Nuclear magnetic resonance - chest; Chest MRI; NMR - chest; MRI of the thorax
A chest MRI uses powerful magnets and radio waves to construct pictures of the body.
Unlike conventional radiography and Computed Tomographic (CT) imaging, which make use of potentially harmful radiation (X-rays) that pass through a patient to generate images, magnetic resonance imaging (MRI) is based on the magnetic properties of atoms.
A powerful magnet generates a magnetic field roughly 10,000 times stronger than the Earth’s. A very small percentage of hydrogen atoms within the body will align with this field. Radio wave pulses are broadcast towards the aligned hydrogen atoms in tissues of interest, returning a signal of their own. The subtle differing characteristics of that signal from different tissues enables MRI to differentiate between various organs, and potentially, provide contrast between benign and malignant tissue.
Any imaging plane, or “slice”, can be projected, and then stored in a computer or printed on film. MRI can easily be performed through clothing and bones, however, certain types of metal in or around the area of interest can cause significant errors in the reconstructed images.
How the test is performed
Since MRI makes use of radio waves very close in frequency to those of ordinary FM radio stations, the scanner must be located within a specially shielded room to avoid outside interference. The patient will be asked to lie on a narrow table which slides into a large tunnel-like tube within the scanner.
If contrast dye is used, it will be injected into a small vein of the hand or forearm. A technologist will operate the machine and observe you during the entire study from an adjacent room.
Several sets of images are usually required, each taking from 2 to 15 minutes. A complete scan, depending on the sequences performed, and need for contrast enhancement, may take up to one hour or more. Newer scanners with more powerful magnets utilizing updated software and advanced sequences may complete the process in less time.
How to prepare for the test
No preparatory tests, diets, or medications are usually needed. An MRI can be performed immediately after other imaging studies.
Because of the strong magnets, certain metallic objects are not allowed into the room. Items such as jewelry, watches, credit cards, and hearing aids can be damaged. Pins, hairpins, metal zippers, and similar metallic items can distort the images. Removable dental work should be taken out just prior to the scan. Pens, pocketknives, and eyeglasses can become dangerous projectiles when magnet is activated and should not accompany the patient into the scanner area.
Because the strong magnetic fields can displace or disrupt the action of implanted metallic objects, people with cardiac pacemakers cannot be scanned and should not enter the MRI area. MRI also should not be used for people with metallic objects in their bodies such as inner ear (cochlear) implants, brain aneurysm clips, some artificial heart valves, older vascular stents, and recently placed artificial joints.
Sheet metal workers, or persons with similar potential exposure to small metal fragments, will first be screened for metal shards within the eyes with X-rays of the skull. The patient will be asked to sign a consent form confirming that none of the above issues apply before the study will be performed.
A hospital gown may be recommended, or the patient may be allowed to wear “sweats” or similar clothing without metal fasteners.
Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:
- infant test or procedure preparation (birth to 1 year)
- toddler test or procedure preparation (1 to 3 years)
- preschooler test or procedure preparation (3 to 6 years)
- schoolage test or procedure preparation (6 to 12 years)
- adolescent test or procedure preparation (12 to 18 years)
How the test will feel
There is no pain. The magnetic field and radio waves are not felt. Some people experience a claustrophobic feeling from being inside the scanner. The table may be hard or cold, but you can request a blanket or pillow.
The machine produces loud thumping and humming noises during normal operation. Ear plugs are usually given to the patient to reduce the noise. A technologist observes the patient during the entire procedure and may be spoken to through an intercom in the scanner. Some MRI scanners are equipped with televisions and special headphones to help the examination time pass.
Excessive movement can blur MRI images and cause errors in the image. If the patient has difficulty lying still or is very anxious, an oral or intravenous sedative may be given. There is no recovery, unless sedation was necessary. After an MRI scan, you can resume normal diet, activity, and medications.
Why the test is performed
An MRI provides detailed pictures of tissues within the chest cavity, without obstruction by overlying bone. It may be used to:
- Clarify findings from previous X-rays or CT scans.
- Show the structures of the chest from multiple planes.
- Help diagnose abnormal growths and provide information for the staging (such as the size, extent, and spread) of tumors in the chest cavity. MRI can distinguish tumors or other lesions from normal tissues.
- Show lymph nodes and blood vessels
- Evaluate blood flow.
- Avoid the dangers of angiography or of repeated exposure to radiation.
A normal MRI would not show any abnormalities in the size or position of organs in the chest cavity, as viewed from any plane. The MRI would not reveal any new growths or lesions. Organs would appear to be functioning normally (for those organs where MRI can indicate function).
What abnormal results mean
The sensitivity of MRI depends, in part, on the experience of the radiologist.
A chest MRI may reveal disorders including:
- thymus tumor
- lung masses
- esophageal mass
- other masses (aggregations of cells) or tumors of the chest
- abnormal lymph nodes
- swollen glands and enlarged lymph nodes in any location of the chest
- staging of tumors including invasion of blood vessels
- alveolar bullae (COPD)
- bronchial abnormalities
- cystic lung lesions
- pleural abnormalities, including thickening or pleural effusion
- abnormal pulmonary vessels
- coarctation of the aorta
- aortic stenosis
- aortic dissection
- pericardial effusion
- thoracic aortic aneurysm
Additional conditions under which the test may be performed:
- atrial myxoma - right
- atrial septal defect
- Cardiac tamponade
- ischemic cardiomyopathy
- mitral regurgitation - acute
- mitral regurgitation - chronic
- mitral valve prolapse
- pericarditis - bacterial
- pericarditis - constrictive
- pericarditis - post-MI
- pulmonary edema
- restrictive cardiomyopathy
- skin lesion of histoplasmosis
- SVC obstruction
What the risks are
There is no ionizing radiation involved in MRI, and there have been no documented significant side effects of the magnetic fields and radio waves used on the human body to date.
However, because the effects of strong magnetic fields on a fetus are not well documented at this time, pregnant women are usually advised to avoid MRI scans.
The most common MR intravenous contrast agent, gadolinium, is very safe, and although there have been documented allergic reactions to it, it is an extremely rare occurrence.
If sedation is used, there are associated risks of over-sedation. The technologist monitors the patient’s vital signs, including heart rate and respiration as needed.
People have been harmed in MRI machines when they did not remove metal objects from their clothes or when metal objects were left in the room by others.
MRI is more accurate than CT scan or other tests for certain conditions, but less accurate for others. The disadvantages include the high cost, long duration of the scan, and sensitivity to movement. People with claustrophobia or who are confused or anxious may have difficulty lying still for the relatively long scan times.
MRI is not portable and is incompatible with metallic implants, life support-devices, traction apparatus, and similar equipment.
MRI is superior in most cases where differentiation of soft tissues is necessary. It can view organs without obstruction by bone and foreign bodies. It is capable of showing the tissues from multiple viewpoints and is a noninvasive way to evaluate blood flow. Currently, MRI is not valuable in the evaluation of subtle changes of the lung tissue since the lungs contain mostly air and are difficult to image.
by Sharon M. Smith, M.D.