IMAGE CONSTRUCTIONS: SLICE SELECTION

Chapter 7

We shall see:

• Image Construction•Slice Selection

•Slice selection•Slice thickness •Slice-select gradient •RF pulses •Bandwidth

IMAGE CONSTRUCTION

• The Fourier transform (FT) is a mystery to most radiologists. Although the mathematics of FT is complex.

• Basically, the FT provides a frequency spectrum of a signal. It is sometimes easier to work in the frequency domain and later convert back to the time domain.

SLICE SELECTION

• The signals received from a patient contain information about the entire part of the patient being imaged. They do not have any particular spatial information. So, we cannot determine the specific origin point of each component of the signal.

• This is the function of the gradients. One gradient is required in each of the x, y, and z directions to obtain spatial information in that direction.

• Depending on their function, these gradients are called

a) The slice-select gradient b) The readout or frequency-encoding gradient c) The phase-encoding gradient

SLICE THICKNESS • If we pick a frequency bandwidth of a certain range, we

would then get a slice of a certain thickness. Therefore, we transmit an RF pulse with a specific frequency bandwidth and no frequencies outside of this range (ideally).

• Bandwidth = range of frequencies (determines the slice thickness)

• The magnetic field strength everywhere outside this slice is going to be either more or less than the magnetic field strengths that correspond to the Larmor frequencies of the RF bandwidth.

SLICE-SELECT GRADIENT • The slice thickness can be decreased by

decreasing the bandwidth of the RF pulse, or Increasing the slice select gradient.

• There is an electronic limitation as to how much we can decrease the bandwidth. There is machine limitation as to how much we can increase the gradient. These factors set a limit on how thin a slice can be.

• The slice is selected with a frequency range in the RF pulse that corresponds to the slice location and its thickness. The echo signal that we get back from the slice is from the entire slice.

RF PULSES • There are two types of RF pulses:

1)nonselective2)selective

• A nonselective RF pulse excites every part of the body that is in the coil (used in three dimensional [3D] imaging).

• A selective RF pulse excites selected part of the body that is in the coil (used in teo dimensional [2D] imaging).

BANDWIDTH

• Bandwidth is a measure of the range of frequencies.

• For a 1.5-T magnet, the Larmor frequency is about 64 MHz.

• In MRI, the center frequency is the Larmor frequency.

• The bandwidth of frequencies in the RF pulse is very narrow.