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Magnetic Resonance Imaging: Understanding the Principles and Recognizing the Basics


  • Magnetic resonance imaging (MRI) is a diagnostic modality capable of producing both anatomic and physiologic data that utilizes the molecular composition of tissues, especially water, to generate images with extraordinary contrast between soft tissues, surpassing the sensitivity of other imaging modalities.

  • To achieve this sensitivity , MRI studies are designed using specific scanning protocols based on the clinical question that allows the signal from certain tissues to be maximized while minimizing competing signal from other tissues.

  • Some understanding of these protocols is necessary to comprehend how these contrast differences are generated and then to recognize what imaging protocol was used in order to correctly interpret the images.

How MRI Works

  • MRI uses a very strong magnetic field to manipulate the electromagnetic activity of atomic nuclei in a way that releases energy in the form of radiofrequency signals , which are recorded by the scanner’s receiving coils and then computer-processed to form an image.

    • Clinical MRI scanners utilize the properties of hydrogen nuclei (which contain one proton) due to their abundance in the human body.

  • Each proton has a positive electrical charge , and because protons also have a spin, this charge is constantly moving. You might remember that a moving electrical charge is also an electrical current, and because an electrical current

    Case Quiz 20 Question

    This patient is a 61-year-old female who presented with a large mass in her anterior left thigh that could not be fully characterized by ultrasound. These are both axial T1-weighted images, but what additional technique was added to the sequence in (B), and how does that help us make the diagnosis? The answer is at the end of this chapter.

    induces a magnetic field, each proton has its own small magnetic field (called a magnetic moment ).

  • When a patient enters an MRI scanner, the mini-magnet protons all align with the more powerful external magnetic field of the MRI magnet. Most of these protons will point parallel to the field and others point antiparallel to the field, but they will all align with the external magnetic field of the MRI.

  • Protons do not like to be couch potatoes, so they precess (i.e., wobble like a spinning top) along the magnetic field lines of the MRI ( Fig. 20.1 ).

    Fig. 20.1, Relaxing on the Job.

  • We will return to our little wobbling protons in a (magnetic) moment.

Hardware that Makes up an MRI Scanner

Main Magnet

  • The main magnet in an MR scanner is usually a superconducting magnet.

  • Superconducting magnets contain a conducting coil that is cooled down to superconducting temperatures (4° K or −269° C) in order to carry the current. At temperatures that low (close to absolute zero), resistance to the flow of electricity in the conductor practically disappears.

Important Points

  • An electric current sent once through this ultra-cold conducting material will flow continuously and create a permanent magnetic field. The main magnet in an MRI scanner is always “on” whether there is a patient present or not.

  • Most scanners today have a magnetic field strength between 0.5 and 3 Tesla (T). Open MRI scanners, those that do not completely encircle the patient in the scanning circle, have lower field strengths of 0.2–1.0 T, which results in decreased image quality compared to closed-bore MRI. By comparison, the earth’s magnetic field is only about 50 microTesla.

Coils

  • The coils placed within the magnet are an important part of the MRI scanner. These coils are responsible for either transmitting the radiofrequency (RF) pulses (transmitter coils) that excite the protons, or receiving the signal (or echo ) given off by these excited protons (receiver coils) ( Fig. 20.2 ).

    Fig. 20.2, Scanner Schematic.

  • These coils are subjected to strong electrical currents and exist within a strong magnetic field that produces the repetitive “knocking” sound during an MRI scan.

Computer

  • A computer dedicated to the MRI scanner processes the radiofrequency signals obtained by the receiver coils and converts them into an image.

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