Robert C. Smith, R. Todd Constable,

MD #{149}Caroline Reinhold, MD2 #{149}Thomas R. McCauley, MD PhD #{149} Ruben Kier, MD #{149}Shirley McCarthy, PhD, MD

Multicoil High-Resolution MR Imaging ofthe Female A fast spin-echo pulse sequence was combined with multiple surface coils used simultaneously in the form of a “multicoil” in magnetic resonance imaging studies of the female pelvis. This combination allowed maximal resolution with maintenance of the signal-to-noise ratio (S/N) at an acceptable level, and the S/N with the multicoil system was substantially better than that achieved with a body coil. Excellent image quality and demonstration of anatomic detail were afforded by use of this technique. Index

terms: Magnetic resonance (MR), coil #{149} Magnetic resonance (MR), high-reso#{149} Magnetic resonance (MR), multicoil #{149} Magnetic resonance (MR), rapid #{149} Pelvic organs, MR. 85.1214

arrays lution imaging imaging Radiology

1992;

184:671-675

M

resonance

of the

imag-

pelvis

with pulse

conventional sequences

and

that

with a body coil, fast spin-echo provides T2-weighted

of quality

superior

a

to that

achieved with a standard-resolution CSE sequence. The inplane resolution of both of these sequences was 1.1 mm (frequency) by 2.2 mm (phase). The the

FSE technique, however, possibility of performing

allows much

higher-resolution imaging because of the marked reduction in imaging time. In an ideal MR imaging sequence, resolution is maximized while the signab-to-noise ratio (S/N) is maintained near a certain threshold level. An increase

in S/N

beyond

this

level

in little perceptible image quality but

improvement usually increases

imaging

of S/N

time.

Use

results

below

in this

level results in perceptible image degradation (2). When the FSE sequence is cornbined with multiple surface coils used simultaneously in the form of a “multicoil” (3-5), the S/N is improved so that high-resolution images can be obtained in less than 5 minutes. When nonstationary

structures

are

being

imaged, however, an increase in resobution does not necessarily improve image quality. The increased imaging time may From the Department of Diagnostic Imaging, Yale University School of Medicine, 333 Cedar St. New Haven, CT 06510. From the 1991 RSNA scientific assembly. Received February 18, 1992; revision requested May 1; revision received May 18; accepted May 20. Address re1

print

requests

with higher-resolution result in a substantial

physiologic piratory

PhD

phase encoding. In addition, smaller voxel size increases phase ghost artifact (6). Despite improved S/N, use of multicoil imaging itself can result in image degradation due to increased in-

is cur-

(body) coil. A prior issue of Radiology (1)

demonstrated standard-resolution (FSE) sequence images

(MR)

female

rently performed spin-echo (CSE) a whole-volume study in this

C. Lange,

Fast Spin-Echo Pelvis’

AGNETIC

ing

Robert

#{149}

motion motion,

sequences increase

artifacts bowel

from

in

res-

peristalsis,

and vascular pulsation. It is important to note that the FSE technique is not compatible with respiratory-ordered

tensity occurs

of phase as a result

ghost artifact. of both the

This in-

creased signal from subcutaneous fat immediately adjacent to the surface coils and the image reconstruction algorithm employed (3,4). A prior study compared CSE images obtained with a body coil to CSE images obtamed with a mubticoil system (7). Multicoil images obtained with a small field of view (FOV) showed increased motion artifact in comparison with body-coil images obtained with a standard FOV. The purpose of this study was to determine if the increase in imaging time, decrease in voxel size, and multicoil-related artifacts in high-resolution FSE multicoib imaging would result in significant image degradation. We therefore performed MR imaging in a series of patients, by using a standard-resolution body-coil FSE sequence and a high-resolution multicoil FSE sequence that required a longer imaging time. The body-coil FSE

images

were

obtained

by using

parameters that provide images superior to those obtained with body-coil CSE sequences (1). These images served as a baseline for determination of multicoil image quality. Direct comparison of high-resolution body-coil imaging with high-resolution multicoil imaging with identical

imaging

parameters

was

not

undertaken. A preliminary evaluation indicated that body-coil images obtamed with parameters identical to those used to obtain multicoil images

to R.C.S.

Current address: Department of Radiology, Montreal General Hospital, Montreal. © RSNA, 1992 See also the article by Smith et al (pp 665-669) in this issue. 2

Abbreviations: FSE

=

fast

CSE spin-echo,

=

S/N

conventional spin-echo, E-space = signal-to-noise ratio.

=

echo

spacing,

FOV

=

field

of view,

671

b.

a. Figure clearly

1. T2-weighted (4,500/126) high-resolution shows four distinct zones of signal intensity

nal. A surrounding palmatae (arrow).

Figure

2.

intermediate (c) Axial image

(a) Body-coil

c. multicoil MR images obtained in a 29-year-old within the cervix. An inner zone of increased

zone most likely represents mucosa through the corpus demonstrates

T2-weighted

(arrow). a septate

(b) An axial image uterus (arrow).

woman. (a) Sagittal signal intensity most demonstrates

the

image likely

individual

of the uterus represents the cafolds

of the

plicae

(2,900/

126) sagittal decreased endometrium

MR image shows a linear area of signal intensity within the fundal (arrow). (b) Multicoil highsagittal image (4,500/126) obtained patient shows far greater detail and the diagnosis of blood clot (arrow).

resolution in same suggests

The

finding

ages

obtained

was seen

tent

with

to be resolved

3 weeks

later,

the diagnosis

in this

study

quality,

not

proval

of diagnostic

of the

MATERIALS This

lowered

AND

study

was

of the

Yale

is consis-

of clot.

were

because

on im-

which

S/N.

METHODS

performed

with

University

the

ap-

Human

In-

a.

vestigation Committee. The patient population consisted of 32 consecutive women referred for pelvic MR imaging over an

8-week from

period.

The women

17 to 76 years

formed

consent

ranged

(mean,

was

obtained

from

tients during our routine preimaging terview. All imaging was performed a l.5-T system (Signa; GE Medical

tems, Milwaukee). Prior to imaging, patients bladder. Patients underwent the prone or supine position, on

their

preference.

After

weighted

bocalizer

msec/echo

time

tamed,

sagittal

images

were

with

the

28-cm

and

axial

2.5-mm

intersection

matrix, length

time

These

provide

parameters

53 seconds.

To

anatomy,

was

used

672

#{149}

ensure

Radiology

MR imaging

coverage 18 sections

localizer

pelvis,

either

of the

of

in

a prototype

Medical

pelvic

or sagittal

was

initially

RESULTS

T2-weighted Because imag-

high-resoluwere obtained

plane 4,500/126,

FOV anterior and posterior pulses (8) were used. The saturation the sagittal

sec-

ters

provide

with

Overall reduction

in

the

20-cm

saturation position of these

pulses was determined localizer image. These

10 sections

Imwas

Systems).

with a unit in time available depending on the

needed, images

On the basis of the body-coil center 10 section locations

were chosen for the multicoil images. aging in the axial and sagittab planes performed in eight of the 32 patients.

under-

nab averages, echo-train length of 16, and E-space of 17.5 msec. Superior and inferior saturation pulses, no-phase wrap, and in-

rele-

acquisition in 106

and

imaging

parameters:

49 seconds. images, the

FOV, 4-mm section thickness, 2-mm intersection spacing, 512 x 256 matrix, four sig-

x 128

sections

the axial

following

inferior saturation wrap were used.

nine

FSE

(GE

TI-

all patients

bladder

with

system

FSE 2,900/126,

a concatenated

to provide

went

imaging, their

clinical information tion FSE multicoil

two signal averages, echo-train of 16, and echo spacing (E-space) and

emptied

axial

obtained.

ing was being performed clinical use, total imaging was limited. Therefore,

thickness, 256

were

performed. High-resolution images were then obtained.

ob-

the

spacing,

18 msec. Superior pulses and no-phase

vant

was

conventional

body-coil

again multicoil

Ti-

body-coil

section

After

inwith Sys-

(repetition

parameters:

5-mm

all pa-

this, images

Sagittal

through

following

FOV,

After

weighted

emptied their imaging in depending

image

obtained

onds.

In-

a coronal

500/20 msec)

in age

34 years).

b.

in 4 minutes

from parame-

and

with

the

omy

of

quality were

and motion consistently

mubticoil

images.

the

uterine

artifact excellent

Zonal

corpus

and

anatcervix

were visualized in such greater detail on the multicoil images that a cervical zone not described previously with CSE

pulse

60%

(12

sequences of

20)

was

noted

of the patients

in

imaged

the sagittal plane (Figs 1, 2). Ovarstructure was depicted in far better detail (ie, individual follicles and cortical and medullary stroma) than previously shown with body-coil imin

ian

ages

(Figs

cysts

were

3, 4).

Similarly,

seen

in far

nabothian greater

detail

September

1992

k-space data, which are Fourier-transformed to separate images. These images are then combined into a single composite image by using a sum of squares

combination

reconstruction

algorithm (3,5). With this technique, the signal intensity of each pixel in the composite image is equal to the square

b. Figure 3. (a) Body-coil T2-weighted (4,500/126) multicoil image obtained cystic ovarian disease. Notice that ated on the multicoil image.

(2,900/126) axial MR image and (b) high-resolution in a 19-year-old woman with a clinical diagnosis the individual cysts and ovarian stroma are clearly

of polyappreci-

b. Figure 4. Definitive hon image

on the

(a) Body-coil T2-weighted (2,900/126) image identification of the right ovary is not possible. obtained in the same patient clearly delineates

multicoil

individual venous (Fig 6).

images

(Fig 5). The

veins of the perivaginal plexus could be identified

obtained in an 18-year-old (b) Multicoil (4,500/126) the right ovary (arrow).

in a receive-only

An example body-coil

DISCUSSION In regions close to the coil, S/N with local (or surface) coils is markedly better than that provided by body coils. The local coils are used only in the receive mode, and the transmit excitation pulse is provided by the body coil. The sensitive region of a local coil is severely limited, typically on the order of the coil diameter. When used individually, these coils can

provide

When

multiple

simultaneously

“multicoil,” Volume

184

only

small-FOV

surface in the

coils form

images.

are used of a

however,

large-FOV

#{149} Number

3

im-

woman. high-resolu-

ages with improved S/N can be obtamed, as previously described (3-5). This requires that each coil act independently

mode.

of high-resolution

and

mubticoil

images

ob-

tamed in the same patient and with identical imaging parameters is shown in Figure 7. This demonstrates the substantial that is afforded

The

improvement in S/N by the mubticoil.

mubticoil

used

consisted

of two

coils

two

(Fig

and

8). The

adjacent adjacent

lapped such tance is zero generated by during signal erate a current Each individual

in this

adjacent

posterior are

through

of the

sum

of the

squares

the

creased

coils

that their mutual induc(ie, the magnetic field the current in one coil reception does not genin the adjacent coil). coil generates its own

image

(3,5).

voxel

size

in high-resolution

multicoil FSE imaging. In fact, even though glucagon was not administered and the imaging time of the high-resolution sequence is close to 5 minutes, even large bowel anatomic detail is consistently sharp in the multicoil images. In addition, the in-FOV saturation pulses were extremely effective in suppressing the signal from subcutaneous fat and in preventing ghost artifact. Correct placement of these pulses, especially anteriorly, is essential. The position of these pulses is determined

bocalizer

from multicoil

the uterus show detail. A distinct has

not

been

study.

images

spin-echo during

axial)

This

canal)

through

zone

previously

most

on

MR images, the course of of intermediate

signal intensity (immediately to the inner high-intensity cervical

(or

the zonab anatomy in cervical zone, which

described

conventional was identified this

a sagittal

image.

Sagittal

over-

entire

In addition to those artifacts related to the multicoil and the image reconstruction algorithm, the decreased voxel size (6) and increased imaging time with a higher-resolution sequence will also have a tendency to degrade image quality. The purpose of this study was to determine if these factors resulted in significant degradation of high-resolution FSE multicoil images. The inplane resolution of the multicoil images is 0.4 mm (frequency) x 0.8 mm (phase). This represents a decrease of close to a factor of 3 in voxel size in both the phase and frequency directions, compared with standard-resolution images. On the basis of the results of this study, it appears that there is not a perceptible increase in motion artifact from increased imaging time or de-

study

anterior

coils

root

of corresponding pixel values in the individual-coil images. This method does not use weighting factors to compensate for the increased signal intensity from voxels close to a coil. This reconstruction method has two important consequences: (a) objects located close to the coil have markedly increased signal intensity and (b) phase ghost artifacts are poorly suppressed, being propagated

likely

adjacent zone of the represents

Radiology

673

#{149}

Figure

a.

b.

Figure

5.

shows

ill-defined

nabothian

(a) Body-coil cysts

tient allows

T2-weighted

increased (arrow).

definitive

signal (b) Multicoil

identification

(2,900/126)

intensity

High-resolution T2-weighted multicoil image demonstrates vessels of the penvaginal-periure-

individual

image

within

6.

(4,500/126) obtained

the external

high-resolution

(4,500/126)

of the individual

in a 35-year-old

woman

Os, which may represent image obtained in same

thral

venous

Figure

8.

plexus.

pa-

cysts.

A multicoil

phantom.

nor

Notice

coils

are

torn by straps

a.

is placed

that

held

around

the anterior

in opposition

with

fabric

and to the

a

postephan-

fasteners.

b.

Figure 7. FSE images obtained at 4,500/126, 20-cm FOV, 4-mm section spacing, 512 x 256 matrix, no-phase wrap, and in-FOV tion pulses. The only difference between the imaging techniques the image in a was obtained with a body coil and that in b was patient has a septate uterus.

section

anterior used obtained

thickness,

and

posterior

2-mm

inter-

satura-

to obtain these is that with a multicoil. This

patients.

the mucosa. Its corrugated inner margin, the plicae palmatae (9), was apparent. The high-resolution images enabled visualization of individual vessels of the perivaginal-periurethral venous plexus. Availability of this much detail would obviate possible confusion with high-signalintensity lesions such as urethral diverticuba (10). Sometimes, because of the limited resolution of images obtained with a 256 x 128 matrix, ovarian stroma or individual cysts cannot be discretely seen, making definitive identification of the ovaries impossible. Of 34 ovaries identified on axial images, nine were evident only on high-resolution images (P < .005). This study was not intended as a comparison of high-resolution multiRadiology

#{149}

coil imaging and imaging performed with an optimized body-coil FSE sequence. The body-coil FSE images were obtained to have a reference by which to evaluate the multicoil images. Therefore, on the basis of results of this study, we cannot conclude that high-resolution multicoil imaging is superior to body-coil FSE imaging. It is clear, however, that the high-resolution parameters used with multicoil imaging in this study cannot be employed in a body coil without image degradation occurring because of the lower S/N (Fig 7). It has been shown that improvement in S/N beyond a certain threshold will result only in a small improvement in image quality (2).

S/N

in the

high-resolution

coil images obtained near this threshold

multi-

in this study in average-sized

was

Therefore,

in obese

patients,

an increase in FOV or decrease in matrix size may be required to maintain S/N

674

the

at an

acceptable

level.

Multicoib imaging formed with standard tems

unless

cannot be perimaging sys-

additional

hardware

is

obtained. Each coil of the multicoil array requires a separate preamplifier, amplifier, receiver, and memory. Thus, multicoil imaging capability is costly.

In addition,

since

performance

of this study, we have noted significant image degradation when one or more coils of the multicoil array is not functioning properly. It is therefore important to check the function of the individual coils on a regular basis. This is accomplished by obtaining images of a phantom and comparing the received signal intensities of the mdividuab coils. The individual-coil signal intensities

are

accessible

during

per-

formance of the preimagrng process. This study indicates that multicoil imaging can provide improved visualSeptember

1992

ization of anatomic structures in pebvic MR imaging. This should result in improved ability for detection of abnormalities as well as determination of their extent. The application of multicoil imaging to specific disease entities as well as tumor staging needs to be formally investigated to determine if the benefits outweigh the additional cost. U

2.

3.

4.

Smith RC, Reinhold C, Lange RC, et al. Fast spin-echo MR imaging of the female

I. Use of a whole-volume

with MR phased

arrays.

son Med 1991; 18:309-319. Hayes CE, Roemer PB. Noise in data simultaneously acquired

tiple surface

6.

coil. Radi-

ology 1992; 184:665-669. Owens RS, Wehrli FW. Predictability of SNR and reader preference in clinical MR imaging. Magn Reson Imaging 1990; 8:737745. Hayes CE, Hattes N, Roemer PB. Volume

imaging

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McCauley TR, McCarthy S. Lange RC. Pelvic phased array coil: image quality assessment for spin echo MR imaging. Magn Reson Imaging 1992 (in press). Edelman RR, Atkinson DJ, Silver MS. Loaiza

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Radiology 1988; 166:231-236. Bloom W, Fawcett DW. A textbook of histology. Philadelphia: Saunders, 1975; 894. Hricak H, Secaf E, Buckley DW, Brown JJ,

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Volume

184

Number

#{149}

3

Radiology

675

#{149}