AAOS Comprehensive Orthopaedic Review

Section 2 - General Knowledge

Chapter 16. Musculoskeletal Imaging

I. Radiography

A. Principles of radiography

 

1. Radiographic images are obtained by projecting x-ray beams through an object onto an image detector.

 

2. The image produced is a projectional map of the amount of radiation absorbed by the object along the course of the x-ray beam.

 

3. The amount of whiteness of the image is a function of the radiodensity and thickness of the object.

 

4. The denser the object, the more radiation is being absorbed, and hence a lighter or whiter image is produced. Metal objects and bone are very radiodense and appear white on radiographs.

 

5. Digital radiography

 

a. Commonly used now

 

b. Image processing and distribution are achieved through a picture archiving and communication system.

 

c. The process allows the images to be portable and transferable via computers or compact discs.

 

B. Digital radiography versus conventional film screen radiography

 

1. Film screen radiography has higher spatial resolution.

 

2. Improved contrast resolution for digital radiography means the technique is comparable in terms of diagnostic efficiency.

 

C. Radiation dose measurements

 

1. The scientific unit of measurement of radiation dose, commonly referred to as "effective dose," is the millisievert (mSv).

 

   *C. Benjamin Ma, MD, or the department with which he is affiliated has received research or institutional support from OREF and NIH.

 

2. Other radiation dose measurement units include the rad, rem, roentgen, and sievert.

 

D. Radiation exposure

 

1. Continual from natural sources

 

2. The average person in the United States receives an effective dose of 3 mSv/year from naturally occurring radioactive materials and cosmic radiation.

 

3. The average radiation dose from a standard chest radiograph is 0.1 mSv.

 

E. Advantages of radiography

 

1. Most commonly used medical imaging modality

 

2. Relatively inexpensive

 

3. Real-time radiographic imaging, or fluoroscopy, allows instantaneous feedback on stress radiographs, angiography, and orthopaedic interventions.

 

F. Disadvantages of radiography

 

1. Radiation is transmitted to the patient.

 

2. It is not effective for soft-tissue imaging because of poor contrast resolution.

 

3. The images obtained are always magnified. Measurement "standards" can be placed with the object to allow determination of magnification.

 

4. Although most medical x-ray beams do not pose a risk to a fetus, there is a small likelihood that serious illness and developmental problems can occur. The actual risk depends on the type of imaging study and the trimester of pregnancy.



II. Computed Tomography

A. Principles of CT

 

1. Uses x-ray beams to produce tomographic images, or slices of an object.

 

2. Multiple images are obtained and reassembled to generate a three-dimensional image.

 

3. X-ray densities are measured in Hounsfield units (HUs) or CT numbers.

 

a. Water is assigned a value of 0 HU; air, a value of -1,000 HU.

 

b. Images are displayed as grayscale; denser objects are lighter.

 

c. Good for soft-tissue imaging; grayscale can be modified ("windowed") to show data that fall within a fixed range of densities, such as bone windows or lung windows.

 

B. Advantages of CT

 

1. Tomographic nature of the images

 

2. Higher contrast resolution

 

3. The latest generation of CT scanners uses multiple detector row arrays, which leads to improved resolution and shorter acquisition times.

 

4. Images are processed digitally; images in a plane other than the one in which the original images were obtained can be reconstructed to give a different perspective of the object/tissue of interest.

 

5. Magnification artifacts that occur in plain radiography do not occur with CT; direct measurements can be performed on the scans.

 

6. CT can be combined with arthrography or myelography to evaluate specific joint or spinal abnormalities.

 

7. CT is useful for injections, biopsies, and aspirations of fluid collections.

 

8. CT provides better detail of cortical and trabecular bone structures than MRI.

 

C. Disadvantages of CT

 

1. Most CT slices require about 1 second, which is longer than the time required for a radiographic image; thus, scans are subject to motion artifacts.

 

2. CT is subject to artifacts due to metal objects.

 

a. Metal has high x-ray density, which prevents sufficient x-ray beams from being transmitted through the body part.

 

b. These result in an artifact called "beam hardening." Beam hardening appears as streaks of white or black that can obscure the anatomy adjacent to the metal object.

 

3. CT can be impractical for obese patients.

 

a. Most scanners have a weight limit.

 

b. Above the limit, the table that carries the patient through the scanner may not move or may break.

 

4. CT produces higher radiation exposure than plain radiography.

 

5. It is generally contraindicated for pregnant patients, except in life-threatening circumstances.



III. MRI and MR Arthrography

A.

General principles of MRI

1. MRI is similar to CT scanning in that images are produced by reconstruction of a data set.

 

2. MRI does not use radiation or have the tissue-damaging properties of radiation-based imaging modalities. It uses a strong magnet that generates a magnetic field and multiple coils that send and/or receive radio frequency (RF) signals.

 

3. All clinical MRI scans image the protons in hydrogen atoms. In a strong magnetic field, the protons line up like countless compasses.

 

4. A brief RF pulse is applied to the tissue that deflects the protons. When the pulse is terminated, the protons realign, or relax, along the strong magnetic field. The protons relax at different rates depending on their atomic environment.

 

5. The weak signal that the proton emits during relaxation allows the detector to detect the properties within the tissue.

 

6. Contrast on MRIs can be manipulated by changing the pulse sequence parameters. Two important parameters are the repetition time (TR) and the echo time (TE). The most common pulse sequences are T1-weighted and T2-weighted sequences. The T1-weighted sequence uses a short TR and short TE; the T2-weighted sequence uses a long TR and long TE. Different structures are identified more easily on each sequence (

Figure 1 and

Table 1).

 

7. Signals from fat also play an important role in providing contrast. Fat-suppression techniques add a useful dimension to the manipulation of image contrast. Fat-suppressed images are helpful in delineating various structural abnormalities.

 

8. The strength of the magnet is expressed in tesla (T) units. The stronger the magnet, the higher the intrinsic signal-to-noise ratio, which can improve imaging speed and resolution.

B.

Types of MRI scanners

1. Conventional

 

a. Requires a large room and a small bore

 

b. Has weight limit for patients

 

c. Takes longer than CT scanning

 

d. Patients with claustrophobia may not tolerate the scan well.

 

2. Open scanners or extremity scanners

 

a. Usually are lower field-strength machines

 

[Figure 1. The appearance of different anatomic structures on T1- and T2-weighted coronal MRIs of the knee. A, On the T1-weighted image, fat and bone marrow are bright, whereas the menisci and tendon are dark. B, On the T2-weighted image, joint fluid and blood vessels are bright in contrast to other structures.]

[Table 1. Relative Signal Intensities of Selected Structures in MRI]

 

b.

Can image the extremity well

 

c.

Can accommodate claustrophobic patients

 

d.

Images can be of adequate quality despite the lower field strength but generally provide less resolution than conventional closed MRI scanners.

 

D.

MR arthrography

1. Commonly used to augment MRI to diagnose soft-tissue problems.

 

a. In direct MR arthrography, a dilute gadolinium-containing solution is percutaneously injected into the joint.

 

b. In indirect MR arthrography, gadolinium is administered intravenously and allowed to travel through the vascular system to the region of interest.

 

2. MR arthrography is commonly used for diagnosis of labral tears in the shoulder and hip joints and postoperative evaluation of meniscus repairs.

E.

Advantages of MRI

1. Superior images of soft tissues, such as ligaments, muscle, and fat

 

2. Can give tomographic images of the object of interest

 

3. Can be more effective than CT at detecting changes in intensity within the bone marrow to diagnose osteomyelitis, malignancy, and stress fractures

 

4. MRI contrast (gadolinium) is safer than iodine-based media.

F.

Disadvantages of MRI

1. Prone to large and more severe types of artifacts from metal and motion

 

a. Metal screws or pellets can produce significant artifact, obscuring anatomic structures.

 

b. Metal suppression sequences can be used, but with loss of resolution.

 

2. Usually takes longer than CT

 

3. Patients need to remain still throughout the scanning process.

 

4. Sedation often is needed for pediatric patients.

G.

Dangers associated with MRI

1. Because of the strong magnet in the scanner, extreme caution is needed when a patient, physician, nurse, or technician enters the room. Electrical appliances such as pacemakers and mechanical pumps can malfunction.

 

2. Metal objects brought into the scanner can turn into dangerous projectiles.

 

3. Metal foreign bodies within the eye or brain can migrate and cause blindness and brain damage.

 

4. Patients with metal implants in their joints or body can have a MRI scan if the implants are secured in bone or stable. Discussion with the physician and technician before the scan is important to avoid potentially disastrous outcomes.

H.

Considerations in pregnant women

1. Although MRI does not use radiation, the effect of RF and magnetic field on the fetus is unknown.

 

2. It is usually recommended that a pregnant woman not have an MRI scan.

I.

Use of gadolinium as contrast

1. Gadolinium behaves like iodinated contrast media, accumulating in highly vascular and metabolically active tissues.

 

2. It should not be administered to patients with a creatinine < 33 mg/dL because it can lead to nephrogenic fibrosing dermopathy.

 

IV. Ultrasonography (Ultrasound)

A. Principles of ultrasonography

 

1. Uses high-frequency sound waves to produce images, analogous to using sonar waves to obtain images of the ocean.

 

2. A transducer produces sound waves that travel through the patient; echo waves are deflected back by the tissue to the same transducer.

 

3. The echo waves are then analyzed by the time traveled and amplitude, and the information is converted into an image.

 

4. Image resolution and beam attenuation depend on the wavelength and frequency.

 

5. A lower frequency ultrasound beam has a longer wavelength and less resolution but deeper penetration.

 

6. A higher frequency ultrasound beam can give higher resolution for superficial structures such as tendons and ligaments.

 

7. Doppler ultrasonography can be used to image blood vessels for flow velocity and direction. Color maps can be generated for color Doppler ultrasound.

 

B. Advantages of ultrasonography

 

1. Noninvasive at the frequencies used for diagnostic imaging

 

2. Commonly used for imaging in children and pregnant women

 

3. Shows nonossified structures such as the femoral head to diagnose hip dysplasia and dislocation

 

4. The equipment is portable and inexpensive compared with MRI and CT equipment.

 

5. Highly echogenic structures, such as a foreign body that may not be visible on radiographs, can be easily detected using ultrasonography.

 

6. Can be used for targeted therapy, such as injections and ablations

 

7. Useful for injections and aspirations of fluid collections

 

C. Disadvantages of ultrasonography

 

1. Image quality and interpretation depend on the experience of the ultrasonographer and radiologists.

 

2. Cannot image inside bone because bone cortex reflects almost all sound waves

 

3. Internal joint structures are not well visualized.



V. Nuclear Medicine

A. Principles of nuclear medicine

 

1. Uses radioisotope-labeled, biologically active drugs.

 

2. The radioactive tracer is administered to the patient to serve as markers of biologic activity.

 

3. The images produced by scintigraphy are a collection of the radiation emissions from the isotopes.

 

B. Bone scintigraphy (bone scan)

 

1. Generally performed using diphosphonates labeled with radioactive technetium Tc 99m.

 

2. Phases

 

a. The initial (transient) phase is characterized by tracer delivery to the tissue, which represents the perfusion images.

 

b. The second (blood pool) phase follows the initial phase.

 

c. The final (delayed) phase shows tracer accumulation in tissues with active turnover of phosphates, mostly in bone undergoing growth and turnover.

 

C. Positron emission tomography (PET)

 

1. PET using the metabolic tracer FDG is widely used in clinical oncology.

 

2. FDG accumulation reflects the rate of glucose utilization in tissue.

 

a. FDG is transported into tissue by the same mechanisms of glucose transport and is trapped in the tissue as FDG-6-phosphate.

 

b. Use of FDG in evaluation of the musculoskeletal system is based on an increased glycolytic rate in pathologic tissues.

 

c. High-grade malignancies tend to have higher rates of glycolysis than low-grade malignancies and have greater uptake of FDG than do low-grade or benign lesions.

 

D. Advantages of nuclear medicine imaging

 

1. Scintigraphy has high sensitivity for bone pathology.

 

2. Scintigraphy allows imaging of metabolic activity. Most metabolic processes involving bone have slow metabolic activity compared with that of soft tissue, such as kidney and liver. Fortunately, most radioisotopes are relatively long lived.

 

3. White cell scintigraphy can be used to diagnose osteomyelitis.

 

4. Scintigraphy can be used to diagnose metastasis, stress fracture, or occult fractures.

 

E. Disadvantages of nuclear medicine imaging

 

1. Lack of detail and spatial resolution

 

2. Has limited early sensitivity to detect acute fractures in patients with slow bone metabolism; it may take several days for the bone scan to be positive to diagnose occult femoral neck fracture.

 

3. Low sensitivity can occur with lytic diseases such as multiple myeloma.

 

4. Scintigraphy has low specificity for bone pathology.



VI. Radiation Safety

A. Children and fetuses are especially susceptible to ionizing radiation.

 

B. Radiography, CT, and bone scintigraphy produce ions that can deposit energy to organs and tissues. The energy can damage DNA.

 

C. Radiation for radioactive-labeled tracers in scintigraphy primarily affects the patient. Some tracers (eg, iodine-131) have half lives of several days and can concentrate in excreted body fluid and breast milk.

 

D. Rapidly dividing tissues are the most susceptible to radiation-induced neoplasia (

Table 2).

 

1. Bone marrow

 

2. Breast tissue

 

3. Gastrointestinal mucosa

 

4. Gonads

 

5. Lymphatic tissue

 

E. Risk of cancer is approximately 4% per sievert (100 rem).

 

F. Risk of fetal malformation

 

1. Greatest in the first trimester and with doses > 0.1 Gy (10 rad)

 

2. Late in pregnancy (≥ 150 days postconception), the greatest risk is an increase in childhood malignancies such as leukemia.

 

[Table 2. Threshold Acute Exposure Doses for Effects in Humans]

3. A 10-mGy (1-rad) dose increases childhood leukemia risk as much as 40%.

 

4. It is important to ensure that the patient is not pregnant when obtaining any imaging examinations other than ultrasonography. Performance of other types of imaging examinations can be discussed in consultation with the radiologist and physician.

 

G. Protection

 

1. Sensitive organs such as gonads should be shielded.

 

2. It is best to follow the principle of ALARA ("as low as reasonably achievable") dosing for pregnant women and children.

 

3. Exposure to radiation decreases as an inverse square of the distance from the source.

 

4. Medical personnel should wear lead aprons and be monitored using devices such as film badges.

 

5. CT delivers the highest amount of radiation dose among all medical imaging procedures (5-15 mSv versus 0.1-2.0 mSv for plain radiography).



Top Testing Facts

1. All clinical MRI scans image the protons in hydrogen atoms.

 

2. It is extremely important to screen patients with metallic objects before entering the MRI machine. Ferromagnetic objects in or on the body can be pulled toward the magnet and cause serious injuries.

 

3. Patients with advanced kidney failure should not receive gadolinium-containing contrast agents because exposure to the agent can cause development of nephrogenic fibrosing dermopathy.

 

4. A lower frequency ultrasound beam has a longer wavelength and less resolution but deeper penetration.

 

5. A higher frequency ultrasound beam can give higher resolution for superficial structures such as tendons and ligaments.

 

6. Caution is advised when ordering nuclear medicine tests for women who are breast-feeding; some of the pharmaceuticals can pass into the mother's milk and subsequently into the child.

 

7. Exposure to radiation decreases as an inverse square of the distance from the source.

 

8. CT delivers the highest radiation dosage of all imaging modalities.

 

9. The risk of cancer is approximately 4% per sievert (100 rem).

 

10. It is important to ensure that the patient is not pregnant when obtaining any imaging examinations other than ultrasonography. Performance of other types of imaging examinations can be discussed in consultation with the radiologist and physician.



Bibliography

Board on Radiation Effects Research, Commission on Life Sciences, National Research Council, Beir V: Committee on Biological Effects of Ionizing Radiation, in Health Effects of Exposure to Low Levels of Ionizing Radiation. Washington, DC, National Academy Press, 1990.

Brent RL, Gorson RO: Radiation exposure in pregnancy, in Current Problems in Radiology: Technic of Pneumoencephalography. Chicago, IL, Year Book Medical, 1972.

Johnson TR, Steinbach LS (eds): Imaging modalities, in Essentials of Musculoskeletal Imaging. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2003, pp 3-30.

Vaccaro AR (ed): Musculoskeletal imaging, in Orthopaedic Knowledge Update 8 Home Study Syllabus. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2005, pp 119-136.