BASIC PRINCIPLES OF MRI

In the absence of a strong magnetic field, hydrogen nuclei are randomly aligned as in (a). When the strong magnetic field, Bo , is applied, the hydrogen nuclei precess about the direction of the field as in (b).

The basis of MRI is the directional magnetic field, or moment, associated with charged particles in motion. Nuclei containing an odd number of protons and/or neutrons have a characteristic motion or precession. Because nuclei are charged particles, this precession produces a small magnetic moment.
When a human body is placed in a large magnetic field, many of the free hydrogen nuclei align themselves with the direction of the magnetic field,as shown in the figure above... The nuclei precess about the magnetic field direction like gyroscopes. This behavior is termed Larmor precession.
The frequency of Larmor precession is proportional to the applied magnetic field strength as defined by the Larmor frequency, Wo
Wo = Y x Bo
where Y is the gyromagnetic ratio and Bo is the strength of the applied magnetic field. The gyromagnetic ratio is a nuclei specific constant. For hydrogen, Y =46.2 MHz/ Tesla.
To obtain an MR image of an object, the object is placed in a uniform magnetic field, Bo, of between 0.5 to 1.5 Tesla. As a result, the object's hydrogen nuclei align with the magnetic field and create a net magnetic moment, M , parallel to Bo. This behavior is illustrated in Figure Below

Figure (a) The RF pulse, Brf, causes the net magnetic moment of the nuclei, M, to tilt away from Bo. (b) When the RF pulse stops, the nuclei return to equilibrium such that M is again parallel to Bo. During realignment, the nuclei lose energy and a measurable RF signal

Once the RF signal is removed, the nuclei realign themselves such that their net magnetic moment, M , is again parallel with Bo. This return to equilibrium is referred to as relaxation. During relaxation, the nuclei lose energy by emitting their own RF signal (see Figure 2.2b). This signal is referred to as the free-induction decay (FID) response signal. The FID response signal is measured by a conductive field coil placed around the object being imaged. This measurement is processed or reconstructed to obtain 3D grey-scale MR images.

To produce a 3D image, the FID resonance signal must be encoded for each dimension. The encoding in the axial direction, the direction of Bo, is accomplished by adding a gradient magnetic field to Bo. This gradient causes the Larmor frequency to change linearly in the axial direction. Thus, an axial slice can be selected by choosing the frequency of Brf to correspond to the Larmor frequency of that slice. The 2D spatial reconstruction in each axial slice is accomplished using frequency and phase encoding. A ``preparation'' gradient, Gy , is applied causing the resonant frequencies of the nuclei to vary according to their position in the y-direction. Gy is then removed and another gradient, Gx , is applied perpendicular to Gy . As a result, the resonant frequencies of the nuclei vary in the x-direction due to Gx and have a phase variation in the y-direction due to the previously applied Gy. Thus, x-direction samples are encoded by frequency and y-direction samples are encoded by phase. A 2D Fourier Transform is then used to transform the encoded image to the spatial domain.
The voxel intensity of a given tissue type (i.e. white matter vs grey matter) depends on the proton density of the tissue; the higher the proton density, the stronger the FID response signal. MR image contrast also depends on two other tissue-specific parameters:
The longitudinal relaxation time, T1 , and
the transverse relaxation time, T2
T1 measures the time required for the magnetic moment of the displaced nuclei to return to equilibrium (ie. realign itself with Bo). T2 indicates the time required for the FID response signal from a given tissue type to decay.
When MR images are acquired, the RF pulse, Brf , is repeated at a predetermined rate. The period of the RF pulse sequence is the repetition time, TR. The FID response signals can be measured at various times within the TR interval. The time between which the RF pulse is applied and the response signal is measured is the echo delay time, TE. By adjusting TR and TE the acquired MR image can be made to contrast different tissue types.

The MR images used in this article were all acquired using a Multiple Echo Spin Echo pulse sequence in which two images are acquired simultaneously. TR and TE are adjusted such that tissues with a high proton density appear bright in the first image and tissues with a long T2 appear bright in the second image. The two images are said to be proton density-weighted (PD-weighted) and T2-weighted respectively. Figure 2.3 shows 2D slices from the weighted MRI volumes.

Figure (a) A proton density (PD) weighted MR image slice. (b) The same T2-weighted slice.

Spin-spin relaxation or T2 Relaxation

Immediately after a 90 degree RF pulse, the individual magnetic moments are all located in the transverse plane ; more important. However , they are coherent ( pointig in the same direction at nearly the same frequency). In other words , all protons have the same rotating phase.
Because the FID (free induction decay) is the sum of voltage from many protons at each spatial location,the protons must precess at the sane frequency and with the same alignment so that their magnetization work together. As the time goes on ,following the RF excitation pulse the individual magnetic moments begin to become spread out in the transverse plane. One cause of this spread is due to small local variation in the otherwise static magnetic field resulting out of random interaction from the magnetic moments of nearby protons. It is the spreading out that causes the vector sum to decrease. With time ,therefore, the individual spins

Spin-lattice relaxation or T1 Relaxation

When a 90 degree RF pulse is applied, it rotates
net magnetization vector M into the x-y or transverse plane , entire vector is located in that plane and its value along the z-axis is zero. Following cessation of the RF pulse , the nuclei returns towards their equilibrium magnetization. M rotates back from the x-y plane to the z-axis and M returns to to the initial magnetization at some finite time latter. The process of returning towards the equilibrium magnetization costitutes T1 rlaxation or longitudinal relaxation. The process is cused by release of energy into the surrounding tissue ( i.e., into the molecular environment or lattice around the proton ). Hence this relaxation is also known as spin-lattice relaxation.

STEPS IN MRI IMAGING

There are four basic steps involved in MR imaging :
1.Placing the patient in a strong magnetic field.
2.Excitation process by sending a Radiofreqency signal through RF coils.
3.Relaxation process which sends back signals in RF coils.
4.Signal are send to computers for data processing and image display.

BASIC PHYSICS OF MRI

MRI stands for Magnetic Resonance Imaging.
MR imaging is based on imaging the hydrogen nuclei (or Proton) which is present in abundance in human body. Protons are positively charged particles and posses a rotatory movement called SPIN. Any charge which moves produce a current. Every current is associated with a small magnetic field around it. So every spining proton has a small magnetic field around it.