Clare F. Wallner
COMPUTED TOMOGRAPHY
Computed tomography (CT) creates a cross-sectional image of a patient by placing a radiograph source and a detector on a gantry that rotates around a patient. An image is displayed after computer reconstruction.
CT can best be used to differentiate calcified from noncalcified tissue. Major advantages over MRI include availability, speed, and decreased cost.
Major disadvantages include its associated ionizing radiation, and potential allergies, or renal insult with IV contrast.
Hounsfield units (HU) or CT numbers quantify the amount of attentuation through tissue. Air is most negative and cortical bone is the most positive.
The image can be manipulated to differentiate between structures. Window width refers to the range of HU for image processing; densities above the chosen range will be white and those below will be black. A narrow window width will look at a structure with little difference in tissue densities, such as the brain. Window level refers to where the range lies (low with densities close to air, like the lungs, or higher density to visualize bony structres).
Spiral (helical) scanning allows for continuous image acquisition by rotating the gantry around a moving patient table.
Multidetector array CT scanners can provide multiple (64, 128, or 256) images at the same time, reducing time and contrast needed. Combined with software improvements, they also allow for multiplanar and 3-D reconstruction.
Electron beam CT (EBCT) uses a stationary beam and anode to produce a very high resolution, rapidly obtained image. It is most used for cardiovascular imaging, but is limited by large size, higher cost, and limited availability.
GENERAL USES AND LIMITATIONS OF CT
Head CT is the primary study for screening ED patients for acute intracranial bleeding, trauma, and stroke.
CT is the imaging study of choice for many disorders of the abdomen, pelvis, retroperitoneum, and thoracic cavity including evaluation of the vasculature.
Fractures and other bony abnormalities are usually well demonstrated by CT, especially cervical spine, facial, and pelvic fractures.
Body areas that are poorly visualized with CT include the posterior cranial fossa and the pituitary fossa; the spinal cord cannot be well differentiated from cer-ebrospinal fluid without contrast (CT myelogram).
USE OF CONTRAST
Contrast material can be administered by the oral, intravenous, rectal, intra-arterial, intra-articular, or intrathecal route.
Oral contrast for abdominal CT scanning improves the likelihood of detecting bowel wall abnormalities, including hematoma, edema, mass, or laceration.
If contrast is to be administered to trauma patients, a water-soluble agent should be used to avoid the extravasation of barium-containing agents.
Adverse reactions include (1) idiosyncratic anaphy-lactoid reactions that occurs with increased frequency in those with prior adverse reactions, asthmatics, diabetics, those on beta blockers or metformin, renal or cardiac failure, extremes of age, and those with a history of atopy and (2) allergic reactions, including minor side effects, intermediate reactions such as hypotension or bronchospasm, and severe reactions including laryngeal or bronchial edema, severe bronchospasm, or cardiac dysrhythmias and arrest.
Pretreatment with steroids and diphenhydramine hours prior to IV contrast can reduce risk of reaction by a factor of 10.
Contrast-induced nephropathy (CIN) is an increased risk for those with preexisting renal insufficiency, diabetic nephropathy, or those taking nephrotoxic drugs. Hydration may help reduce the risk of CIN.
Patients who are on metformin may have an accumu lation of metformin resulting in potentially fatal lactic acidosis. Metformin should be held for 48 hours after the injection of IV contrast and then restarted after renal function is rechecked.
IONIZING RADIATION
There is a linear dose-response relationship between exposure to ionizing radiation and the development of solid cancers. One in 100 persons exposed to 100 mSv will develop cancer (whole-body CT scan is an exposure of 10 mSv).
Children are at greatest risk.
Alternative imaging modalities should be utilized in the pregnant patient. If CT is necessary, the abdomen and pelvis of pregnant patients should be shielded whenever possible to reduce radiation to the fetus.
MAGNETIC RESONANCE IMAGING
MRI takes advantage of the fact that the nuclei of hydrogen atoms in water and fat molecules act like spinning magnets. When placed in a strong magnetic field, these “magnets” align with the field, and when a short pulse of a radio wave of specific frequency is applied, the magnets change alignment and then realign with the external magnetic field. The resultant small voltage can be measured and displayed as an image.
T1 and T2 images are generated by net recovery in different planes and change with different tissues. T1 images clearly delineate different tissues and are considered “anatomy” scans. T2 images reveal bright fluid with most other tissues gray allowing edema to be easily seen. They are considered “pathology” scans.
The major advantages of MRI over other imaging modalities are (1) it does not use ionizing radiation and no short- or long-term side effects have been demonstrated, (2) it can produce image slices of any orientation through the body, and (3) in many body areas, it produces better contrast resolution and tissue discrimination than radiographs or ultrasound.
MRI SAFETY AND OTHER CONSIDERATIONS
CONSIDERED SAFE
There is no known increased risk to pregnant patients, although no definitive evidence of lack of teratogenic or acoustic effects on the fetus. MRI is prefered over imaging involving ionizing radiation in weeks 2 to 20 of pregnancy.
Many cardiac implanted devices are nonferromagnetic or only weakly ferromagnetic, and are considered safe for MRI. These include most cardiac stents, although additional protection is conferred 6 weeks after place ment due to epithilialization. Prosthetic heart valves are also considered safe; although those with stainless steel components may experience strong magnetic forces, they are less than the normal stress from car diac function and gravity.
CONTRAINDICATIONS AND SAFETY
Internal cardiac pacemakers and transvenous pacers may experience electrical currents or heating and may malfunction.
Hemodynamic support and monitoring devices, such as pulmonary artery catheters (Swan-Ganz), an intra-aortic balloon pump, or a ventricular assist device, may experience electrical currents or heating and may malfunction.
Certain (ferromagnetic) aneurysm clips may be affected, causing brain injury.
Steel slivers in the eye (asymptomatic in some metal workers) may injure the retina.
Cochlear implants may be damaged or cause injury.
Other implanted devices (neurostimulators, bone growth stimulators, etc.) may malfunction or cause injury.
Life-support equipment containing steel is attracted to the magnetic field and could injure the patient and the system.
Any ferromagnetic object carried into the room can also become a potential missile.
The examination can take from 30 to 60 minutes, and there must be no motion (except breathing). This may be difficult for patients who are claustrophobic or in pain and unsafe for an unstable patient.
ADVERSE EFFECTS
There is an unclear effect on a fetus. Gadolinium does cross the placenta and is relatively contraindicated during pregnancy.
Loud noise caused by the gradient coils may cause acoustic damage.
Gadolinium may lead to contrast nephropathy at high doses, although unlikely at standard doses.
Allergy reactions do occur, but are less common than with iodinated contrast.
Gadolinium may cause nephrogenic systemic fibrosis in patients with severe renal insufficiency (GFR <30 mL/min/1.73 m2), resulting in an FDA warning. Renal impairment due to hepatorenal syndrome or perioperative liver transplant patients may also lead to nephrogenic systemic fibrosis.
APPLICATIONS
MRI of the brain and spinal cord provides better diagnostic quality images than CT. Intravenous contrast agents (that are less toxic than agents used for CT) are sometimes required.
MRI has a major role in imaging the musculoskeletal system because it visualizes soft tissue with better resolution than CT and because it is sensitive to marrow and trabecular bone changes. This aids in the detection of osteomyelitis and avascular necrosis.
MRI is useful in evaluation of other soft tissue musculoskeletal trauma such as muscle or tendon tears, hemorrhage, and edema, and injuries to medium-sized nerves and the brachial plexus.
MRI can detect metastatic disease with high sensitivity and specificity.
Because it is fast, readily available, and compatible with life-support equipment, CT is still the imaging technique of choice for suspected head, spine, and abdominal injuries.
MRI SCANNING IN THE EMERGENT SETTING
In the ED setting, MRI is the imaging modality of choice for (1) evaluation of suspected spinal cord compression of any cause, (2) evaluation of radiographically occult femoral intertrochanteric and femoral neck fractures, and (3) evaluation of posterior cranial fossa pathology.
Potential uses include (1) evaluation of aortic dissection as well as carotid and vertebral artery dissection, (2) evaluation of appendicitis in pregnancy if ultrasound is unavailable or nondiagnostic, (3) evaluation of pulmonary embolism, (4) evaluation of cerebral venous sinus thrombosis, and (5) evaluation of CVA (higher sensitivity than CT). MR diffusion weighted imaging can detect ischemic injury within minutes and MR perfursion imaging can identify the ischemic penumbra.
For further reading in Tintinalli’s Emergency Medicine: A Compresensive Guide, 7th ed., see Chapter 299.2, “Computed Tomography,” by William Roper and Stephanie Abbuhl, and Chapter 299.3, “Magnetic Resonance Imaging,” by Esther K. Choo and Robert F. DeMayo.