Introduction:

MRI Like CT, MRI produces images, which are the visual equivalent of a slice of anatomy. MRI, however, is also capable of producing those images in an infinite number of projections through the body. MRI uses a large magnet that surrounds the patient, radio frequencies, and a computer to produce its images. As the patient enters a MRI scanner, his body is surrounded by a magnetic field up to 8,000 times stronger than that of the earth. The scanner subjects nuclei of the body's atoms to a radio signal, temporarily knocking select ones out of alignment. When the signal stops, the nuclei return to the aligned position, releasing their own faint radio frequencies from which the scanner and computer produce detailed images of the human anatomy. Patients who cannot undergo a MRI examination include those people dependent upon cardiac pacemakers and those with metallic foreign bodies in the brain or around the eye.

MRI is a new technique for imaging human body. It makes use of magnetic fields and radio-frequency waves to generate intensity-modulated images from specific sections of the body. The single most distinguishing feature of MRI, when compared with other modalities, is the extraordinarily large inherent contrast. Contrast is defined as the relative signal intensity difference in an image between two adjacent anatomic structures. The physical basis for MR involves the interaction of the nuclei (protons) of an isotope, which may be a part of many molecules in many human cells, with external magnetic field and an external oscillating RF. One of the exciting factors about MRI is that it makes use of non-ionizing radiation and therefore, it appears to be a safe technique.

 

  • The basic phenomena involved in MR experiments may be classified into the following four groups:

  1. Inducing a magnetic field in the sample

  2. Reorienting this magnetic field by a known amount (the spin flip).

  3. Allowing this field to return to its original orientation (relaxation)

  4. Detecting and measuring the field after relaxation is underway.

 

  • The atomic magnetic fields are due to the combination of three subatomic fields:
  1. The magnetic field of the nucleus

  2. The magnetic field due to the electron spins

  3. The magnetic field due to the orbital motions of the electron.

 

  • Basic Principles of MRI Systems:

  1. External magnetic field provided by resistive, permanent or super-conducting magnet of strength ranging from 0.08 to 2 T

  2. Gradient coils

  3. RF transmitter coils

  4. Receiver coils

  5. Computer system and interface

  6. Image display system.

 

*The MR signal is emitted by those atoms in the sample (or patient) that were excited by the 90 or 180 degree pulses and is detected with a " pick- up" coil or receiver coil in the system. The signals are then converted to form image and processing is done by reconstruction.

 

  • Reconstruction:

  1. Reconstruction by back-projection

  2. 2D back-projection imaging with selective excitation of the slice.

  3. Direct imaging by gradient control

  4. 2D FT method

  5. 3D F

 

  • The spin Flip:

The nuclear angular momentum is called spin. The nuclear magnetic field strength is called magnetic moment. Rotation of the nuclear magnetic field out of alignment with the external field is called spin flip. The smooth rotatory motion of the gyroscope around the vertical (the direction of gravity) is called precession. When the nuclear magnetic fields are moved out of alignment with the external field, they precise smoothly around the direction of the external field forever unless acted upon by other forces. This smooth precession is called Larmour Precession, and the frequency of this precession is called Larmour Frequency. LP is the key phenomenon for understanding MRI.

 

  • Relaxation:

The return of the macroscopic spin magnetization to equilibrium is determined by two independent processes, called relaxation, which are characterized by a single time constant. Relaxation is an exponential decay. It takes a constant time t.

T1 is a relaxation time constant describing the growth of nuclear magnetic moment to a final value. T1 is sometimes called spin-lattice relaxation time.

T2 is the relaxation time constant describing the decay of nuclear magnetic moment to a final value of zero. T2 is also called spin-spin relaxation time.

In general, T2 is less than T1.

 

  • Radio-frequency Pulse Sequence:

Several different pulse sequences are available on most commercial MRI systems. These include

  1. Partial saturation

  2. Inversion recovery

  3. Spin echo

  4. Multi-echo

  5. Calculated T1 & T2.

 

  • Image Display:

MRI is based on computer processing and depends on its powerful image manipulation ability. The primary clinical diagnosis is made by visual inspection of the displayed image. In most clinical systems, the computer displays are used for two operations:

  1. Performance of the clinical study

  2. Performance of the final image.

The numerical information representing the specific physical quantity of interest (tissue density for CT; T1 or proton density for MRI) is displayed as changes in intensity in the image. Both CT and MRI use a window and level method of displaying the image so that the dynamic range of the image can be handled more adequately.

 

  • Physiologic Basis of Magnetic Relaxation:

The contrast that is apparent in magnetic resonance images of soft tissues usually arias from the heterogeneous distribution of tissue proton densities and relaxation times. The sensitivity of MRI to pathological changes and variations in tissue composition most often relies on detecting small changes in tissue-water relaxation rates.

Interpretation of MR Images:

Interpretation of MR images depends on observation of anatomic abnormalities and also abnormalities in signal intensities from the tissues. The distinction between an abnormal structure and surrounding normal tissue is achieved by contrast in intensity between the two. Each of the various organs yields characteristic signal intensities relative to surrounding organs under various pulse sequences.

 

  • There are at least four factors that currently are recognized as being important in producing contrast among different tissues:

  1. Hydrogen density (spin density)

  2. T1 relaxation time

  3. T2 relaxation time

  4. Blood flow and blood concentration within an organ.

* In general, both T1 and T2 weighted sequences are used for studying most regions of the body.