Biomedical Engineering Reference
In-Depth Information
Cellular phones/
pagers
AM
CB
FM
Shortwave
Television
Channels 2-6
Channels 7-13
Channels 14-83
MRI
0.2T
3T
0.5T
1.5T
4T
NMR spectrometers
1
2
4
8 10
20
40
80
100
200
400
800 1000
Frequency, MHz
Figure 6.1-2 Magnetic resonance frequencies.
that reflect the molecular environment of the protons. In
an ethanol molecule (CH
3
CH
2
OH), for example, the
protons in the CH
3
group precess at a slightly different
frequency from those in the CH
2
group or the OH group.
Spectroscopy can determine structural information from
the Fourier spectrum of the received signal.
To factor out the signal's dependence on the static
magnetic field, NMR measurements are often given in
a unitless quantity called chemical shift, which is typi-
cally measured in parts per million. It is the difference
between the precession frequency of protons that are
part of a particular molecular group and precession
Gradient Coils
Gradient
Amplifiers
Transmit/
Receive
Switch
Receiver
Blanking
Frequency
Synthesizer
RF
AMP
RF
AMP
Pre-Amp
Atten
F
o
F
o
F
o
+
δ
f
F
o
-IF
Waveform Generator
Timing Boards
IF
Filter
IF+
δ
f
Pulse Programmer
Software Interface
Host
Computer
IF
RAM
Low-Pass
Filter
Quadrature
Detection
2-D FFT
A-to-D
Converter
f
δ
Baseband
AMP
Image
Display
Image
Archive
Figure 6.1-3 Block diagram of an NMR system.
