MRI

Study type: MRI; related body system: Circulatory, Digestive, Endocrine, Musculoskeletal, Nervous, Reproductive, and Respiratory systems.

MRI is useful when the area of interest is soft tissue. The study can be performed with or without the contrast medium gadopentetate dimeglumine (Magnevist), which is administered IV to enhance contrast differences between normal and abnormal tissues. The technology does not involve radiation exposure and is considered safer than other imaging methods, such as radiographs and computed tomography (CT). MRI uses a magnet and radio waves to produce an energy field that can be displayed as an image of the anatomical area of interest based on the water content of the tissue. The magnetic field causes the hydrogen atoms in tissue to line up, and when radio waves are directed toward the magnetic field, the hydrogen atoms absorb the radio waves and change their position. This change in the energy field is detected by the equipment, and an image is generated by the equipment’s computer system using assigned values that correspond to the strength of the signal produced; the anatomical images are represented in various shades of gray.

MRI produces cross-sectional images of the vessels in multiple planes without the use of ionizing radiation or the interference of bone or surrounding tissue. Images can be obtained in two-dimensional (series of slices) or three-dimensional sequences. Standard or closed MRI equipment has the appearance of an open tube or tunnel; open MRI equipment has no sides and provides an alternative for people who suffer from claustrophobia, pediatric patients, and patients who are obese. Some open MRI units are designed to allow the patient to stand or sit while images are taken in various body positions. IV gadolinium-based contrast media may be used to better visualize the vessels and tissues in the area of interest. Clear, high-quality images of abnormalities and disease processes significantly improve the diagnostic value of the study.

MRA

MRA is an application of MRI that provides images of blood flow and diseased and normal blood vessels. In patients who are allergic to iodinated contrast medium, MRA is used in place of angiography. MRA is particularly useful for visualizing vascular abnormalities, dissections, and other pathology. Special imaging sequences allow the visualization of moving blood within the vascular system, and two common techniques are used to obtain images of flowing blood: time-of-flight and phase-contrast MRA. In time-of-flight imaging, incoming blood makes the vessels appear bright, and surrounding tissue is suppressed. Phase-contrast images are produced by subtracting the stationary tissue surrounding the vessels where the blood is moving through vessels during the imaging, producing high-contrast images. MRA is the most accurate technique for imaging blood flowing in veins and small arteries (laminar flow), but it does not accurately depict blood flow in tortuous sections of vessels and distal to bifurcations and stenosis. Swirling blood may cause a signal loss and result in inadequate images, and the degree of vessel stenosis may be overestimated.

MRI Abdomen

Abdominal MRI is performed to assist in diagnosing abnormalities of abdominal and hepatic structures. Contrast-enhanced imaging is effective for distinguishing peritoneal metastases from primary tumors of the GI tract. Primary tumors of the stomach, pancreas, colon, and appendix often spread by intraperitoneal tumor shedding and subsequent peritoneal carcinosis (condition in which cancer advances throughout large areas of the body).

Magnetic resonance cholangiopancreatography (MRCP) is an imaging technique used specifically to evaluate the hepatobiliary system that is composed of the liver, gallbladder, bile ducts, pancreas, and pancreatic ducts. MRCP is a less invasive way than endoscopic retrograde cholangiopancreatography (ERCP) to investigate abdominal pain, suspected malignancy, gallstones, or pancreatitis.

MRI Brain

Standard brain MRI can distinguish solid, cystic, and hemorrhagic components of lesions. This procedure is done to aid in the diagnosis of intracranial abnormalities, including tumors, ischemia, infection, and multiple sclerosis, and in assessment of brain maturation in pediatric patients. Rapidly flowing blood on spin-echo MRI appears as an absence of signal or a void in the vessel’s lumen. Blood flow can be evaluated in the cavernous and carotid arteries. Contrast-enhanced imaging is effective for enhancing differences between normal and abnormal tissues. Aneurysms may be diagnosed without traditional iodine-based contrast angiography, and old clotted blood in the walls of the aneurysm appears white.

Functional MRI (fMRI) is a neuroimaging application of MRI used to study how the brain is working. It identifies changes in blood flow, reflected by changes in the level of blood oxygenation, in response to activity. fMRI can identify metabolic changes in normal, diseased, or injured brain tissue. It is also used in research to study which parts of the brain are responsible for speech, physical movement, thought, and sensations; this type of research is also called brain mapping and has significant implications in understanding and managing the effects of stroke, brain tumors, and diseases such as Alzheimer disease. fMRI is based on the blood oxygen level–dependent contrast mechanism that takes advantage of the inherent paramagnetic quality of deoxyhemoglobin. In a properly performed study, the patient is asked to perform a task; the MRI scanner detects changes in the signal strength of brain water protons produced as blood oxygen levels change, and the corresponding strength of the natural paramagnetic signal of deoxyhemoglobin changes.

Magnetic resonance spectroscopy (MRS) is an application of MRI based on the same principles as MRI, but instead of as an anatomical image, the data are displayed graphically as a series of peaks. The peaks represent specific elements and compounds that provide physiological data regarding the tissue of interest. MRS can be performed using MRI equipment with software adapted for the collection and interpretation of spectral data. MRS may be used alone or in conjunction with MRI, whereby anatomical images are first collected by MRI followed by focused MRS images that reflect specific active metabolic processes. The frequency information used in MRS identifies specific chemical compounds, such as amino acids, lipids, and lactate, that are commonly involved in or produced by cellular activity. The presence or absence of different metabolites in the spectral analysis can be used to identify metabolic activity associated with a suspected tumor, differentiate between tumor types, provide information about brain lesions (brain tumors, Alzheimer disease), and monitor response to therapeutic interventions.

MRI Breast

MRI imaging of the breast is not a replacement for traditional mammography, ultrasound, or biopsy. This examination is extremely helpful in evaluating mammogram abnormalities and identifying early breast cancer in women at high risk. Women at high risk include those who have had breast cancer, have an abnormal mutated breast cancer gene (BRCA1 or BRCA2), or have a mother or sister who has been diagnosed with breast cancer. Breast MRI is used most commonly in women at high risk when findings of a mammogram or ultrasound are inconclusive because of dense breast tissue or there is a suspected abnormality that requires further evaluation. MRI is also an excellent examination in the augmented breast, including both the breast implant and the breast tissue surrounding the implant. This same examination is also useful for staging breast cancer and determining the most appropriate treatment.

MRI Chest

Chest MRI scanning is performed to assist in diagnosing abnormalities of cardiovascular and pulmonary structures. Two special techniques are available for evaluation of cardiovascular structures. One is the electrocardiograph (ECG)-gated multislice spin-echo sequence, used to diagnose anatomical abnormalities of the heart and aorta, and the other is the ECG-referenced gradient refocused sequence used to diagnose heart function and analyze blood flow patterns.

MRI Musculoskeletal

Musculoskeletal MRI is performed to assist in diagnosing abnormalities of bones and joints and surrounding soft tissue structures, including cartilage, synovium, ligaments, and tendons. MRI eliminates the risks associated with exposure to x-rays and causes no harm to cells. Contrast-enhanced imaging is effective for evaluating scarring from previous surgery, vascular abnormalities, and differentiation of metastases from primary tumors.

As with brain studies, MRS applications can also be used to provide information about the spine (e.g., demyelinating diseases) or conditions involving skeletal muscle disease (muscular dystrophies), and monitor response to therapeutic interventions.

MRI Pancreas

MRI of the pancreas is employed to evaluate small pancreatic adenocarcinomas, islet cell tumors, ductal abnormalities and calculi, or parenchymal abnormalities. A T1-weighted, fat-saturation series of images is probably best for evaluating the pancreatic parenchyma. This sequence is ideal for showing fat planes between the pancreas and peripancreatic structures and for identifying abnormalities such as fatty infiltration of the pancreas, hemorrhage, adenopathy, and cancers. T2-weighted images are most useful for depicting intrapancreatic or peripancreatic fluid collections, pancreatic tumors, and calculi. Imaging sequences can be adjusted to display fluid in the biliary tree and pancreatic ducts.

MRI Pelvis

Pelvic MRI is performed to assist in diagnosing abnormalities of the pelvis and associated structures. Contrast-enhanced MRI is effective for evaluating metastases from primary tumors. MRI is highly effective for depicting small-volume peritoneal tumors, carcinosis, and peritonitis and for determining the response to surgical and chemical therapies. Oral and rectal contrast administration may be used to isolate the bowel from adjacent pelvic organs and improve organ visualization.

MRI Pituitary

Pituitary MRI shows the relationship of pituitary lesions to the optic chiasm and cavernous sinuses. MRI has the capability of distinguishing the solid, cystic, and hemorrhagic components of lesions. Rapidly flowing blood on spin-echo MRI appears as an absence of signal or a void in the vessel’s lumen. Blood flow can be evaluated in the cavernous and carotid arteries. Suprasellar aneurysms may be diagnosed without angiography, and old clotted blood in the walls of the aneurysms appears white.

MRI Venography

Magnetic resonance venography (MRV) is an accurate, noninvasive technique used to detect deep vein thrombosis (DVT). This application of MRI provides images of blood flow in diseased and normal veins. In patients who are allergic to iodinated contrast medium, MRV is used in place of venography or CT venography. MRV is particularly useful for visualizing vascular abnormalities, thrombosis, and other pathology. MRV can be accomplished with a contrast-enhanced (CE) or non–contrast-enhanced method. Special imaging sequences allow the visualization of moving blood within the venous system. Two common techniques to obtain images of flowing blood are time-of-flight (TOF) and steady-state free precession (SSFP). In TOF imaging, incoming blood makes the vessels appear bright, and surrounding tissue is suppressed. SSFP is generally used for assessment of veins in the chest, abdomen, and pelvis. Although the initial evaluation of the iliac and lower extremity veins is usually accomplished with sonography, MRV is more efficient in detecting venous thrombus in the pelvic and calf veins, especially in obese patients and those with chronic asymptomatic thrombus.