2 Fluke Corporation MRI adjustments with a Fluke ScopeMeter®190 Series
tant. For this purpose an oscillo-
scope is a most valuable piece of
test equipment, if it can display
the RF carrier frequency and is
capable of showing the envelopes
of the bursts properly. These
envelopes show you everything
that needs to be known about
the process of activating and
terminating the RF bursts.
Many oscilloscopes, however,
have difficulty working properly
in the vicinity of the MRI-equip-
ment because of the extremely
high static magnetic fields.
By nature, the CRT of the
oscilloscope is sensitive to mag-
netic influences as it is based
upon electrons travelling
through the vacuum of the CRT.
The Fluke ScopeMeters are all
able to operate as compact, high
performance oscilloscope that
are well equipped to acquire
and display electrical signals of
various frequencies and that are
not influenced by magnetic
fields (in contrast to a traditional
CRT-based oscilloscope) since
the display used is an LCD.
ScopeMeters used
in MRI
The Fluke ScopeMeters are
available in different bandwidth
and with different sample rates
(see table 1). Given the RF-
frequencies commonly used in
MRI of 42.58, 63.8 and 127
MHz, the Fluke 190 Series
ScopeMeters are the most
appropriate instruments for
test and alignment purposes.
All Fluke ScopeMeters are
equipped with a function called
‘Glitch Detect’. This function
activates a digital overscan
mechanism to find high-fre-
quency signal content even
when working at low speed
timebase settings. An example
of this is given in Figure 4.
At higher timebase speed, the
number of cycles that is con-
tained in an individual burst is
quite limited, and it becomes
more difficult to recognize the
overall envelope of the signal
(see Figure 5). Furthermore, at
higher RF-carrier frequencies,
the effects of aliasing may also
influence the quality of the dis-
played bursts.
Here, some of the additional
functions found in the ScopeMe-
ters come in handy to enhance
the quality of the displayed
waveform.
Envelope function
The Fluke 190B Series ScopeMe-
ters are equipped with a so-
called “Envelope”- function that
builds the envelope over succes-
sive waveform-acquisitions on
the display. See Figure 6. For
this, the extreme values (that is
the most positive and the most
negative value for each of the
horizontal positions over succes-
sive traces) are stored, and
these are overwritten only when
newer values are found in a
next trace that exceed the ear-
lier ones at the same position.
After just a few acquired
traces, the envelop function
shows a good representation of
the overall envelope of the
burst. On top of the envelope
pattern, the most recently
acquired waveform is then also
displayed.
Digital persistence
Next to the envelope function,
the Fluke 190C Series of Color
ScopeMeters also includes a so-
called ‘Digital Persistence’ mode.
In that mode, individual traces
are stored in a dedicated display
memory and each waveform
fades away slowly. The decay
time, or fade-out time, can be
selected for short, medium or
longer persistence, or it can be
switched to have an ‘infinite
persistence’.
Figure 3: When placed in a static magnetic
field B0under the influence of an alternating
field perpendicular to the static field, the
nuclei start precessing.
Fluke type number Bandwidth Max. sample rate (single shot)
Fluke 123 20 MHz 25 MS/s
Fluke 124 40 MHz 25 MS/s
Fluke 192B 60 MHz 500 MS/s
Fluke 196B, 196C, 196BM, 196CM 100 MHz 1 GS/s
Fluke 199B, 199C, 199BM, 199CM 200 MHz 2.5 GS/s
Table 1: ScopeMeter models and their bandwidth.
Figure 4: At lower timebase setting, the ‘glitch
detect’ function builds a solid envelope of a
high frequency burst (Fluke 199C).
Figure 5: A short duration burst is captured,
but the envelope is more difficult to identify
(Fluke 199C).
Figure 6: The envelope function of the Fluke
199B is used here with the same signal as in
Figure 5.