The Basics of MRI

Chapter 14

Your MRI Exam



Introduction

Many readers of this hypertext book have asked what they can expect during their first MRI exam. This chapter is intended to help those readers prepare for their MRI exam. Although some familiarity with the preceding chapters of this text is recommended, it is not essential to understand this chapter. Your MRI exam may not proceed exactly as described here, but it will be similar enough so that this description will help you prepare for your exam.

Your MRI will be performed at a location which will be referred to as the MRI Center. When you first arrive at an MRI Center, you will report to the reception/waiting room. Here you will check in and fill out any necessary paperwork for your exam.

Jewelry, watches, coins, keys, and credit cards are incompatible with the magnetic resonance imaging procedure. You will need to leave these items at home or leave them in a locker provided in a dressing room at the MRI Center. These objects can be attracted by the magnet on the imager or distort the images. Here is a picture of a metal belt buckle being attracted by the magnetic field of the imager. There is clearly a large force pulling the belt buckle into the magnetic field. Alternatively these objects can inhibit radio frequency waves from getting into the body and thus produce distorted images. Certain articles of clothing, such as metal zippers, rivets, wires, and belt buckles, are incompatible with the imaging procedure for the same reasons. You will need to leave these items of clothing in the lockers and change into an imaging gown which contains no metal.

The MRI Center also has nursing, scan, control, computer, and reading rooms. You will only see the nursing area if you require special nursing assistance, for example if you have an intravenous line for medication. The scan room is where the imager is located and is where you will be for your exam. The technologists operate the imager from the control room. The computer room is where the imaging hardware and computer are located. The last room is called the reading room. This room consists of large light boxes on which a radiologist will read your images.

In addition to the receptionist who greeted you when you arrived, there are many individuals working as a team to make your MRI exam happen. A MRI nurse will take care of your nursing needs which might include administration of any medications or contrast agents that you may need. A MRI technologist will interact with you during your scan. The technologist will take you in and out of the scanner as well as operate the scanner. A radiologist will prescribe which imaging sequences are to be used to record the magnetic resonance images, and will read or interpret the images. Many other individuals work behind the scenes to make the center functions properly. You will most likely not meet these individuals, but they include the custodial staff, service engineers, and scientists, all of whom keep the imager clean and functioning properly.

Screening

Certain anthropogenic, or man-made objects are incompatible with the radio frequency waves and magnetic fields used in magnetic resonance imagers. You left some of these objects, such as keys, pens, and zippers, in your locker or at home. Other objects which may have been placed inside of your body, or certain medical conditions, may also be incompatible with your MRI exam. The process of identifying these contraindicators, as they are called, is called screening. The reason why some of these items are contraindicators are discussed in Chapter 9. The MRI personnel will ask you many questions to determine if you can safely be imaged. You will also be asked you weight and height. This information will allow the pulse sequence to adhere to the specific absorption rate (SAR) limitations described in Chapter 9, and the proper choice of imaging hardware. Some of the screening questions can be found in the animation window.

If you have a pacemaker, you should not be imaged because the pacemaker will not function in the magnetic field of the imager. If you have metal filings, shrapnel, or ferromagnetic clips or pins in your body, you probably will not be imaged. These objects could be twisted by the magnetic field and cause serious injuries. Most new surgical metal implants are made of a high quality stainless steel which is non magnetic. You can be imaged with these, provided they are not in the anatomy being imaged. Dental fillings and bridgework are okay, but some bridgework may prevent the radio frequency waves from getting into the body if the head is imaged.

The Imager

You will be imaged inside a long tubular hole approximately 60 cm (24 inches) in diameter. If your feet or knees are being imaged, your head will be outside of the magnet. If your head, shoulder, or chest is being imaged, your feet will be outside of the imager. Some claustrophobic individuals get anxious in situations like this, but be assured that you are in constant intercom contact with the technologists in the control room and can get out at any time. Some imaging sites will allow a friend or relative to accompany you through your scan. This individual may sit in the scan room while you are being imaged and talk with you.

When you first lie on the bed of the imager, the technologist will position an imaging coil around the anatomy being imaged, in this example a head coil. The technologist will the move your body to position a specific piece of anatomy at a crossed light beam. This spot on your body will be advanced to the isocenter of the magnet before the scan begins. This process is called landmarking.

The Exam

Your exam will last between 30 and 60 minutes. You will need to lie still for periods of 3 to 10 minutes at a time while the series of images are collected. You can breathe freely during this time. You may, in some cases, be allowed to move slightly between scans, but not so much that your position changes.

The imaging session creates a series of repetitive knocking sounds when the magnetic field gradients are turned on and off. Listen to the following audio segment as an example of the sound generated by a magnetic resonance imager while an image is being acquired.

Because of the volume of these sounds, it is recommended that you wear ear protection. The ear protection consists of foam ear plugs which you roll between your fingers and insert into your ears This form of protection provides upwards of 26 dB of noise suppression. Some imaging sites provide an airplane-like audio system for those being imaged. These systems provide some noise suppression, and also mask out the imager sound with music.

Some individuals experience magnetophosphenes when placed in the magnetic field. Magnetophosphenes are a visual sensations of flashes of light on the retina. They were first reported by the French physiologist Jaques-Arsène d'Arsonval in 1896. They are caused by induced electric currents in the retina when moving through a static magnetic field, or when stationary in an changing magnetic field. In either case, the field strength must be greater than ~ 7 mT.

Other sensations which have been reported by some individuals during their magnetic resonance scan include vertigo or dizziness, a metallic taste, and nausea. The exact physiological cause of these effects is not fully understood. The reader is directed to the following references for additional information.

Your Results

In medicine, the diagnosis of disease is rarely the result of a single exam or test performed by a single individual. The physician, who interacts directly with you, takes advantage of input from many specialists. One of these specialists is the radiologist. A radiologist is a medical doctor, trained to interpret the information in magnetic resonance images. A radiologist will read the magnetic resonance images from your scan, and provide your physician with a report. Your physician will share the findings from the radiologist and other medical specialists with you.


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