One size does not fit all….
When we image patients, radiation matters!
Making the correct diagnosis is the most important goal of medical imaging.
A radiologist is a physician who is an expert in interpreting images of the body obtained with x-ray, fluoroscopy, ultrasound, CT (computed tomography) or MRI equipment. Choosing the most appropriate imaging exam involves a collaborative decision between the radiologist and the referring healthcare provider based on the patient’s suspected illness and the imaging resources available at the health care facility.
Let’s Image Gently® by:
- using MRI or ultrasound when it is more likely to reveal the correct diagnosis than an imaging test like CT, fluoroscopy, or radiography that exposes the patient to ionizing radiation
- when CT is the best way to make the correct diagnosis, using the lowest dose of radiation that will reliably produce diagnostic images
What is MRI?
Magnetic resonance imaging (MR or MRI) is a technique that uses a powerful magnet to look inside the body. MRI does not expose the patient to ionizing radiation. An MRI examination usually takes a relatively long time (30 to 90 minutes), and requires that the patient stay very still for nearly the entire time to get the best images. Therefore, young children or others who are unable to stay still for this period of time are sedated or receive general anesthesia so that the MRI examination can be performed. MRI reveals detailed anatomy of many parts of the body and is especially useful for imaging of the brain, spine, heart, abdominal solid organs, bowel, bones, joints, and muscles (figures).
Examples of MR images of the brain, abdomen, heart and foot.
Depending on the expertise and MRI equipment available at a particular institution, specific MRI scanning protocols are designed to acquire the best possible images of certain body parts and diseases. A few of these indications are discussed below.
Patients with hydrocephalus (dilated fluid-filled ventricles in the brain) often undergo numerous brain CT examinations to determine if there has been any detrimental change in size of the ventricles in the setting of suspected ventriculoperitoneal shunt dysfunction. Very fast (<10 minute) brain MRI protocols have been designed as an alternative to CT so that the degree of hydrocephalus may be rapidly determined in children of all ages without the use of sedation or ionizing radiation.
Rapid brain MRI protocols are generally not appropriate when looking for intracranial hemorrhage, tumor, stroke, infectious disease, or a seizure focus. When hydrocephalus is not the primary concern, a complete brain MRI should be ordered instead. These are better addressed by more traditional length examinations.
Tumor Diagnosis, Therapy Response Assessment & Surveillance
In many cases, tumors are initially diagnosed by CT or US. MRI is then sometimes used to provide greater anatomic definition of the tumor and surrounding brain or other organ system or to characterize the type of tissue in the tumor. MRI is the imaging examination of choice for assessing tumor response to treatment (arrows in following figure) and for surveillance for recurrence of certain tumors, especially those of the brain, spinal cord, bone and muscle.
Inflammatory Bowel Disease
MRI is the preferred imaging examination for monitoring patients with inflammatory bowel disease (IBD). Inflammation of the bowel wall, abnormal connections between bowel loops, and infected fluid collections outside the bowel can be detected with MRI. Abnormal peristalsis (motion of the bowel) can even be detected with a technique known as cine MRI.
Ultrasound is most often used as the initial imaging examination for children with suspected appendicitis. When the ultrasound examination is inconclusive, computed tomography (CT) is often performed. However, MRI is emerging as a viable alternative to CT for the evaluation of appendicitis (enlarged inflamed appendix shown by arrow in following figure) in children with an inconclusive ultrasound examination.
Musculoskeletal Infectious and Inflammatory Conditions
MRI is the preferred imaging examination for diagnosing infections of the bone, growth cartilage, and muscle. While bone scintigraphy (nuclear medicine study) can also be used to diagnosis osteomyelitis, MRI better shows associated conditions such as septic arthritis and abscesses in and around bones and muscles that may require surgical drainage (arrows in figure below).
MRI is also very useful for demonstrating the extent of arthritis, such as juvenile idiopathic arthritis, and the extent of inflammation not due to infection in a setting such as juvenile dermatomyosis. This helps to guide treatment and monitor therapy response to anti-inflammatory drugs (see arrows in the following figures).
Tendon, Ligament, Cartilage and Bone Injury
Due to its excellent demonstration of different structures such as fat, blood vessels, muscles, tendons, ligaments, cartilage, bone and bone marrow, MRI is the preferred imaging examination to detect injury to these tissues (arrows in figure below). The extent and severity of these injuries can be graded by MRI to assist in treatment planning by the orthopedic surgeon or sports medicine physician.
MRI is also especially good for detecting bone bruises and stress injuries that are not evident on radiographs (arrows in figure below).
Cardiac MRI (CMR) is an ideal imaging examination for many patients with acquired and congenital heart disease. CMR images readily show the anatomy of the heart, the aorta, and adjacent structures, enables examination of cardiac function and blood flow and characterization of abnormalities in the heart muscle (myocardium) including wall motion. CMR is the most accurate imaging method for measuring cardiac chamber volumes and ejection fraction and therefore provides crucial information for surgical planning in patients with congenital heart disease. CMR is very helpful for examining patients with various types of cardiomyopathy or cardiac injury as characteristics of the cardiac muscle and muscle mass may be determined.
Magnetic resonance angiography (MRA) with GBCA is often utilized as an adjunct to CMR exams as the technique is ideal for producing 3 dimensional reconstructions of cardiac anatomy.
Contrast Material for MRI examinations
Intravenous (IV) gadolinium-based contrast agents (GBCAs) are used in many MRI examinations, particularly when there is concern for tumor, infectious/inflammatory disease, or vascular abnormality. GBCAs improve the delineation of tumors, infected fluid collections, and inflamed soft tissues. GBCAs also enhance depiction of the arteries with magnetic resonance angiography (MRA) and veins with magnetic resonance venography (MRV). GBCAs may also be injected into joints to better delineate intra-articular structures with MR arthrography.
Anaphylactoid or other serious adverse reactions to GBCAs are very rare. Due to the risk of nephrogenic systemic fibrosis, renal insufficiency is a contraindication to the administration of certain linear GBCAs. This risk is lessened by using macrocyclic GBCAs. Please refer to the policies in place at your institution for recommendations regarding the use of GBCAs in patients with renal insufficiency. Deposition of GBCAs in the brain and other tissues is an area of current investigation, the possible detrimental effects of which are unknown at this time.
Sedation and General Anesthesia for MRI examinations
Many children, especially those less than 8 years of age, and even some adolescents and adults, are unable to remain still for the typical 30-90 minute duration of an MRI examination. Therefore, sedation or general anesthesia is often required in these patients to obtain a diagnostic quality MRI examination uncompromised by motion artifact. There are possible cognitive deficits from sedation/anesthesia in children, especially with prolonged and recurrent exposure, that are still unclear. This issue should be considered when deciding about imaging strategies.
- Because MRI involves a strong magnetic field, certain metallic materials or devices may become hazardous projectiles, malfunction or cause artifacts that degrade image quality.
- For safety, items that may become projectiles are never allowed into the MRI scanner room.
- Some metallic items, such as braces, are safe but may cause artifacts that degrade image quality (see figure below).
- Although exceptions exist, patients with pacemakers, defibrillators, and certain other electronic devices may be unable to undergo an MRI examination.