GE Healthcare

MR Field Notes
closer a coil’s position to the area of
interest, the stronger the signal will be,
therefore, the SNR will be higher.

Optimal coil designs differ depending on

the body part being imaged. Let’s look
at some basic differences in coil design
Vol. 1, No. 2 Spring 2005 and technology.
RF Coils ... Receive-Only vs. Transmit/Receive
Contents
1 RF Coils ... They’ve Come a
They’ve Come a Imaging coils receive and/or transmit
the RF signal. Receive-only coil designs
only receive the MR signal, using the
Long, Long Way
A brief history of coils, how far
Long, Long Way body coil as a transmitter. These de-
signs come in a variety of shapes, con-
they’ve come, where they’re figurations, and sizes. They include sur-
From single loop, to quadrature pair, to face and phased array coils. Receive-
headed and the advantages they phased array, radio frequency (RF) im- only coils are effective because, with a
provide. aging coils have grown in variety and relatively simple design, are used, to-
technology, while applications for them gether with the (transmit) body coil, to
4 Coil-induced Artifacts have proliferated. The latest break- provide uniform excitation over the
Artifacts happen. Here’s why and through: coils dedicated exclusively for entire volume of interest. The downfall
high density imaging. These coils target of this approach is a higher whole-body
what you can do about them.
specific anatomy and make it possible SAR, leading to
to achieve the highest fewer slices and
6 Raise Your Coil Consciousness possible signal-to-noise susceptibility to
Nuggets of basic information, too ratio (SNR). Will the artifacts due to sig-
often forgotten. increase in coil nal from excited tissue
choices ever outside the volume of
7 Coil Reliability Improvements cease? Not as interest.
long as tech- Transmit/receive
The right coil with the right sys-
nology keeps coils transmit RF
tem at the right time. improving. then change to a
receive mode to
7 What’s New? Having a variety of imaging coils receive the MR
New coils, new applications. on site makes it possible for you to han- signal. This kind of coil reduces whole
dle a wide range of imaging situations. body SAR, allowing acquisition of more
Making the correct coil selection for a slices and significantly reducing arti-
10 Coil FAQs
given exam plays a critical role in deter- facts due to tissue outside the volume
You have questions, we have mining the quality of MR images you of interest. The coil design is more
answers. acquire. Your job is to find the right coil complex, however, and yields reduced
for the specific application you’re per- B1 field uniformity over the volume of
11 Useful Links forming. interest.
Good sites for more information
A coil’s geometry always plays an im- Surface Coils
about coils.
portant part in optimizing your results. It A surface coil is a receiver coil with a
must be large enough to fit around the definite area of sensitivity from which it
patient comfortably and obtain a satis- receives signal. Outside this area it re-
factory field-of-view (FOV). Yet the larger ceives minimal signal. This configura-
a coil is, the less sensitive it will be as a tion improves SNR because the signal,
receiver, hence SNR may be lower. The received from a smaller, specific area,
competes with less noise than it would
with a larger coil, such as the body coil.

The sensitivity area relates to the di-

ameter of the coil. By Increasing coil
size, the area of sensitivity also in-
creases. But as with most MR princi-
RF Coils Have Come a Long Way (continued from
ples, you pay for gain. Not only is the Signal Detection with Linear Coils elements must be uniform over the vol-
amount of MR signal increased, but the As Vector 1 becomes ume of interest, however, or the SNR
amount of noise the area receives in- aligned with the coil at benefit is reduced and uniformity may
creases as well. Coil engineers must 360° and 180°, the suffer. Volume coils, such as the 1-
strike a balance between the size of the signal is strong.
coil area and increased SNR. One ad- When vector ori- Quadrature Combiner
vantage of surface coils is their opti- entation changes
to the 90°/270°
mum sensitivity for tissue close to the
axis, signal from
coil. Image intensity fades, however, as Vector 2 is lost.
distance between the coil and the ROI
increases. Two opposed
Signal Detection with Surface Coils linear coils
The spins produce a net field vector that can be con-
rotates in a nected to-
plane around gether for
more uniform A quadrature combiner allows two quad-
the magnet’s
coverage of rature channel signals to be received by
Z-axis. Since
the volume, one receiver.
the vector
rotates, the but they are still only sensitive to one oscil-
signal is de- lating vector.
channel transmit/receive head coil, are
scribed as
“circularly
well suited for quadrature operation.
polarized”. Signal Detection with Quadrature Coils
Phased Array Coils
The net field vector can be resolved into The illustration on A better solution for quad-
two orthogonal (right angle) oscillating the left shows the rature signal reception
vectors. These can be oriented in any di- quadrature pair
connects each of the
rection in a with the two
signals not in
quadrature channels to a
plane orthogo- separate receiver. Called
nal to the mag- phase.
a quadrature phased ar-
net’s Z-axis as
long as the two
ray design, this arrange-
vectors remain ment, though not circularly polarized
orthogonal. has the same sensitivity as one that is.
When one vec-
tor is maximum, The illustration on the right Another way to create a quadrature
the other is minimum. shows the quadrature sig-
element pair adds an additional “figure
nal. Note that when one
vector is at maximum signal, eight” or “butterfly” element, sensitive
Linear Coils the other vector is at mini- only to the horizontal component of the
Linear coils are receive-only devices mum signal. field. This arrangement can be found in
sensitive to only one oscillating vector. flat coils such as those used to image
As the rotating vector aligns with the the spine.
coil, the signal peaks. When the mag- Signal Detection with Phased Array Coils
netization vector unaligns with the coil,
the signal is at its weakest. A coil oper-
ating in this way is said to be “linearly The simplest way around this uses a
polarized”. special quadrature combiner, which
combines the signals while taking the
Linear coils are now found mostly as phase difference between them into
components in phased array units such consideration. A coil operating in this
as CTL spine coils way is “circularly polarized”.

Quadrature Coils SNR can be improved with quadrature

Quadrature coils employ two pairs of coils by as much as 40% for volume
coils arranged about the signal source designs. Since these coils require only
to further improve performance. This one receiver, this technology is simple With Phased Array Coils, each quadrature
configuration presents a dilemma be- and relatively inexpensive. Quadrature channel is connected to a separate receiver
cause it delivers signals from two vec- coils are also less sensitive to artifacts channel.
tors of opposite phase to manage. when they are tilted to accommodate
the patient. Sensitivity of orthogonal Continued on page 3
2
RF Coils Have Come a Long Way (continued from page 2)
The design choice producing the best tors will need to be considered as coils with more and more channels are designed.
image quality brings the signal from On the other hand, reducing coil diameter and adding elements can deliver a dra-
each element in the quadrature pair out matic increase in SNR, the 8-channel brain phased array coil is one example of this.
to a separate receiver. When the num- More elements also allow for the design of coils optimized for ASSET imaging and
ber of receivers is limited, combining two can enhance the ASSET reduction factor. Furthermore, coil utility can be improved
quadrature elements into a single signal with the addition of elements without sacrificing SNR. For example, in imaging the
routed to one receiver provides an ac- peripheral vasculature or the spine, regions of interest can be more precisely de-
ceptable trade-off. fined to suit a patient’s size and the anatomy being imaged.
Signal Detection with Phased Array Coils
Clinical Impact of 8-Channel Coils
Eight-channel surface coils can help you improve productivity, a crucial considera-
tion in today’s competitive scanning environments. These devices can be optimized
for parallel imaging techniques, improved SNR, and can provide better image resolu-
tion. Parallel imaging techniques, like ASSET, reduce scan times, which can decrease
patient exam times. Reduced coil diameter together with the 8-channel phased
L array elements over a given volume increase SNR and thereby resolution.
Left illustration shows a linear coil only How does this impact your routine imaging? Let’s compare the 1-channel transmit/
sensitive to the vertical component of the
receive Head coil and the EXCITE 8-channel High Resolution Brain coil.
field. The illustration on the right shows an
addition of a butterfly element, providing 1-Channel Transmit/Receive Head 8-Channel High Resolution Brain Coil
sensitivity only to the horizontal component Coil
24 cm diameter— increases SNR
28 cm diameter
Twelve Quadrature Elements to 8 Receivers 30 cm S/I coverage – increases SNR
The EXCITE HD 8-channel CTL spine ar- 38 cm S/I coverage
Brain and TMJ
ray coil features twelve elements lead- Brain, Neck, and TMJ
ing to 8 receivers. Receiver switching is ASSET x2 optimized – reduces scan time
accomplished within the coil interface
multiplexer.

Phased array technology takes advan-

tage of multi-channel imaging, which
produces increased SNR over linear and
quadrature coils. A phased array coil
achieves the sensitivity of a small sur-
face coil over a larger FOV. The develop-
ment of phased array coils has im-
proved MR image quality and expanded
applications for MR imaging.
Routine T1-Weighted Imaging SNR Comparison
The image on the left was acquired with the 1-channel transmit/receive head coil and the
right image was acquired with the 8-channel high resolution brain coil. Note the higher SNR
on the image acquired with the 8-channel coil.

The 8-channel brain coil’s decreased diameter provides increased SNR. The coil is
intended only for brain imaging, and you can use it with ASSET to help optimize your
scan time.

Using National Electric Manufacturers Association (NEMA) methodology, SNR meas-

Are more channels always better? urements over different diameters of acquired images with both coils were recorded.
More channels equal better imaging, At 10 cm, a 40 to 45% SNR increase of the 8-channel brain coil versus the 1-channel
right? That depends. As the number of head coil was measured. At 20 cm, that percentage rose to 80 to 85%. On average,
channels increases, the elements be- the 8-channel brain coil has 40% more SNR than the 1-channel head coil, which de-
come smaller and penetration may be livers higher image quality for an improved diagnosis.
compromised. Additionally, SNR may
increase near the surface of the coil, but
not deep within the patient. Such fac-
Continued on page 4
3
RF Coils Have Come a Long Way (continued from page 3)

Coil-induced Artifacts
At the edge of an image, comparisons of NEMA SNR measure-
ments showed that the 8-channel brain coil has 90 to 95%
more SNR. This is useful for applications such as fMRI for mo-
tor or visual strip mapping.
It is important for you to recognize, understand, and manipu-
late the coil to reduce or avoid coil-induced artifacts that
may appear in your images. The improper use of coils …
wrong coil configuration, improper patient positioning, poor
FOV selection … is a major cause of image artifacts. Appropri-
ate coil selection along with an awareness of how these arti-
facts happen can help you eliminate them.

Shading
Shading artifacts, displaying as areas of reduced signal inten-
fMRI Imaging SNR Comparison sity or bands of signal cancellation, often result from im-
The fMRI images above were acquired with the finger-tapping mo- proper coil or patient
tor paradigm. The EXCITE 8-channel coil image (right) shows a positioning. These arti-
noticeable increase in SNR over the 1-channel head coil image facts can be easily cor-
(left), resulting in a more comprehensive demonstration of neu- rected by repositioning
ronal activation. the coil and/or the pa-
tient so that the coil re-
The new 8-channel coils for 1.5T include a breast and knee ceives signal that best
coil as well as a 16-channel, 32 element, lower leg array. represents the patient.
These new coils expand GE’s current offering, which include a The anatomy of interest
brain coil, neurovascular coil, 8-channel torso array, cardiac, should be placed in the
and a CTL spine array. 3.0T users can also take advantage of center of the magnetic
8-channel coils with the 3.0T brain, neurovascular, and car- field, within the center of Axial abdomen image with bands of
the coil, and within the signal cancellation (shading) due to
diac coils. incorrect patient positioning.
group of sections to be
Coil Selection Tips acquired.
GE offers a wide variety of coils from which to select for the
variety of imaging exams you may encounter. As long as you Inhomogeneous bright spots or a drop in coil signal on the
take the time to use your expertise and your imagination, image can also be prevented by making sure the patient
you’ll find a coil for every imaging application. During your does not come in direct contact with the coil. If the patient is
coil selection, remember these basic rules of thumb: likely to come in contact with the coil, it is recommended that
• Match the coil to the anatomy or area you’re going to you place a pad or folded sheet between the patient and the
image – this will let you optimize the SNR for the desired coil.
scan time.
• Match the FOV to the size of the coil or number of coil Tips for coil positioning:
elements selected • Choose the coil most appropriate for the corresponding
>If the receiver coil is larger than the FOV, signal from anatomy of interest and required FOV.
tissue outside the FOV can be aliased into the FOV. • Landmark on the coil marker, not on the patient’s anat-
Workaround – use No Phase Wrap to oversample in the omy. The landmark line(s) on the coil indicate the center
phase direction. Although this removes the aliased sig- of the coil or each coil configuration. Imaging coils will
nal, the overall SNR will not improve. function most accurately when placed at the magnet’s
>Smaller coils reduce the area of coverage, yet increase isocenter.
the inherent SNR of images and therefore fewer signal • If the coil has multiple configurations, select the appro-
averages are needed. priate number of elements according to the area that
>Individual patient anatomy may sometimes make it needs to be covered.
difficult to use the appropriate coil. It may be necessary
to use an alternative coil (such as a Flexcoil) to get the
• If the coil has multiple configurations, center the coil ele-
ments corresponding to the coil configuration chosen
best image possible. While image quality may suffer,
over the region of interest.
the alternative is no image at all.
>Always read the manual for the particular coil configu-
ration you’re using. And always be aware of and follow
safe MR scanning procedures. Ω
Continued on page 5
4
Coil-induced Artifacts (continued from page 4)
• Select a coil configuration and FOV based on the number RF Inhomogeneity
of elements chosen. Failure of an RF coil can cause intensity variation across an
image. This often indicates the failure of a coil element or the
• Consult individual coil manuals for FOV coverage and be presence of ferromagnetic material in the imaged object.
conscious of the coil’s limitations when selecting FOV. Check with the patient to make sure that nothing on them or
in them is causing the artifact. If you suspect a faulty coil ele-
Non-Uniformity of Signal ment, check each element using manual prescan. If you iso-
The RF receiver detects signals closest to it most efficiently. late a faulty element discontinue using the coil and consult
This characteristic may cause a non-uniformity of signal in with your service engineer.
the image. The effect is more pronounced with surface coils
than with volume coils, appearing as localized bright areas
close to the coil. Signal variability may also result in incom-
plete fat suppression when chemical fat suppression tech-
niques are used. To minimize the chance of this happening,
try a different coil or use a STIR sequence rather than trying
additional fat saturation techniques.

Coil intensity correction techniques can also be applied to

correct the non-uniformities in signal. Phased array UnifoRm-
ity Enhancement (PURE) and Surface Coil Intensity Correction
(SCIC) are two techniques designed to minimize surface coil
Defective Coil Element Metal on Patient
intensity variations. PURE or SCIC can be used with compati- Note the loss of signal intensity Axial shoulder image displaying
ble surface coils. PURE can also be used with the 8-channel in the lower right corner of the a drop in signal intensity caused
transmit/receive high resolution knee coil by MRI Devices. axial abdominal image due to a by a ferromagnetic object on the
failure in a coil element. patient.

Peripheral Signal Artifacts

Peripheral signal artifacts appear as either bright spots or as
Uncorrected ribbons of signal smeared through the image. We sometimes
refer to these artifacts as Star Artifacts (bright spot) or Anne-
facts (ribbon). Basically, both have the same root cause: Sig-
nals are generated outside the desired FOV and the receiver
is able to detect them.
FSE sagittal thoracic spine im-
age presenting a Star artifact.
The image was acquired with
CTLMID, 38 cm FOV, and the
phase and frequency swapped.
Corrected with SCIC The artifact could have been
prevented by not swapping
phase and frequency and using
a 3-coil selection, such as
USCTS234 so the surface coil
coverage would closer match
the scan FOV.

FSE sagittal cervical spine with

annefact. The image was ac-
quired with CTLOP, 24 cm FOV,
Corrected with PURE and the phase and frequency
swapped. The artifact could
have been prevented by not
swapping phase and frequency
and using a 2-coil selection,
such as CS12 so the surface coil
coverage would closer match
the scan FOV.

Continued on page 6
5
Coil-induced Artifacts (continued from page 5)
Star artifact signals, which appear as a bright star close to the If you operate a TwinSpeed system, you should be aware of a
middle of the image, originate very far from isocenter. In that zipper artifact that can occur while using Zoom mode. Most of
non-linear region, the free induction decay (FID) signal com- these artifacts are caused by the ineffective dephasing of the
ing off the RF 180 pulse or from a SAT pulse is not crushed out magnetization outside the effective length of the gradient coil.
and aliases back into the image. Although the prescribed FOV and its orientation are well con-
trolled in the Zoom mode, the spatial saturation (particularly in
Annefact appears in Fast Spin Echo (FSE) scans as smeared, the A, P, R, and L directions) cannot be controlled and may
bright, ghosting signals through the image in the phase direc- cause zipper artifacts. Other saturation techniques such as fat
tion. It typically appears on sagittal spines or pelvis scans saturation and magnetization transfer (MT) can also produce
using a phased array surface coil. Like a Star artifact, its ori- this type of artifact. By selecting the correct receive coil and/or
gin is far from isocenter, where the gradients are non-linear. placing the spatial saturation band carefully, you can eliminate
Uncompensated eddy currents in this area cause phase er- or minimize the zipper artifact. Ω
rors in the compressed signal and smear it through the im-
age.

By selecting the receive coils that match the imaging FOV (i.e.,
Raise Your
LS45, LS56, CS12, etc.), you can lessen the likelihood of picking
up the peripheral signals that are generated outside the FOV. Coil Consciousness
Coil Malfunctions RF energy from scanning can cause localized warming at con-
Coil decoupling mecha- tact points between the patient/bore and patient/RF coil, caus-
nisms are circuits activated ing discomfort, tingling sensations, or skin irritations similar to
by diodes to prevent radio- sunburn. RF can heat non-compatible surface coils, damaged
frequency currents from surface coils, surface coils that are not properly plugged in, and
flowing in the receive-only improperly routed coils, which can result in patient heating. If
coil during transmission you are operating a scanner and your patient tells you he or
from the body coil. This she is experiencing a burning sensation, stop the scan.
results in local distortion of
the transmit field and sig- People are always talking about not closing the loop on quality,
nal intensity variations but when it comes to MR safety, there are loops you will want
within the image (right). If to keep open to prevent patient discomfort. To help prevent a
you suspect a coil malfunc- patient burn from closed loops formed by clasped hands, hands
tion, consult your service touching the body, from thighs touching, or from the patient’s
engineer and discontinue breasts contacting the chest wall over a small area, insert non-
use of the coil. conducting pads at least 0.25 inches thick between touching
parts.
Zipper Artifact with Zoom Mode on TwinSpeed Systems
Patient positioning and coil awareness can affect the safety of
the scan procedure. The following safety precautions should
always be taken.
• Position the patient properly.
• Use the supplied coil pads with the coil at all times. The coil
should never come into contact with the patient.
• Do not allow the patient to directly contact the surface of
the bore; use non-conductive padding.
• Never let the coil’s RF cables come into contact with the pa-
tient. Position cables under a cushion whenever possible.
• Do not loop or cross cables. Keep them straight, positioned
The left image displays the axial c-spine prescription with an angled
anterior SAT pulse. The 3-coil CTL (CS123) was used in the Zoom
down the center of the magnet, directly out the bore.
mode to acquire the right axial image showing the zipper artifact. • Use only approved, undamaged RF coils.
This was caused by signal being received from tissue excited by the • Inspect coils for damage and wear. Do not use a coil that is
anterior saturation pulse outside the FOV that wrapped into the
not functioning properly, e.g., tuning problems or intermit-
imaging volume in the phase direction. This artifact could be elimi-
nated by switching to Whole mode or by using the 2-coil CTL (CS12). tent poor quality images. Ω

6
Coil
Reliability Improvements
With the introduction of Signa EXCITE HD, several improve- 8- or 16-Channel EXCITE HD System (left) and 4-Channel or Up-
ments in coil reliability have been made. These additions in- graded 8-Channel EXCITE HD System (right)
clude Coil Identification (ID), Automatic Identification, and The left port is for transmit/receive coils introduced in EXCITE HD or
changes to surface coil connections. later. The left and right ports are for 8-channel receive-only coils.
16-channel coils must be plugged into both the left and right ports.
ID Check All coils plugged into the outer ports must have a Coil ID chip in-
Coil ID confirms that the coil you have plugged into the system stalled. The middle port is a legacy port for 1.5T systems. The up-
matches the coil se- per slot is for legacy transmit/receive coils and receive-only single
channel surface coils. All coils plugged into the upper middle slot
lected in the scan
must have a Coil ID chip. The bottom slot is for phased array coils
prescription, also as- and may or may not have a Coil ID. A 3.0T system has only a single
suring that the coil is slot for the middle port that can be used for head or phased array
properly seated in the coils with Coil ID chips installed. Ω
port.

Coil indicator lights

next to each coil port
What’s New?
let you know when
your coil connection The EXCITE HD architecture presents a real breakthrough for
is secure. When a coil musculoskeletal MR imaging with the 1.5T EXCITE HD 8-
is connected to any of the three ports, both light-emitting di- Channel Knee Array coil. With its unmatched SNR, its ability to
odes (LEDs) illuminate, then one will stay on. provide the most uniform fat saturation, and its ASSET and
PURE compatibilities, this coil outperforms both the Quadra-
• If the red light stays on, the coil is faulty or there is some ture Extremity and the 4-channel Phased Array Knee coils.
other problem. Check the message area and follow the
directions to correct the problem. Scanning is not allowed. SNR
• If the green light stays on, the first level of Coil ID related The Quadrature Extremity coil
checks have passed and further checks will be performed has limited SNR for applica-
during scan prescription. Even though the green light is on, tions such as cartilage os-
you may still be prevented from scanning. The correct coil teoarthritis with T2 mapping
must be selected from the Scan Rx Desktop in order to suc- (a work-in-progress) and ul-
cessfully begin scanning. tra-high resolution of the car-
tilage structure. The 4-
Automatic Identification channel Phased Array Knee
Automatic coil detection activates if the coil plugged in has a coil provides a slight improvement in SNR performance.
Coil ID chip. When a coil is automatically detected, it appears
in the Coil Names window. You must still select the correct coil
configuration before downloading the acquisition, except if a
receive-only coil is plugged into either port. In this case you
can scan with the Body coil.

Surface Coil Connections

Coils are plugged into the coil port carriage. Your system’s car-
riage port will have one of two configurations, depending on if
you have an 8- or 16-channel Signa EXCITE HD system or if
you have a 4-channel or upgraded 8-channel Signa EXCITE HD Phased Array Knee and Quadrature Extremity Coils - SNR Comparison
system. The image on the left was acquired with the 4-channel Phased Array
Knee coil and the right image was acquired under the same condi-
For Signa EXCITE 3.0T systems, use only Signa EXCITE 3.0T coils. tions with the Quadrature Extremity coil. Note the improved SNR
These coils are labeled “3T”. If you plug a 3.0T coil into a 1.5T with the Phased Array Knee coil.
system, you will not be able to scan.

Continued on page 8
7
What’s New? (continued from page 7)
The new EXCITE HD Knee coil delivers a significant SNR gain
over existing 4-channel designs. Because of its 8-channel
design, each RF element in this coil is smaller. While it gathers
the same amount of signal, the smaller phased array ele-
ments significantly reduce the amount of noise collected.
The EXCITE HD Knee coil provides up to 100% more SNR than
the single channel Quadrature Extremity coil.
Quadrature Bird Cage Phased Array Elements
Some of the SNR improvement results from making the coil The bird cage component of the The eight phased array elements
smaller, yet it still accommodates a large patient body habi- coil is used for RF transmission are used during data acquisition
and signal reception for Auto- for signal reception.
tus. The internal diameter of the coil tapers to the knee. The
shim and PURE calibration.
head end features a flared contour for the thigh, while the
foot end has a flared contour for the calf. The coil is designed
to fit 95% of the patient population. PURE
The increasing number of smaller elements in phased array
coils leads to more signal attenuation at depth and therefore
more signal-to-noise variation across the imaging volume.
The EXCITE HD Knee coil, with eight RF elements, has 32%
more signal attenuation from the periphery to the center of a
volume. To avoid seeing a hypersensitive signal around the
knee, you must correct the images with PURE. PURE applies a
correction based on the coil’s sensitivity profile and does not
change the SNR or contrast in the image.
EXCITE HD Knee and Phased Array Knee Coils – SNR Comparison
The image on the left was acquired with the 8-channel EXCITE HD
Knee coil and the right image was acquired under the same condi-
tions with the 4-channel Phased Array Knee coil. Note the significant
SNR increase with the EXCITE HD Knee coil.
SNR Improvements with the
EXCITE HD Knee Array Coil
This sagittal FSE image was ac-
quired with the EXCITE HD Knee
Array coil in just 4 minutes. Imag-
ing parameters include: 24 slices,
10 cm FOV, 3.5 slice thickness, and
chemical fat saturation. The image Uncorrected Image Image Corrected with PURE
was corrected with PURE. Sagittal T1-weighted image of Same sagittal T1-weighted im-
the knee with no correction dis- age corrected with PURE. Note
plays increased signal intensity the decrease in signal variation
Hybrid Technology at the periphery of the knee. across the image.
The EXCITE HD Knee coil is a transmit/receive coil incorporat-
ing unique hybrid technology that allows eight phased array ASSET
elements and a separate quadrature bird cage to function The EXCITE HD Knee Array coil is optimized for ASSET imaging
together in one coil. The bird cage coil is used for RF power in the A/P and R/L directions. ASSET scans use the same cali-
transmission and for signal reception during prescan and bration scan, collected with the bird cage coil, as PURE. ASSET
PURE calibration sequences. The bird cage has a twisted de- may be applied to decrease scan time in examinations where
sign for a more uniform RF deposition within the excitation of short scan times are crucial … with claustrophobic patients,
the volume, thus providing signal uniformity across the whole for example.
imaging volume. The eight phased array elements, tapered to
the knee anatomy for optimum SNR performance, receive the Chemical Fat Saturation
MR signals. The EXCITE HD Knee Array’s hybrid technology, which uses
the bird cage coil to collect the autoshim data in the transmit/
With the transmit/receive coil design and hybrid technology, receive mode, means this coil provides unmatched chemical
you no longer have to worry about aliasing from the opposite fat saturation. You’ll see uniform signal within the field and
leg as you would with conventional phased array receive- uniform fat suppression across the entire imaging volume.
only knee coils. Just make your patient comfortable and con-
centrate on the leg you’re scanning!

Continued on page 9
8
What’s New? (continued from page 8)
8-Channel CTL Spine Array Impacts 3.0T Users
The new EXCITE HD 8-Channel CTL Spine Array coil sets a
new standard for full spine clinical 3.0T imaging. Its quadra-
ture phased array design
includes 12 elements that
provide excellent SNR and
uniform coverage resulting
in high-resolution images of
the spine from the cervical
through the lumbar spine.

ASSET Image Comparison The cervical region is designed for imaging the cervical spine
The axial FSE knee image on the left was acquired with ASSET x2 in and neck, and for neck MRA applications.
44 seconds. The right axial FSE knee image was acquired under The coil covers extended regions of interest including the
same conditions without ASSET in 1 minute, 26 seconds. Both im-
thoracic and lumbar anatomical areas. It accommodates
ages are uncorrected and display matching window widths and
window levels as determined by the ROI.
the full range of patient sizes, including the taller patient
population.

Chemical Fat Saturation The use of SCIC is recommended with this coil to reduce sur-
Fat suppression Improve- face coil intensity variations. Protocols for the 8-channel CTL
ments with the EXCITE Spine coil can be found in the GE Protocol Menu on your sys-
HD Knee Array Coil Sagit-
tem. These protocols are routine clinical protocols developed
tal Fast Spin Echo im-
ages acquired with Fat in collaboration with clinical test sites.
SAT. Note homogene-
ous fat suppression Other New Coils
across entire volume of
images. These images
were corrected with
PURE calibration.

Protocol Adjustments T1 FLAIR Sagittal Cervical T2 FRFSE Sagittal Thoracic

The higher SNR in the EXCITE HD Knee Array coil enables you Spine Spine
to make changes in your protocol strategy. To take advan-
tage of the additional SNR and increase spatial resolution,
you may want to reduce the FOV somewhat and slightly in-
crease the frequency matrix size and receive bandwidth.

For example, if you previously used the Quadrature Knee Coil

by Medical Advances, you may adjust an FSE sequence in the
following way. Instead of a 14 FOV, lower the FOV to 12. If
the matrix was 256x256, raise the frequency matrix to 320.
By not adjusting the phase matrix, you’ll be able to keep the
same scan time. And, since you have more signal, you can
raise the receive bandwidth a little (from 16 to 21), while T2 FSE Lumbar Spine T2 FRFSE Lumbar Spine
maintaining the same echo spacing. Try similar adjustments
with your existing knee protocols to find a new protocol that
meets your imaging needs with the 8-channel EXCITE HD
Knee Array coil.

Continued on page 10
9
 What’s New? (continued from page 9)
In addition to the new 1.5T coils, HD Breast Array and HD
Lower Leg Array, which were highlighted in the last issue of MR
Field Notes, GE Healthcare has several new imaging coils de-
Coil FAQs
signed for 3.0T systems.
Q: We have a 9.0 system and are upgrading to EXCITE HD.
The 3.0T HD Shoulder Array coil is new tech- Will we still be able to use all of our coils?
nology for 3.0T EXCITE HD users, providing A: You’ll still be able to use the same coils, however, they will
them with an additional orthopedic imag- need to be updated. Connectors in the HD system are differ-
ing application. The 3-channel Shoulder ent and the connectors on the coils you’re using now will
Array is a receive-only coil. The coil de- have to be modified to fit.
sign enables you to obtain high resolu-
tion images of the shoulder and con- Q: When can I use the body coil for localizing with another
tiguous anatomy. The coil is optimized for ASSET appli- coil plugged in?
cations in the R-L and A-P directions. A: As long as the system allows you to scan, go ahead and
use the body coil for localizing. Make sure, however, that the
The 3.0T 8-Channel HD Cardiac Array is coil you’ll use for scanning is plugged in and its ID has been
a high-resolution coil used to image selected when you do the localizer scan.
the cardiovascular system on GE
Signa EXCITE HD 3.0T systems. It is Q: My imaging facility has a 1.5T EXCITE II system and is
capable of ASSET scans to reduce the interested in purchasing the new EXCITE HD Knee Array
required breath-hold time, which im- coil. Is this coil compatible with our system?
proves patient comfort and helps pro- A: No, the EXCITE HD Knee Array coil is only compatible with
duce more diagnostically useful images. 1.5T EXCITE HD systems due to the system architecture. The
EXCITE HD Knee Array coil has a unique transmit and receive
The 3.0T 8-channel HD Neurovascular Array is a receive-only design that prevents aliasing from outside the knee anatomy.
coil designed to give optimum SNR and uniform coverage of It incorporates a hybrid technology that uses a separate bird-
the head and neck. This coil has an excellent ergonomic de- cage coil for transmission and a set of phased array elements
sign, which incorporates soft, flexible components that con- for reception. The system architecture in previous systems
form to the patient anatomy to accommodate various body does not allow use of the body coil when a transmit/receive
contours while improving patient comfort. The coil geometry is coil is inside the bore. The hardware system was modified
optimized for ASSET applications in R-L and A-P directions. with the EXCITE HD systems to accommodate this technology
and allow coils to operate in this way.
Medrad is introducing a new 1.5T 8-Channel Neurovascular
(NV) coil that incorporates 8 elements in the head section and Q: When using the Medrad NV8 coil with PROPELLER DWI,
4 elements in the retractable chest section designed for opti- the sequence did not reduce motion. Is this a coil problem
mal neurovascular imaging. The full mode provides a maxi- or a PSD problem?
mum 46 cm FOV and allows you to acquire high SNR images of A: Neither. PROPELLER DWI is not intended to nullify motion.
the carotid arteries, soft tissue neck, cervical spine, brachial The PROPELLER DWI sequence is used to reduce susceptibility
plexus, and aortic arch. The Head mode activates 8 receivers artifacts from metal. Use the PROPELLER T2 sequence or
and 8 elements in the head configu- PROPELLER T2 FLAIR to reduce patient motion artifacts.
ration to image the brain and Circle
of Willis, acquire EPI images, and Q: The Body Array coil has a Coil ID sticker and I have a
use the PROPELLER imaging tech- green light on the coil port but I receive an error when I
nique for reduced motion artifact download the acquisition, what is preventing me from
and enhanced contrast-to-noise downloading?
properties. A: A coil with Coil ID is automatically detected and shown in
the Coil Names window, but you must still select the correct
The cervical spine and anterior coil elements can be activated coil. The system will not allow you to click the Download but-
with the Neck mode. The Neck mode can acquire images of ton unless the coil plugged in matches the coil selected in the
the soft tissue in the neck and cervical spine up to a 24 FOV. Coil Names window. An error message posts if there is no
The Full, Head, and Neck modes are designed for ASSET imag- match. Change the coil selection in the protocol to match the
ing, but they are not compatible with spectroscopy. There is a actual coil that’s plugged in. The exception to this situation is
special Spectroscopy mode that activates the head section for if a receive-only coil is plugged into either port. In this case
both single and multi-voxel spectroscopy imaging. you can scan with the Body coil. Ω
The coil also features a Head/Neck configuration for extended
superior/inferior coverage. Ω

10
Useful Links
The University of British Columbia has web pages that include MR images with artifacts and expla-
nations. You can also view small images of these artifacts with no explanations in order to quiz
yourself on possible causes.
http://www.rad.pulmonary.ubc.ca/stpaulsstuff/MRartifacts.html

Frank Shellock, PhD, is a physiologist with more than 17 years of experience conducting
laboratory and clinical investigations in the field of magnetic resonance imaging. As a
commitment to the field of MRI safety, bio effects, and patient management, he cre-
ated and maintains a great MR safety web site. www.MRIsafety.com

USA Instruments, Inc. (USAI) engineers, manufactures, and markets RF coils

for MRI, providing technically superior products with higher quality and
reliability at competitive prices. Check out their website to find cur-
rent products and work-in-progress coils.
www.usainstruments.com

Invivo was formed by Intermagnetics General Cor-

poration Medical Advances and MRI Devices. You
can access MRI Devices Corp. and Medical
Advances, Inc. separate websites from their
home page. Invivo leads the markets for
MRI devices and flexible patient moni-
toring by maintaining a competitive
edge through research and tech-
nology.
http://
www.invivoresearch.com/
index.html

MEDRAD offers a wide

range of MR products,
including MR compati-
ble injection systems,
surface coils, patient
monitors, sound sys-
tems, syringes, and
disposables. Check
out their site to view
their latest products.
www.medrad.com Ω

11
©2005 General Electric Company—All rights reserved.

General Electric reserves the right to make changes in

specifications and features shown herein, or discontinue
the product described at any time without notice or
obligation. Contact your GE Representative for the most

For more than 100 years, scientists and industry

leaders have relied on General Electric for
technology services and productivity solutions.
So no matter what challenges your healthcare
system faces — you can always count on GE to help
you deliver the highest quality services and support.
For details, please contact your GE Healthcare
representative today.

GE Healthcare
Waukesha, Wisconsin
U.S.A.

imagination at work