Slice Selection

Slice selection is effected by two factors. The first is the bandwidth of the B1 RF pulse, and the second is the gradient throughout the field of view. The smaller the bandwidth [shorter the pulse], the thinner the slice selected. The FOV gradient (z axis) intersects at its null point with the slice selection pulse to find the origin of the slice. Slice selection occurs simultaneously with the B1 pulse.

Spatial Encoding

The spatial encoding of the selected slice is effected by means of encoding the x and y co-ordinates (anatomical long and short axis) in two different ways. Within the transduced signal there are aspects of frequency, phase, and amplitude. It is in fact these three parts which are used to transmit the information. Spatial encoding of short anatomical axis information is performed on the x axis using the phase encoding method. This involves post B1 RF pulse, which started precession in the given mode, the application of a magnetic field gradient alters the Larmor equation. It will be remembered that frequency of precession increases linearly with increase in magnetic field strength, and so the angular velocity of precession too increases with field strength. In those parts of the applied magnetic field gradient which have the highest field strength, the precessions will hence become faster, gaining in phase compared to those with the weaker field gradient. Stopping after a time the magnetic field gradient, the x axis location is stored within the phase of precession, and can be decoded from the transduced signal. Spatial encoding of the long anatomical axis location along the y axis is performed using the frequency encoding method. It has already been mentioned that frequency of precession increases with an increase in magnetic field gradient, and this effect again is used. During the measurement of the signal a gradient is applied in the y axis such that frequency increases in those parts with a greater magnetic field density, and y axis data becomes the frequency transduced. first the slice is selected, and then data is encoded for both axis:V F MurphyIn these ways first the slice is selected, and then data is encoded for both axis. To reduce the effect of inhomogeneities a 180o pulse of four times the B1 pulse is applied to flip the precessions, such that those which were gaining are losing, and those which were losing are gaining. In that way, the leading edge drags, the trailing edge catches up, and the pulse again converges to a cleaner echo, showing T2 more than T2*.

Contrast Application Example

Distinguishing between:
(i) normal brain (either white or gray matter)
(ii) a cyst (fluid filled lesion) and
(iii) a lipoma (tumour with high lipid content)?

The two principle factors affecting image contrast are T1 and T2 weighting. Water has a longer T2 than lipids. The time before the echo pulse (TE) is important in detecting T2. Using a long TE the lipoma will become dark in comparison to the cyst, as the transverse magnetization has had time to fade during that time. The longer T2 of water will mean that the increase in TE will not affect image contrast to such a great extent. The composition of the cyst and the brain is similar, both containing essentially fluid. The CSF however is expected to be less densely distributed than the fluid within the cyst. Water carries with it a high spin density, and so the best way to distinguish these two situation is based upon spin density. The way in which spin density is measured is by using a long TR, the repetition time, as T1 (spin-lattice) time is greatly exceeded, and spin density becomes prominent. The second way of differentiating the CSF from the fluid filled cyst is by the use of a contrast agent such as Gadolinium DTPA. Cysts do not rapidly exchange fluid with the surrounding and so the uptake of the Gadolinium tracer will be somewhat lower than that in the brain.