NMR and MRI Selftest – Some Details
Robert Gilchrist Huenemann,
M.S.E.E.
(831) 635-0786
January 26, 2006
There is a class
of electronic devices and systems which can test themselves using little or no
external equipment. The classic example is the modem (modulator/demodulator).
The output of the modem can be connected to the input, and a loopback test can then be performed. The external equipment
consists of a nothing more than a short piece of wire. And computers can test
themselves by writing various patterns to memory and reading those patterns
back.
Other examples
include devices or systems that consist of a radio transmitter and receiver.
Radar sets test themselves whenever they are in operation. The radar operator
can spot many kinds of failures immediately by simply observing his screen.
Sometimes ‘corner reflectors’ are set up in the field of view of a radar set to
give a stable reflection for test purposes.
Various kinds of
two way radios could test themselves, although it is more common to use a
‘radio check’ for this purpose. Your cell phone tests itself when you turn it
on, by looking for a response from an external cell tower. Police radar sets
are tested using a simple tuning fork to simulate the Doppler offset at a known
speed.
NMR and MRI
systems consist of transmitters and receivers that can, and have been, used as
part of a selftest. In principle, all the transmitter
outputs and receiver inputs are tied together at one common point so that any
transmitter can be used to test any receiver that shares a frequency
range. In general, the tests use
information recovered from the spectrum of an acquisition.
In practice, it
is preferable to make these connections using simple power dividers at each
connection branch, to maintain a reasonable impedance match. Also, various
levels of attenuation can be inserted into the transmitter to receiver path to
exercise transmitters over their range of levels and
receivers over their range of gains.
Lock
transmitters and receivers can be included in this connection scheme, but it
may be desirable to connect the lock transmitter directly to the lock receiver
for some kinds of tests.
An operator may
be required to set up these connections and to vary the attenuation in the
paths for the gain and level tests. So the tests will not be fully automatic.
It is important, however, to use one fixed set of connections and attenutations for as many of the tests as possible so that
a large number of tests can be run repeatedly for hours or days, to identify
intermittent failures.
The tests can be
extended to include shim and gradient supplies by adding a data acquisition
system controlled by the host computer.
The tests which
can be made with this approach include:
TransmittersGates Output Level
and Stability Phase and
Phase Stability Internal
Modulations External
Waveforms Homodecouplers Frequency Probe Tune
Circuits Amplifiers (c.w. mode) Lock Pulsing
and Bleed |
ReceiversGates Gain Bandwidth Preamplifiers Transmit/Receive
Switches Unbalance Quadrature Error Phase Cycling Image
Responses |
AccessoriesShim Currents PFG Currents Gradient
Waveforms Eddy Current
Compensation Coordinate
Rotation Z0/B0 Current
vs. Lock Phase Magnet Leg Signal
Paths |
These tests have
proven to be sufficiently complete that few additional system level tests are
required in manufacturing, and no NMR/MRI tests are required until the system
reaches the customer installation. Intermittent failures in the field can
essentially be eliminated by running these tests for approximately 24 hours in
the factory.
Power supply
voltages should be verified before other tests are run, and it is useful to
have a simple initial test to verify that the host computer and acquisition
computer are communicating properly.
There are two
tests other which must be run manually, as they cannot conveniently be
incorporated into selftest. Amplifier power levels
can only be measured by using calibrated power meters and accessories, and
preamplifier noise figures can only be measured by using calibrated noise
sources. All other tests can be run automatically as part of selftest, which of course generates its own test records as
part of the process.
My Experience With Fully Automatic, Semi Automatic, And Self Test