growing bone is called the epiphysis. Initially composed of cartilage, the epiphysis undergoes gradual ossification,
eventually fusing with the shaft. This signifies the end of the growth period. The primary function of the epiphysis is to produce and support the articular cartilage. Pathologically, it is prone to dysplasia, ischemia, arthritic deformation, and specific neoplasms such as chondroblastoma and giant cell tumor. Apophyses of bone should also be thought of as epiphyses, despite their function as muscle attachments.

Physis

The cartilage growth plate between the epiphysis and metaphysis is known as the physis. Alternate terms are epiphyseal plate, epiphyseal growth plate, and bone growth center. It is made up of layers of progressively maturing cartilage and developing bone. Adjacent to the physis, a stable zone of resting cells is found, followed by sequential zones of proliferation and hypertrophy. These layers are responsible for providing longitudinal growth of a long bone and remain radiolucent during skeletal development. Altered hormonal or vascular dynamics may decrease or increase growth, producing abnormalities of length, such as dwarfism and giantism.

Zone of Provisional Calcification

At the junction of the physis and metaphysis, a thin line of increased density is identifiable, known as the zone of provisional calcification (ZPC). This represents the region of calcification of the physis cartilage that is the precursor to bone formation. Calcium disorders, such as rickets, will disturb the appearance of this line. Residuals of intermittent growth arrest from systemic disease may be seen later in life as transverse, opaque metaphyseal lines (growth arrest lines).

Metaphysis

Between the ZPC and diaphysis lies the metaphysis. The metaphysis is the most metabolically active region of a bone and, as a result, is the most common site for tumors and infections of bone. In this location calcified cartilage is transformed into definitive weight-bearing stress trabeculae (primary trabeculae) and supporting transverse or oblique trabeculae (secondary trabeculae). In addition, as part of this process, the constricting tubulation of the bone occurs by periosteal resorption. Abnormalities of tubulation result in underconstriction or undertubulation of the metaphysis (Erlenmeyer flask deformity). (Fig. 7-17)

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