The contrast in an MR-image is controlled by the choice of measuring method. For example, we call an image T2-weighted if the acquisition parameters are chosen so the image contrast mainly reflects T2-variations. One must understand, however, that even in a heavily T2-weighted image, the contrast will often reflect more than just T2-variation. To provide an example, variation in water content always results in some contrast. The echo time, TE, is the period from we rotate the magnetization into the transversal plane until we decide to measure the radio waves. Meanwhile, a loss of magnetization and signal will occur due to T2-relaxation. The echo time is thus the period within the measurement which gives T2-weighting in the images. A long TE compared to T2 will thus result in considerable T2-contrast, but only little signal. The greatest sensitivity to T2-variation will be achieved when TE ~=T2. Often, we will repeat similar measurements several times, e.g. once per line in an image.
The repetition-time, TR, is the time between these repetitions. Every time we make such a measurement, we (partially) use the longitudinal magnetization present (the magnetization is rotated into the transversal plane which results in emission of radio waves while the transversal component gradually disappears). If we use the magnetization often (short TR), every repeat will therefore only produce a small signal. If, on the other hand, we wait longer between repetitions (long TR), the magnetization will nearly recover to equilibrium between repetitions. If the magnetization is completely rebuilt between measurements for all types of tissue in the scanner, meaning if TR is significantly longer than the maximum T1, the T1-contrast will disappear.
Thus, we may conclude that using a long TR results in limited T1-weighting but a strong signal. If we apply a shorter TR, the signal is reduced for all types of tissue, but the signal becomes more T1-weighted, meaning that the images will be less intense, but with a relatively greater signal variation between tissues with differing T1. So, we can summarize as follows:
T1-weighted images are made by applying short TR and short TE, since T1 contrast is thereby maximized and T2 contrast is minimized. An example is shown in Fig 4.5
T2-weighted images are made by applying long TR and long TE, since T1 contrast is thereby minimized and T2 contrast maximized. An example is shown in Fig 4.6.
Fig 4.5: A T1-weighted image
Fig 4.6: A T2 weighted image Slice selection
By using gradient coils, the magnetic field strength can be controlled so that it, for example, increases from left to right ear, while the direction is the same everywhere (along the body). This is called a field gradient from left to right. By making the field inhomogeneous in this way, the resonance frequency varies in the direction of the field gradient. If we then push the protons with radio waves at a certain frequency, the resonance condition will be fulfilled in a plane perpendicular to the gradient as shown in Fig 4.7. The spins in the plane have thus been rotated significantly, while spins in other positions simply vibrate slightly. Thus we have achieved slice selective excitation of the protons and a sagittal slice has been chosen. This can be repeated for selecting the coronal and axial plane by giving the field gradient to the corresponding perpendicular directions.
Fig 4.7:slice selection in sagittal plane Phase and frequency encoding
If one gradient coil is used for slice selection, the other two can be used one for phase encoding and the other for frequency encoding. The phase encoding result in all the protons precessing in the same frequency, but in different phases. The protons in the same row, perpendicular to the gradient direction, will all have the same phase. In frequency encoding, it modifies the Larmor frequencies in the horizontal direction throughout the time it is applied. It thus creates proton columns, which all have an identical Larmor frequency. The phase encoding is given before the data acquisition where as frequency encoding is given during data acquisition.
When the RF pulse given is removed, the magnetization return back to the equilibrium position and there will be rate of change of magnetic flux and the RF coil will be present in this changing magnetic flux and there will be a voltage induced in the coil due to Faraday’s law. The induced voltage is proportional to the strength of the magnetic field which produces the Magnetic resonance to happen and also the difference in the number of parallel and anti-parallel protons.
A “sequence” is an MR measurement method as shown in Fig 4.8. Sequences include elements such as:
Excitation: Turning of the magnetization away from equilibrium.
Refocusing pulse: After excitation and dephasing, part of the signal that has been lost due to inhomogeneity can be recovered. This is achieved by sending a 180-degree radio wave pulse (in this context known as a refocusing pulse) that turns the magnetization 180 degrees around the radio wave field. The effect is that those spins which precess most rapidly during dephasing are put the furthest back in evolution and vice versa (mirroring around the radio wave field causes the sign of the phase angle to be reversed). In the following “refocusing period” where the nuclei are still experiencing the same inhomogeneity, they will therefore gradually align again (come into phase). The lost signal is recovered, so that an “echo” is measured.
Readout: Measurement of MR signal from the body.
Waiting times: Periods wherein the relaxation contributes to the desired weighting.
After a phase roll has been introduced in the object by applying a gradient, the signal we receive is a measure of whether there is structure in the object matching the phase roll. Different phase roll patterns in the body are drawn one after the other. The resulting radio wave signals are recorded for each of the patterns. The size of the signal for a particular phase roll pattern tells us whether there is similarity to the structure of the body. Rather than showing arrows pointing in different directions, phase rolls will be shown as an intensity variation as in the second figure, where the color reflects the direction of the arrows.