Biomedical Engineering Reference
In-Depth Information
and
I
or by redefining the functions in terms of the new variables,
s
,
ðð
I
ð
x
y
Þ
e
ik
x
x
e
ik
y
y
dxdy
s
ð
k
x
,
k
y
Þ¼
,
ð
16
:
94
Þ
where
I
s
(
x
,
y
) is equal to the term in the braces outside the exponentials in Eq. (16.93) and
is redefined as well. Similarly, Eq. (16.86) becomes
ðð
I
ð
x
k
y
Þ
e
ik
x
x
e
ik
x
x
e
ik
y
y
dk
x
dk
y
,
y
Þ¼
ð
s
ð
k
x
,
ð
16
:
95
Þ
Previous results have now been recast in
-space and
-space variables.
xy
k
16.3.7 Magnetic Resonance Imaging Systems
The main parts of an MRI system are shown in Figure 16.49. A large superconducting
magnet provides a homogeneous static
B
0
magnetic field within its interior. Typical clinical
field values vary from 0.15 to 2 T, but there are systems available in the 3 to 5 T range.
Inserted within the main magnet are pairs of surface coils for producing the
-axis
gradients. These typically produce variations of only a few percent of the value of the static
field for spatially localizing the magnetic spin signals of the body. These signals are picked
up by rf transmit/receiver coils that are sometimes in the form of surface coils placed close
to the body. The rf signals are directed by a computer (CPU) through the rf pulse generator
and sequencer. A switch routes the received rf signals through an amplifier and into a data
acquisition unit, after which they are processed and Fourier transformed to create an image.
Modern systems allow the operator considerable flexibility to alter the gradients and pulse
sequences to achieve different image effects and weighting. Typical image acquisition times
are on the order of 50 to 100 ms, though faster acquisitions are becoming available.
x
-,
y
-, and
z
person to be imaged
super-
conducting
magnet
surface
coils
slide in
rf coil
x,y,z gradient
coil sequencer
& power amp
rf switch
rf amp
data
acquisition
superconducting
magnet control
and power amp
rf pulse
generator &
sequencer
cpu
display
image processing
FIGURE 16.49
Block diagram of an MRI system.
