196CM/003 - Fluke Electronics

Application Note

From the Fluke Digital Library @ www.fluke.com/library

Basics of MRI

NMR is based on the fact that

the atoms of many chemical ele-

ments and their isotopes have

some degree of magnetic orien-

tation. Under normal conditions,

the individual magnetic fields of

individual atoms show a random

orientation, and the net field is

zero (Figure 1).

When placed in a very strong

static magnetic field, the mag-

netic fields of the individual

atoms line up, resulting in a net

magnetization (Figure 2). The

magnetic fields used to accom-

plish this are around 0.1 to 4 tesla

(1 T = 10000 gauss) with a ten-

dency to the ever stronger fields.

Once the nuclei are aligned

by this static magnetic field, an

alternating magnetic field in a

direction perpendicular to the

In modern medicine, Magnetic Resonance Imaging or

“MRI” is one of the techniques used to study the inner

structure of a living body on a non-invasive basis. The

technique is an application of Nuclear Magnetic

Resonance or NMR which is also used beyond the

medical field to study the structure of matter. The main

advantage of MRI over X-ray based techniques is in the

better imaging capability of soft tissue, where X-ray

techniques are more useful for study of the bone

structure, for instance in case of fractures.

main field will make the protons

precess around the axis of their

static orientation.

When this occurs, the atoms

turn out to have a resonance

frequency in the RF-range. For

instance the hydrogen atom (1H)

has a resonance frequency of

approximately 42.58 MHz when

placed in a static magnetic field

of 1 T.

Given this resonance, the

nuclei can be thought of as res-

onators that absorb energy from

the applied RF-field.

Nuclei of different elements

and different isotopes all have

different resonance frequencies.

For as long as the RF-field is

applied, all nuclei will resonate

in phase, but once this stimulus

ends the individual nuclei will

lose synchronism, or de-phase,

MRI adjustments with

a Fluke ScopeMeter®

190 Series

Figure 1: Normal randomly arranged

magnetic orientation of nuclei.

Figure 2: When placed in a strong magnetic

field, the magnetic orientation of nuclei align

with the external field.

and return to their original posi-

tion aligned with the static mag-

netic field. This process of

returning to the original orienta-

tion takes a length of time that

is again characteristic of each

different type of nucleus.

By measuring the resonance

frequencies and the decay times,

the individual elements present

in the body can be determined.

Oscilloscopes used

in MRI

For the proper functioning of the

MRI-equipment, it is essential

that the RF stimuli are switched

on and off in a well controlled

manner, and with good repro-

ducibility. During development

and adjustment of the equip-

ment, a proper representation of

these RF-bursts is most impor-

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.

3 Fluke Corporation MRI adjustments with a Fluke ScopeMeter®190 Series

The resulting display shows a

number of successive wave-

forms, superimposed and at dif-

ferent intensity levels in which

the most recent ones stand out

more clearly. See Figure 7.

When used on burst signals,

as encountered in MRI-equip-

ment, this Digital Persistence

gives the display quality one

would expect from a traditional

high-frequency oscilloscope

with its phosphor ensuring the

slow trace fade-out.

An additional benefit of the

Digital Persistence mode is that

it automatically adapts the

waveform representation if the

overall amplitude drops, e.g. as

a result of an alignment made to

the equipment under test.

The envelop mode, on the

other hand, will continue to

show the maximum amplitude

that the envelope has ever

attained since the functions was

initiated. Should a new ampli-

tude of the waveform be seen or

expected, a simple key-press

(e.g. of the attenuator, 1 step up

and back) will delete the cre-

ated envelope and start building

a new one.

Conclusion

The Fluke 190 Series Scope-

Meters are well equipped to

capture and display the short-

duration bursts of RF-energy as

used in the MRI-medical imag-

ing equipment. On top of the

basic functionality of these high

performance handheld oscillo-

scopes, some additional func-

tions are built-in that help to

optimally display the waveforms

of such short duration bursts.

Fluke Corporation

PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa (31 40) 2 675 200 or

Fax (31 40) 2 675 222

In Canada (800) 36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-51

Web access: http://www.fluke.com

Printed in U.S.A. 10/2005 2543288 A-EN-N Rev A

Fluke. Keeping your world

up and running.

Figure 7: The Fluke 199C is used here with

‘Digital Persistence’ to capture a natural enve-

lope of a short duration burst of an RF-carrier.

ScopeMeters for medical equipment

applications

The Fluke ScopeMeter 190M Series offer addi-

tional capabilities targeting the service engineer

dealing with Medical Imaging Equipment and

Video Display Systems. These models have the

following additional capabilities :

Current-over-time measurement giving

mAs read-out

Using the cursors you can measure directly the

amount of radiation produced by X-ray systems

or the total amount of charge supplied to

a system.

Extended video triggering

The 190M Series supports triggering on high-

resolution non-interlaced video systems.

Smart averaging

Smart averaging shows the averaged curve of

repeated signal parts, as well as incidental

deviations.

Extended offset

Allows vertical zoom-in for study of small details

of the larger amplitude signal or when a larger

DC-component is included.

Medical models available are the Fluke 199CM,

Fluke 199BM, Fluke 196CM and Fluke 196BM.

See techical data sheet for full details.