1.2.10 - Prescan

Prescan is performed before every pulse sequence is played out on the scanner, and is used to optimize the image acquisition for the specific body part being imaged by adjustment of the center frequency as well as calibration of the transmit gain (TG) and receive gain (RG) values. The center frequency adjustment controls the tuning of the RF transmitter, and under normal imaging conditions is set to correspond to the most abundant resonant frequency of the water protons in the tissue being imaged. The accurate identification of the water peak for tuning of the central frequency is critical to suppressing fat reliably. For most applications, Autoprescan algorithms are able to reliably detect the water peak, and set the center frequency; however, it is sometimes necessary to resort to using Manual Prescan if Autoprescan fails. Autoprescan can fail for several different reasons, the most common of which is the inability to reliably distinguish the water peak. A guideline for how to reliably set the center frequency using Manual Prescan is given in Appendix A.

Transmit gain determines the RF power emitted by the transmitter and is measured in tenths of decibels (dB). The flip angle produced in the patient is proportional to the square root of the absorbed power. Since power absorption is dependent on patient size, TG must be adjusted for each patient individually. During Autoprescan, TG is calibrated by determining the amount of power that needs to be deposited to produce a 90? flip angle.

Receive gain is a factor that controls the amount of multiplication applied to the received signal by the preamplifier. RG is adjusted such that the minimum and maximum signal intensities at the receiver correspond to the lower and upper limits of the optimal dynamic range of the analog-to-digital converter in the receiver respectively. Too high a RG setting results in overrange signals, and clipping of the signal. (Fig. 1.11) Too low a setting results in increased image noise. TG and RG are adjusted automatically during auto prescan.

Figure 1.11 (click on image for enlarged view)

Artifact resulting from improper RG setting. In this case the signal is too high for the dynamic range selected (i.e. RG is too high) and the signal is clipped.

Even in a perfectly uniform main magnetic field, inhomogeneities are introduced by the presence of the tissue itself. Small gradient fields are often generated (shim fields) that compensate to some degree for the effects of imperfections in the magnetic field homogeneity for the specific FOV being imaged. If the Autoshim feature is selected, then shim currents are calculated automatically during Autoprescan for the tissue being imaged.