IMAGE RECONSTRUCTION IN MRI A BRIEF STUDY

1. IMAGE CONSTRUCTIONS: SLICE
SELECTION
Chapter 7

2. We shall see:
• Image Construction
•Slice Selection
•Slice selection
•Slice thickness
•Slice-select gradient
•RF pulses
•Bandwidth

3. 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.

5. 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.

6. • 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

7. 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.

8. 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.

9. RF PULSES
• There are two types of RF pulses:
1)nonselective
2)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).

10. 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.