Magnetic Hong

N. Yeung,

PhD

Alex

#{149}

Magnetization with Periodic

M. Aisen,

during

presaturation.

MTC

imaging

is dependent on repetition time; this dependence arises from the dual but conflicting effect of the Selective saturation on tissues rich in macromolecular constituents. Substantial changes in tissue contrast were observed with MTC presaturation. The effect was most marked in muscle, as expected from the high protein content of this tissue. Index

terms:

experimental

netization nance, Radiology

I

From

Magnetic resonance (MR), #{149} Magnetic resonance (MR), magtransfer contrast #{149} Magnetic reso-

technology 1992;

183:209-214

the Department

of Radiology,

Imaging

MD

Transfer Contrast Pulsed Saturation’

Magnetic resonance imaging with magnetization transfer contrast (MTC) was implemented on a 0.5-T clinical system by using a stream of binomial radio-frequency pulses for the selective saturation of the bound protons in tissues. Images were obtamed of an egg phantom and of the calf, head, and abdomen in healthy human volunteers. Nonspecific saturation resulting in diminished signal intensity was manifested in the plots of the ratio of MTC signal to control signal for egg yolk, raw egg white, cerebrospinal fluid, and bone marrow fat. This signal degradation resulted partly from the cumulative effects of T2 on the observed transverse magnetization

Resonance

Univer-

sity of Michigan Medical Center, Ann Arbor, MI 48109-0553. Received Iuly 18, 1991; revision requested August 12; revision received September 30; accepted October 30. Address reprint requests to H.N.Y. C RSNA, 1992

S

contrast

IGNAL

in magnetic

tons.

reso-

nance (MR) imaging is known to be determined by a number of interrelated phenomena, with a major factor being the Ti and T2 of the hydrogen nuclei, or protons, of water and fat. Hydrogen atoms in molecules other than water and fat also participate in the MR imaging process, but because of their low concentration, short

T2,

or both,

they

are

not

directly

observed on MR images. For example, hydrogen is abundantly present in molecules such as membrane lipid and collagen, often in relatively high concentrations.

However,

because

it is

chemically in a relatively bound or, loosely speaking, “solid state” form, the T2 is generally much shorter than 1 msec and cannot be readily observed with commonly used imaging techniques. The term “macromolecular” can also be used for such bound protons.

The

spectral

response

or line

width on MR spectra is inversely related to T2, and the spectrum of macromolecular hydrogen is said to be homogeneously broadened, in contrast

to the

line

width

of natural

free

water, which is narrow, though in practice the spectrum is unavoidably broadened somewhat by magnetic field inhomogeneity. The hydrogen nuclear relaxation times in tissue-especially Ti, the tongitudinal relaxation time-have in turn a complex dependence on many fundamental parameters, such as the intrinsic relaxation rate of free water protons, r = i/T1 (where r5, = relaxation rate of free water protons and T1 = the longitudinal relaxation time of water), and the rate of magnetization transfer, r, between protons of free and bound water or between hydrogen nuclei of nearby macromolecules. Thus, though the bound hydrogen nuclei of macromolecules cannot be observed directly, their presence can have substantial effect on MR image

contrast

with

the

through

directly

their

observed

interaction

water

pro-

by

This the

fact

is shown

recent

most

development

vividly (1-5)

of

magnetization transfer contrast (MTC) imaging. With this technique, substantial change in tissue contrast at MR imaging is induced by altering the magnetization of the bound protons

with

use

of continuous,

off-reso-

nance radio-frequency (RF) irradiation. We have applied similar methods to imaging with a whole-body imager by using

a variation

on

the

technique

that may be easier to apply on commercial imagers. The efficacy of the method is demonstrated with MR images sisted

obtained of two

cooked volunteer

and

in a phantom chicken eggs,

one raw, subjects.

MATERIALS

that one

and

AND

con-

in healthy

METHODS

The pulse sequence used for MTC and the presaturation pulse train are shown schematically continuous,

in Figure monophasic,

irradiation,

1. Instead of using off-resonance

of binomial

a stream

pulses

was

employed, as suggested by Hu et al (6). A train of pulses can be easier to im-

plement tinuous

on commercial irradiation

ter of most

imagers

than

con-

since

the RF transmit-

does

not operate

imagers

in

the continuous wave mode and since continuous irradiation would likely trip RF power monitoring safety features.

Binomial

pulses

have

an added

virtue,

which is demonstrated in the Appendix: The dual-phase composite pulse will have a selective

saturation

geneously

broadened

effect

on

the

homo-

macromolecular

proton resonances, which leaves the narrow resonances, such as water, relatively unscathed.

Application

of this

whether applied on-resonance viates a second RF frequency and

substantially

improves

plementation

potential

on many

disadvantage

Abbreviations: contrast, RF time.

MTC =

radio

pulse

train,

or not, obchannel (1,2) the

ease

imaging

of im-

systems.

A

of this approach

=

magnetization

frequency,

TR

is

transfer =

repetition

209

that

there

may

specific image wave

be

a greater

signal

loss,

quality,

than

hence

with

implementation

The

MTC

Heights,

sequence

MTC

was

The

presaturation

repetition (except

number

192; signals

the echo

3.8

minutes

quence single 2,000 aged.

TR =

with

lines

=

and or 7.5 time

in our study was = 600 msec. For TR sections can be imimages were

continuous

The

mode

of saturation.

pulse train I 1 binomial

consists pulses

an interval T, that can msec to longer periods.

rad/sec. Details are included

software

be

varied Variation

Three MTC imaging corresponding control collected from imaging abdomen, and head in man volunteers. In each ages were acquired with

pre-

of the binomial in the Appendix.

0.2 W/kg

by

supplied

with

the

The

use

of

imager, set

which

by

the

U.S.

Standard receive coils were supplied by manufacturer and included a head coil the human head and leg studies and a single-turn surface images. MTC images

tamed terpulse

by

using delay

coil for the were ob-

several values for (and hence different

the

inde-

grees of presaturation) without shifting the presaturation frequency off-resonance. Control images were obtained by using identical

imaging

the

presaturation. signal intensities

MTC Tissue

mined

by using

of interest. sues were

tamed egg

white,

and fat,

were

ratios from

without deter-

without and

regions

of various tisimages ob-

MTC.

cerebrospinal

unaffected by MTC signal in these tissues

Radiology

#{149}

sequences

operator-specified

Intensity calculated

with

relatively tion, the

210

pulse

Since fluid presaturaserves

raw are as a

msec)

and

interpulse

tion,

of averwith strength

with

2,000

ferent

from 5 in this

rate for a body use of the sequence cycle at this field

to be

rectangular abdominal

Periodi,

pulsed

b.

naturation

representation

of imaging

sequences

lose that

demonstrates

the two

types

signal. The likely explanation raw egg white has a long

while

of

the

two

data sets and data sets were of the calf, healthy hudata set, imtwo TRs (600

two

or three

intervals.

sets

of MTC

interpulse

Images Quantitative intensity with and are shown phantom

data

shown in Figure 2. comparisons of signal ratios in images obtained without selective saturation in Figure 3 for the egg and and in Figure 4 for the

cumulative

during

effects

the

saturasignal

best

illustrated

raw

chicken

by

egg.

This

the

images

Since

the

to show

presaturation presaturation. show that while for the raw egg

any

compared Yet the

this is essentially white, the yolk

does

intervals.

proteins

On

heat-

denature

with much larger moThese solid compo-

are

more

and

and

solid matrices tecular weight.

effective

form

in causing

and

relaxation

numerical

from 25 to 10 msec, the saturating duty

image

intensities

linearly. Thus, with selective further with

data

in Figure

which signifies a tive intensities or effect on contrast between the raw Further, decrease

thereby cycle,

decreased

image contrast saturation and cooking. As the

strate, there is a differential saturation on egg white

3 true

sig-

reaggregate

almost changes changes

MTC

control in Figure

comparing

subsequently

the MTC

of the

with data

by

interpulse

egg-white

decreased increasing

fact is

with

appreciated

and

MTC effects, and cooked egg white will show a greater effect with MTC than wilt its uncooked counterpart. The effect of MTC on egg yolk, which contains littte or no protein, is minimal and does not change with cooking. As ‘ri, was

protein

change

transfer contrast

cross

molecules in egg white are too small to contribute much to magnetization transfer, the absolute signal intensities from the raw egg white and the yolk (which consists mostly of small motecules of cholesterol) might not be ex-

pected

The effect of magnetization itself on signal intensities

nents

magnetization

presaturation.

cereand

ing,

of T2 on the

transverse

with longer will emerge

also be applied to the data for brospinat fluid, bone marrow, subcutaneous fat.

ferent

intensity is manifested in the plots of the ratio of MTC signal to control signat for egg yolk, raw egg white, cerebrospinal fluid, and bone marrow fat in Figures 3 and 4. Note that this signat degradation results partly from

the

T2

nat intensities in images of the cooked and raw egg white obtained with both long and short TR and with dif-

are

observed

T2. As dema short

A substance as egg white,

is best

intervals.

human images. Nonspecific tion resulting in diminished

a short

Appendix,

relatively unscathed from the binomiat pulse. The same explanation can

dif-

were acquired for the egg phantom, one with a TR of 1,000 msec and the other with a TR of 10 seconds; each data set was acquired for four differ-

ent

has

in the

saturation. T2, such

In addi-

image

yolk

is T2,

component will not fully restore its longitudinal magnetization at the end of a binomial pulse but rather wilt undergo a diminution in signal proportional to the duty cycle of the pre-

RESULTS

=

and

of a string separated

is still within the guidelines Food and Drug Administration. the for

-a

#{231}#{176}#{231}#{231}#{231}

(a) Schematic

onstrated

parameter will result in different “duty cycles” for the presaturation pulse train and thus will result in differing degrees of presaturation. The RF power level of the pulse train, as measured by means of the nutational frequency imparted by the resulting B1 field to the magnetization, is

is estimated

1.

reference with which to evaluate nonspecific saturation or signal loss resulting from the binomial pulse train.

of MR imager atthrough the period precludthe

specific absorption age weight with the highest duty

6w=

MTC. In both cases, the MTC saturation is applied “before” the 90#{176}-180#{176} imaging sequence, during the intervals that would otherwise be occupied by multisection excitations. The differences between continuous and pulsed saturation are also shown. cw = continuous wave. (b) A 1 1 binomial pulse is illustrated. Note that this pulse can be applied on- or off-resonance. In the latter case, the frequency offset has to be alternated coincidentally with the phase alternation.

the

body coil and limits each transmit to no longer than 10 msec, thus ing the possibility of implementing

pulses

L

Figure

= 600 msec and 2,000 msec. The Se-

used in this study. This particular model lows only RF transmission

2,500

Saturation

2; field of view = 25 cm (human head

implemented section for TR msec, up to four Only single-section

saturation of 5-msec

Continuous

=

for TR

for

Controlled Fn.=;.;os

-

a.

TRs were 1,000 time = 20

of phase-encoding

12.8 minutes

1

fol-

set collected

averaged phantom),

(egg

JH

!

of

imaging

and leg), or 33 cm (human abdomen); section thickness = 5.0 mm (eggs) mm (human subjects); total imaging was

I

consisted

component

data

r

in

(model Highland

The imaging parameters used images presented herein were time (TR) = 600 and 2,000 msec

for one

15 cm

u=a

implemented

spin-echo

egg phantom in which msec and 10 seconds); msec;

continuous-

sequence

by a standard

sequence. for all the

events

diminished

the

MR system International,

Ohio).

towed

Imaging

of non-

of MTC.

a 0.5-T whole-body Vista HP; Picker an

and

degree

3 demonand

effect egg

of yolk,

change in the relatissue contrast; the is quite different and cooked egg.

there is a relatively in the signal of the

greater cooked

April

1992

a.

b.

c.

Figure 2. Representative human head images at (a, b) TR = 600 msec and (c, d) TR = 2,000 msec, all obtained with echo time = 20 msec. (e) T2-weighted image (TR = 90 msec) is also shown to illustrate its superficial re-

4

‘

.. .

,4

t:s

-4

semblance

..‘

masked

.

,

.:7

,$.

changes

to some

at imaging, 4. Intensities MTC, but

extent

TR MTC image in signal

shown

intensity,

by the windows

used

are given quantitatively in Figure of all tissues decrease with the effect is greatest for the my-

elinated white matter, resulting in altered gray-white contrast. Note also in b the increased prominence of the lenticular nuclei

.-

in the image),

(a) TR = 600 msec (control TR = 600 msec (MTC image), (c) TR = 2,000 msec (control image), (d) TR 2,000 msec (MTC image), and (e) TR = 2,000 msec (T2-weighted [echo time = 90 msecj control image).

)

‘I

d.

to the long

in d. Absolute

I

‘

brain.

0,)

=

e.

egg white, which results in a progressive decrease in the intensity ratio of cooked to raw egg white with increasing RF duty cycle. Figure

2 shows

the

representative

MTC images of the head and their control images. In this sequence of images, both Ti-weighted (TR = 600 msec) and proton-density-weighted (TR = 2,000 msec) images were chosen to illustrate the TR dependence of MTC. This dependence arises from the dual but conflicting effect of the selective saturation on tissues rich in macromolecutar constituents. The saturation of the immobile protons from MTC irradiation causes, on the one hand, attenuation of observable signal through magnetization transfer. On the other hand, this same saturating event also brings about a decrease in the apparent longitudinal relaxation time (7). As a result, for pulse seVolume

183

Number

#{149}

1

quences with short fect partially cancels thereby pulse

TR, the latter the former,

compromising sequence with

ef-

the MTC. For tong TR, the

MTC effect (which has a negative impact on signal strength) is not compensated by the Ti shortening (which impacts

therefore

the

signal

positively)

and

is

left unmitigated.

As expected,

in the

human

images

the relative signal drop-off varied from tissue to tissue, being greatest for muscle and smallest for cerebrospinal fluid in the lateral ventricles of the brain. Thus, the images obtained with MTC display dramatically different contrast than do the control images. 3b and

In comparing Figure 4a with

Figure 4b, we

3a with note also

that some tissues, such as cooked egg white and white matter (as discussed above), show moderate to strong TR dependence on signal attenuation,

a

while others, especially those of the abdomen such as back muscle, renal cortex, and liver, show similar amounts of MTC attenuation with varying TR. Figure 2 also demonstrates an interesting effect of MTC irradiation on a Ti-weighted image of the brain. Note the relatively increased signal intensity

in the

lenticular

nuclei

compared

with the signal intensity of the remainder of the brain, the consequence of a lesser degree of saturation in these increased As gray

nuclei. signal matter

The reason intensity structures,

for this is uncertain. lenticular

nuclei have a relative paucity of myelin compared with the surrounding white matter; hence, greater attenuation

of white

matter

could

be ex-

pected. However, this does not explain why the cortical gray matter does not remain equally intense as Radiology

211

#{149}

the

lenticular

trast

nuclei.

mechanisms

clei that

There

in the

are unique.

are connu-

lenticular

For example,

these structures are known to have diminished signal on high-static-field T2*weighted images, an effect that may be a result of iron deposition (8). Perhaps iron or other substances produce a paramagnetic effect that contributes to the overall isointense appearance of the lenticular nuclei on conventional Ti-weighted images, a mechanism not present to the same

degree brain.

elsewhere

in the

visualized

6

0.9

0.9

0.8

0.8

0.7

0.7

0.6

0.6

0.5 0.4

0.3

The

ing.

authors

are grateful

of MTC nomenon.

in demonstrating

this

0.2

10

results

confirm

and

20

‘

Interpulse

Delay,

observed The effect

muscle, protein

with was

10

this

terms

spin-bath

continuous

images.

show signal intensity of MTC-irradiated

(a) Data

for a short

data samples

for cooked to signal

TR (1 second)

factors

2i2

#{149}

affect

Radiology

TR (10 seconds)

are pre-

0.8. pD

0

i

0.8

6

0.6

.

0.4

0

0.6. 0

0.4.

g

#{163} 0.2

10

15

0.2.

10

20

Delay,

15

lnterpulse

msec

Delay,

20

msec

b.

model

to its

is the inherent specifically,

the

properties relative

of the tongi-

tudinal relaxation rates of the freewater protons versus the cross-relaxation rate between the free and bound fractions. The other factor is s, the saturation parameter that, as we show

in the

Appendix,

is dependent

on both tissue properties and machine settings. With the application pulsed saturation, the steady state

of

hence

key

(b) for a longer

as the ratio spin-echo

0

factor tissue,

signal alteration expressed in

saturation

magnetization. ratio from

eggs, expressed of conventional

0

ton

1 and

and

and raw intensities

sented. In both graphs, the ratios for all substances are less than 1, a combination of MTC-specific as well as nonspecific signal loss. Both graphs also demonstrate a dramatic decrease in the ratio of cooked egg white compared with the raw counterpart; this finding is consistent with the effect of increased MTC on the denatured egg-white protein. Note also that the signal loss decreases slightly as the interpulse delay increases, and the amount of MTC irradiation is consequently diminished. The effects of different TRs are modest; the greatest effect is decreased signal in cooked egg white at the longer TR compared with the shorter TR. #{149} = cooked egg yolk, = cooked egg white, 0 = raw egg yolk, = raw egg white.

signal intensity depends not only on the fundamental rate constants of the tissues and the saturation parameter s, but also on a complex dependence on the interpulse interval, T, which effectively controls the duty cycle and

s = the simple It is clear

25

‘

msec

b.

Figure 3. Graphs of signal intensities

unsaturated counterpart can be shown under some limiting conditions (1-5) to be R = {5, + T(1 (r5, + ri), where s is a fraction (0 s 1) that measures the degree of saturation of the bound fraction of the proration,

20

Delay,

high

(iO-i4) in which the free and bound protons are considered to be two mutually coupled thermodynamic reservoirs, each of which relaxes with its own relaxation mechanism. The ratio of the steady state signal intensity

under

#{149}

Interpulse

MTC presaturamost marked in

The reason for can be quantitatively a binary

15

‘

msec

Figure 4. Plot of intensity ratio of MTC/control in images of human tissues as a function of interpulse delay at (a) TR = 600 msec and (b) TR = 2,000 msec. Error bars on individual data points are omitted from the graphs for clarity. They are approximately 15% for = 10 msec, 10%-15% for = 15 msec, and 10% or less for i, = 20 msec. The larger-than-unity ratios of the bone marrow fat are clearly results of measurement errors. = calf muscle, #{149} = bone marrow, A = gray matter, #{149} = white matter, D = cerebrospinal fluid, 0 = subcutaneous fat, = liver, 0 = renal cortex, = back muscle.

(i-5). contrast

as expected from the content of this tissue.

of

....

25

a.

phe-

extend

of other investigators Substantial changes in tissue

were tion.

15

Interpulse

DISCUSSION Our

0.3

#{163}0.1

a.

those

.

#{163}0.1

to an

anonymous reviewer of the initial version of this manuscript for suggesting the intriguing possibility that paramagnetically produced signal was present in the lenticular nuclei and for suggesting the potential role

0.5 0.4

0.2

control

This paramagnetic effect will not be reduced with MTC irradiation, which produces relatively increased signal as the remainder of the brain loses signal. Interestingly, the lenticular nuclei can appear hyperintense on Ti-weighted images in certain disease states, an observation that has been hypothesized to result from the presence of paramagnetic substances in these nuclei (9). Possibly, MTC may play a role in demonstrating subtle paramagnetic effects in clinical imag-

d

For

complete

R becomes T,/(T,

this

MTC

+

satu-

equal

to

Ti).

relation

signal.

that

One

two

As

the

degree

mentioned

MTC sequences tion of continuous tation

over

of saturation above,

use

requires the or pulsed

a lengthy

duration.

(i4). of the

applicaRF exciAt

least

at lower

field

strengths,

the

RF

power level required is reasonably low. Thus, the RF duty cycle presents problems, perhaps not so much for patient safety but for implementation in most, if not alt, commercial MR imagers manufactured for clinical use. This difficulty represents a major

stumbling block to the development of this potentially powerful technique from an investigational method to use in the clinical arena. We have demonstrated

herein

tend MTC

that

the experimental from in vitro

it is possible

to ex-

technique and restrictive

of in

vivo

settings to a clinical environment a pulse sequence that should be easily implemented on most exist-

by using ing

commercial

mentally,

the

may

eliminate

ing

the MTC

MR imagers. use

Experi-

of a binomial

pulse

the necessity of shiftirradiation frequency April

1992

might

M,

be enhanced

with

MTC

irradia-

tion. Second, tissue characterization may be possible. For example, the contrast effect is particularly pronounced in tissues with a high macromolecular content such as muscle. This phenomenon may permit characterization of myelin in the brain or fibrosis in other somatic tissues. For

example,

the

tissue,

which

identification

of fibrous

is difficult

with

Figure Al. Trajectories netization vectors during the

1 T binomial

the various of the

pulse.

trace the tip of magthe time course of

labels along indicate T2 in milliseconds magnetization compo-

lines

corresponding

Number

nent.

Frequency offset = 0, w sec, or B1 = 4.5 .tT (the actual strength of the binomial pulses experiment is = 9.3 p.T).

off-resonance. pulses Figure

= 1,200 rad/ RF field used in our

By applying

off-resonance, i, some of the

binomial

as illustrated nonspecific

in satu-

ration problem described above may be alleviated. Investigation of this possible improvement is currently under way in our laboratory. It should

be noted

that

the

2-4

agree

in terms

that we constants.

off-resonance

saturation,

at (7) demonstrated the cross-relaxation measured.

Grad

et

that both r,, and rate, TX could be

In a related

development

reported elsewhere, we have provided a general quantitative solution to the theoretical problem of the application of selective pulsed saturation to a heterogeneous spin system such

as tissues

by

using

the

spin-bath model; we have vided a means to estimate as well

ters

as the

(ie, the

remaining

binary

also TX

two

pro-

and r5,, parame-

longitudinal rethe molar ratio f of the bound to free protons) that are required to describe fully the theoretical model of coupled spin bath (15). The clinical usefulness of MTC imaging requires further investigation. taxation

Tissue

whether agnostic

There MTC First, Volume

intrinsic

rate,

T,

contrast

, and

is clearly

this

altered,

alteration

efficacy

enhances

remains

but

di-

to be seen.

are at least two possible ways techniques might be of benefit. the conspicuity of certain lesions 183

Number

#{149}

1

is

spin-bath

know

BINOMIAL

=

x)”. The

2 (1 2 1). In our

nance, as shown alternation that

in Figure is coincidental

can the

ii

work,

total

To

very

short

vectors

transverse

we

of the

illustrate

how

saturate free ones,

the bound in Figure

lb.

T2 components,

to realign lose strength

themselves in the

ond

lobe

I 1 pulse

of the

and

long

long

on

coherence,

the

hand,

magnetization tively unaffected. part tion short

the

which

quickly

ter in biologic more nonspecific will

will

vectors

the

maintain

tipped

in the

will

continuously

de-

Thus, the action of a has the effect of reducof the very short T2 hydroleaving the

T2 nuclei accounts

a dynamic

relaxation

with

composite

pulse.

favorable

condition

interplay the

of trans-

RF action

On-resonance,

of the the

in bringing

but not plot the travecof

‘

in Figure is slightly reason

Al, -yB1 less than

strength we

actual

used chose

B1 for

B1 field

T20)

rad/sec, of the

in our experiments. this value instead illustration

happens

to yield

0.45

=

msec

(0.2

msec

This

for

a T2 that

sponse

of the

shows

which zation ignores

is long compared composite pulse,

a final

preserves as expected relaxation for proton continues

the

phase T2 is much

by

taking

into

field in the ation. In the

account

I 1 pulse

diagram,

the

and the

solid

effect

transverse line

of the

RF

relaxrepresents

actual

tip angle

near

the

longitudinal for binomial effects. On

fractions to approach

with the

Al,

of a single

the

effect

zero,

with T2 = 0 regardless

of the RF field. Note shorter than T2,,D,

effect

the re-

magnetipulses and the other T2,, of

that when the saturapulse

di-

B1 field

in Figure

The

(21)

condiT2 (or

the

msec.

equations

the

a 360#{176} flu-

is superfluous. critically damped

tion efficiency of the binomial minishes rapidly unless the strength is increased.

of the magfrom the tran-

of

is that

B1 field in use).

hand,

is

the

1,200 one-half

=

tation on the long T2 protons after the first lobe of the pulse. This fortuity gives the misleading impression that the binomial-

M(t)

applied RF field of the I T pulse. In

Bloch

most

“down”

resonance (in the manner described Fig Ib) is not quite the same as that

to the

relain

T2. magnetization

quency. This diagram represents a tracing of the magnetization vectors of the proton components, with T2 spanning three orders of magnitude from 15 sec to 15

solution

op-

tissues, will tend to show signal attenuation than

show

verse

For

case, the RF irradiation proton-resonance fre-

netization,

sec-

affect

differently.

be

tissues with longer When T2 is short, the

RF action

pulses

the illustrated applied at the

sient

the

longitudinally process. The

of the longer This also

ity of the pulse tion makes the

with a sign to the

by duration

time development M(t), is derived

to-

for the differential nonspecific saturathat tissues will undergo. Tissues with T2 values, in the range found in wa-

actual

used

from equilibrium for the whole

The

of

for

(or the immobile macromolecular gen) nuclei to near zero while

the

i (1 1)

jectory of the tip of the magnetization tor M(t) in the rotating reference frame the proton fractions with different T2 times as they are deflected or nutated the

tilted

which will

other

RF field

corn=

binomial protons Al we

cases msec. pulse,

except

T2 fractions

T2 fractions,

dine longitudinally. complete RF pulse ing the magnetization

RF pulses.

these

are

plane,

tend and

The

duration of 5 rnsec (w = 0 in Fig Ib). also be applied off-reso-

alternation

the

trated which

most

probably

the

longitudinal magnetization (ie, makM, = 0) is the “critical damping” condition or yB1 = 1/(2 T2), where -y is the gyromagnetic ratio. In the example illus-

PULSES

1 T pulse with a applied on-resonance These pulses can

phase

funda-

can have different modulation as of x in the bino-

mial expansions (1 monly used cases are

n

all the

magnetization

the ing

The binomial pulse varieties of amplitude given by the coefficients

and

model

A CONSIDERATION

OF

relaxation

ous

of the binary

APPENDIX:

intrinsic

scte-

what

represent

posite direction and will return toward their initial state. The short T2 fractions,

known about MTC in terms of the theoretical spin-bath model. Tissues rich in proteinaceous fibers, such as muscle, tend to demonstrate a larger reduction in signal intensity than do tissues such as cerebrospinat fluid that have only a small protein content, and this signal reduction increases as the interpulse interval in pulsed saturation decreases. It is also possible to interpret these results quantitatively provided mental

rate of the free-water protons, r5, , cannot be determined simply by measuring Ti with conventional techniques. However, by performing Ti measurements with an inversionrecovery sequence by using continu-

with

the ward

phase

in Figures

qualitatively

of brokenness,

a T2 of 2.67, 0.47, 0.084, and 0.015 After the first lobe of the binomial

The

sequences, would be of great in establishing a differential diagnosis (16-19). One preliminary report suggests a rote for MTC imagof multiple

order

short

conven-

tional benefit

ing in the evaluation rosis (20). The results illustrated

the proton fractions with a T2 of 15 msec, while the rest of the lines, in decreasing

pulse

of an

applied

off-

for shown

on-resonance

pulse. pulse

When the B field of the is applied on-resonance,

called verse

effective field, B1,, lies in the transplane. When the binomial pulse is

applied off-resonance, that brings about

the

binomial the so-

the effective field nutation of the spin

Radiology

#{149} 213

vectors

now

xy)

on

but

result,

to the

tions, be

lies not on the transverse the

an

just

meridional long

ability

pulse

of an

therefore if all the

T2,

immo-

bile fractions will be severely impaired. However, if a sufficient number of such pulses is applied in series, this attenuated saturation will accumulate, which gives a stronger net effect. Use of a train of pulses will also allow for the time required for

saturation

in the immobile

fraction

(

or transfer

to the

free

to communicate

1/r,)

=

fractions. One aspect of the difference between application of continuous and pulsed RF in MTC imaging is the trade-off between use of a weak, long, continuous-RF field and a shorter, more intense, amplitudemodulated field. The key factor in determining the saturation efficiency of the two

techniques tion nus

(measured

by degree

achieved of the immobile that of the free fraction

power)

time

is T2 , the of the immobile

the T2,,, turn-on

the more time and

transverse protons. important the more

RF

complex

as well as on machine level, off-resonance cycle). while

It should also the quantitative

tween MTC and tors is complex,

,

r,

2.

3.

4.

5.

of the

6.

damped

the aforementioned the quantitative

on T2),

(RF power and duty

emphasized relationship

7.

that be-

8.

9.

facrelation10.

214

Radiology

#{149}

13.

14.

15.

Wolff SD, EngI, Balaban RS. Magnetizahon transfer contrast: method for improving contrast in gradient-recalled-echo images. Radiology 1991; 179:133-137. Wolff SD, Chesnick 5, FranklA, Lim KO, Balaban RS. Magnetization transfer contrast: MR imaging of the knee. Radiology 1991; 179:623-628. Wolff SD, Balaban RS. Magnetization transfer contrast (MTC) and tissue water proton relaxation in vivo. Magn Reson Med 1989; 10:135-144. Eng J, Ceckler TL, Balaban RS. Quantitalive 1H magnetization transfer imaging in vivo. Magn Reson Med 1991; 17:304-314. Grad I Mendelson D, Hyder F, Bryant RC.

of nuclear

magnetic

cross-re-

la.xation spectroscopy to tissue. Magn Reson Med 1991; 17:452-459. Hu B, Nishimura D, Macovski A. Pulsed

magnetization

comparison

factors frequency, be

1.

the

dependent T20, , and

11.

U

Application

are beyond the scope of this article. It suffices to point out here that the MTC in tissues is intimately related to this saturation efficiency and is therefore many tissue factors (rn,

(15)

References

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mi-

relaxation The shorter

is the pulsed

formulated

can be calculated numerifundamental constants are

Acknowledgments: The authors express their sincere appreciation for the technical support provided by Picker International. They also benefited greatly from stimulating discussions with many of their colleagues, in particular, Scott Swanson, PhD, and Thomas Chenevert, PhD, of the Department of Radiology, University of Michigan Medical Center, Ann Arbor, Mich.

of saturafraction unit

per

theoretically

12.

bino-

short

known.

been

z pulse.

off-resonance the

has

and cally

will

the

on-resonance

to saturate

ship

pulse

in preserving as the

(or As a frac-

binomial

as effective

the

mial

plane.

free-proton

off-resonance

magnetization But

T2,

(xz)

transfer

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(abstr).

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21.

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vasive parameter of developing liver fibrosis? Hepatology 1988; 8:217-221. Lee fl(, Glazer HS. Controversy in the MR imaging appearance of fibrosis (editorial). Radiology 1990; 177:21-22. Negendank WC, al-Katib AM, Karanes C, Smith MR. Lymphomas: MR imaging contrast characteristics with cinical-pathologic correlations. Radiology 1990; 177:209216. Foust RI, Tanttu II, Sepponen RE, Kinnunen E. Magnetization transfer contrast in multiple sclerosis at 0.1 T. In: Book of abstracts: Society of Magnetic Resonance in Medicine 1990. Berkeley, Calif: Society of Magnetic Resonance in Medicine, 1990; 626. Torrey HC. Transient nutations in fluclear magnetic resonance. Phys Rev 1949; 76:1059-1066.

April

1992