2 surface coil imaging

8/12/2019 2 Surface Coil Imaging

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Authors: Stoller, David W.

Title: M a g n e t i c R e s o n a n c e I m a g i n g i n O r t h o p a e d i c s a n d S p o r t s

M e d i c i n e , 3 r d E d i t i o n

Copyright ©2007 Lippincott Williams & Wilkins

Chapter 2

Surface Coil Imaging

Tom Schubert

Certain nuclei in the human body have a property called magnetic “spin.”This magnetic spin occurs at a specific radiofrequency called the Larmor

frequency . Several of these nuclei are found in the body in enoughabundance to generate an image. 1 The element of interest in most MRapplications is hydrogen. Hydrogen is bound in water and in fat (inhydrocarbon chains), and there is an abundance of fat and water, andtherefore hydrogen, in the human body. In fact, the human body is madeup of 63% hydrogen, making hydrogen an ideal element for imaging. Thebiological abundance of hydrogen is 0.63. 1 Other elements with themagnetic spin property are much less abundant in the body and are thusmuch more difficult to detect and image. The biological abundance ofsodium, for example, is 0.00041. 1

The Larmor frequency of hydrogen is about 42.58 MHz per Tesla.Therefore, when hydrogen is immersed in a magnetic field of 1 Tesla(1T), it resonates and precesses at a frequency of about 42.58 MHz. At1.5 Tesla (1.5T), the precession frequency is approximately 64 MHz;naturally, at 3.0 Tesla (3T), the precession frequency is approximately128 MHz.

Design and Definition of Radiofrequency Coils

MR systems work by depositing radiofrequency (RF) en ergy into thepatient, usually using the MR system body coil. When a tiny portion ofthat RF energy is released from magnetic spins of nuclei, it can bedetected by special radiofrequency antennas called coils. Coils are oftenreferred to as RF coils, surface coils, RF antennas, receiver coils, or array

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coils. The term “RF coils” is used in this chapter.

An RF coil is an electrical circuit. To make the coil very sensitive, it isdesigned to resonate at the frequency of in terest, much like a tuningfork is very sensitive to sounds or vibrations at a certain frequency. Inelectrical terms, a resonant circuit is an RLC circuit —that is, the

resonant frequency of the coil is determined primarily by the resistance(R), the inductance (L), and the capacitance (C) of the circuit elements.By adjusting these three variables, RF coils can be “tuned” and “matched” to 64 MHz. The tune and match are electrical measures of how well an RFcoil is designed to pick up signals at the frequency of interest and deliverthem to a receiver.

When an RF antenna is brought near a patient, the antenna becomes “l oa ded ” by th e pati en t. Larg er pat ients loa d the ant en na mor e heavilyand smaller patients load the antenna less heavily. For optimal signal-to-noise (SNR) performance, the RF antenna must be impedance-matched tothe patient load. In addition, the patient load slightly affects the resonantfrequency of the antenna. A coil must be properly tuned and impedance-matched to a specific patient load to achieve the best possibleperformance. Although early MR systems called for tuning andimpedance-matching the coil on a per-patient basis, this resulted in agreat deal of complexity in both the RF coil and the MR system, with aloss of reliability. Today, therefore, most coils are “fixed tuned”—in other

words, the resonant frequency of each antenna and the impedance-matching to the patient load is set by the electronic circuit elements anddoes not change. The result is a slight loss in performance when imaginga patient whose body habitus differs from the patient load for which thecoil was optimized.

MR system manufacturers are seeking solutions to the complexities of coilarray design outlined in the following discussion. Advances in the nextfew years are likely to include conversion of RF signals at the coilantennas to light signals transmitted to the MR system via optical fibers.It is also possible that the RF signals will be digitized prior totransmission. This will obviate many of the cable issues and decrease theimportance of many of the coupling issues. Another alternative underinvestigation is wireless transmission of the RF signals.

D e s i g n o f R F C o i l s





To better understand the technical challenges of coil design, a briefreview of the components of RF coils is presented. The basic componentsfor an RF coil are:

Resistance

Inductance Capacitance

Therefore, a simple RF coil could conceivably consist of a coat hangerwith a single capacitor between the ends. The wire in the coat hanger hasresistance, the loop of the coat hanger has inductance, and by adding acapacitor an MR image could be made with it. However, RF coils today aremuch more sophisticated.

Decoupling The MR system body coil deposits a large amount of RF energy into thepatient. Since the coil receiving the signal from tissues must be aresonant circuit, it also detects the RF energy from the MR system bodycoil. This energy is many thousands of times stronger than the RF energyemitted by the tissue. If the RF coil is not “turned off” during the bodycoil transmit cycle, it will absorb a large amount of energy from the MRsystem body coil, which can destroy the electrical components of the RF

coil; generate tremendous heat, leading to smoke or fire; or generatehigh electric fields, which can cause an electric field burn in the patient.

Turning the RF coil off is referred to as decoupling . 2 The coil is, in effect,decoupled from the transmit field generated by the MR system body coil.This is done using active electronic components known as diodes . The MRsystem provides a control signal (a voltage or a current) to each RF coilthrough the coil cable. This control signal is used to drive the diodes intoa conducting or nonconducting state. This engages, or disengages, smallcircuits on the RF coil, which shift the resonant frequency of the RF coilaway from the operating frequency of the MR system. This has the effectof minimizing the RF currents on the coil, thereby effectively turning theRF coil off. This is known as active decoupling . 3

It is so important to turn the RF coil off during the transmit cycle of theMR body coil that at least two, sometimes three, redundant methods areemployed. Of course, if the MR system operator fails to plug the RF coilcable in, or the coil plug becomes loose, the electrical signals provided by





the MR system will not reach the RF coil. In that case the coil will not beactively decoupled. There are often diode networks in the RF coil thatturn on passively, without the need for voltage supplied by the MRsystem. These networks extract a small amount of energy from thetransmit field generated by the MR system body coil to engage the diodes

to turn the coil off. This is known as pa ss ive dec oupling .

Diode Technology

Diode technology is a somewhat proprietary art in the RF coil industry.Most diodes used in RF coils today were developed for other electronicapplications, although some RF coils have diodes customized for RF coilapplications. In the future, diodes or other electronic switches maybecome available that are more suitable and robust for RF coilapplications.

One of the major weaknesses of diodes is that they can fail. If a diodefails in the shorted condition, the RF coil is permanently decoupled. TheRF coil will not “see” the MR system body coil, nor will it see the RFsignals emitted from the patient. However, if the MR system body coilcontinues to operate, enough energy may continue to go into the RF coilto melt the diode. In that case, the diode becomes open, and the RF coilbecomes resonant again. For this reason, at least one MR manufacturer

requires each antenna element in an RF coil to have a fuse. If the diodesfail, the fuse melts, permanently turning the RF coil off. This is theultimate fail-safe feature to prevent patient burns. However, fuses bydefinition tend to have a measurable electrical resistance, and thisresistance results in a slight reduction of the performance of the RF coil.

Some MR systems periodically test to see if there is a voltage drop acrossthe diodes, thus checking to see if the diodes are intact. This diode checkmay occur before a scan starts, or even during a scan. This check alsoserves to determine whether an RF coil is plugged in to the MR system.

It is important to understand that coils cannot be turned off completely.The degree of “offness” is usually measured in dB. If an RF coil is notturned off very well, certain image artifacts can result. In particular, notdecoupling a coil well can lead to a distortion in the B 1 transmit field of

the body coil. This can result in a failure of “Fat Sat,” the chemicalsaturation technique inherent in certain pulse sequences.





Coil Cables

A cable connects the RF coil to the MR system. Part of this cable also lieswithin the MR system body coil and is subject to absorption of RF energyfrom the MR system body coil. This can lead to high RF currents orstanding waves occurring along the coil cable, potentially resulting in fireor patient burns. Various techniques are used to suppress standing waveson the coil cable; these are referred to in the coil industry as cable traps or baluns . 4 These devices are most prevalent on 3T RF coils. They usuallycan be seen as plastic barrels in the RF coil cable.

Other Aspects of RF Coil Design

F e r r o u s M e t a l s

RF coils cannot contain ferrous materials (such as iron, nickel, andcobalt) since these materials will distort the B 0 magnetic field of the MR

magnet and result in a hole in the MR image near the coil, commonlyknown as a metal artifact . Occasionally, however, electronic componentscontaining ferrous materials have been unintentionally used in themanufacture of RF coils. Nickel, for example, is a common plating usedfor electronic components, and nickel plating has sometimes been foundto have been applied as a diffusion barrier under the gold or silver plating

specified. This ferrous content can be very difficult to detect, except ofcourse in an MR system.

A n t e n n a s a n d C o i l H o u s i n g

The RF antennas in a coil are generally encased in a coil housing made ofplastic. Since plastic is made of hydrocarbons and contains hydrogen, theplastic chosen for the coil housing must have very little “proton signal”—that is, the number of free hydrogen molecules in the plastic must be sosmall that the coil housing is not visible in the MR image. Each MR

system manufacturer has its own test for proton signal in coil housings.As pulse sequences with short TEs, which can make the proton signalcoming from plastic more apparent, become more popular, this issue isbecoming increasingly important.

In addition to having as little proton signal as possible, the plastic coilhousing must be very robust so it does not crack during use or even ifdropped on the floor. In addition, the plastic must be highly flame-





retardant and must be rated by one of the materials rating laboratories.To achieve the desired flammability rating, the plastic must be of acertain minimum thickness when molded into a complex shape.

The plastic material must also be an excellent insulator. A high electricalpotential test must be passed, and the coil housing must hold off 5,000

volts DC. The seams in the coil housing must also be designed to achievea minimum “creepage distance.” This distance must be long enough sothat the air path through the seam from the patient to the electronics inthe coil provides a minimum amount of dielectric insulation.

The seams in the coil housing must also be designed so that the coil willpass a fluid ingress test. During this test, the housing is sprinkled withwater while several thousand volts are applied to the circuitry inside thecoil housing. If water leaks into the RF coil housing, there will be adischarge of electricity from inside the coil to the outside. The purpose ofthis test is to ensure that the coil housing design prevents electric shockto the patient in the event the patient bleeds, urinates, or regurgitatesduring the scan.

It is important that no part of the coil that comes in contact with thepatient rises by more than a few degrees in temperature during extendeduse. This means that components that dissipate significant energy,especially decoupling circuits, must be carefully designed and placed inlocations that will not significantly raise the temperature of the coil

housing in patient contact areas. Image artifacts may arise from rapid switching of MR system gradientcoils. This typically happens during sequences known as echo planarimaging (EPI). The rapid switching induces current flow in the copper ofthe antenna circuit in the RF coil. Any conductor in which a current isflowing has its own magnetic field. This magnetic field distorts the B 0

magnetic field of the MR system magnet, resulting in what is known as aneddy current artifact . To reduce or eliminate eddy current artifacts, theRF antenna loop has frequent electrical breaks bridged by capacitors, andthe number of copper conductors used to shield components such aspreamplifiers is minimized to the extent practical.

Patient Safety

As mentioned above, decoupling is essential for patient safety. Inaddition, it is very important that the RF coil cable is not coiled into aloop. Since a loop of wire has inductance, a loop in the coil cable can





easily cause the cable to become a resonant circuit at or near the systemoperating frequency (see RLC circuits discussed earlier). Again, this couldresult in the loop absorbing a great deal of RF energy from the MRsystem body coil, and a fire or electric field burns to the patient couldeasily result. This phenomenon may occur whether the coil cable is

attached to the MR system or not. It may also occur with otherelectrically conductive cables, such as ECG leads, or even a bracelet ornecklace or underwire brassiere the patient is wearing.

RF coil cables often have a thick layer of insulation to help prevent aninductive loop from forming and to ensure a minimum distance from thepatient to the cable. In addition, baluns or cable traps may be built intothe RF coil cable at certain locations to suppress surface waves on thecable. Finally, RF coil cables are kept as short as possible, thus making itdifficult for the MR system operator to inadvertently loop the cable.

Coil Designs for 3T MR Systems

Converting a successful RF array coil design from 1.5T to 3T is not simplya matter of retuning the antennas and preamplifiers from 64 MHz to the128-MHz operating frequency of a 3T MR system (although the oppositemay be true). In general, overlapping of adjacent antennas can createsignificant capacitance, causing coupling between the antennas. Thiseffect is much more problematic at 128 MHz, and the higher operatingfrequency gives rise to greater coupling between antennas in the coilarray. Since preamplifier decoupling methods limit only inductive couplingof antennas, great care must be exercised in reducing stray capacitancein antenna arrays operating at 128 MHz.

Cabling for 3T array coils is another important aspect of design, andcables create a serious technical hurdle at 3T. Each antenna loop must beconnected to a receiver in the MR system in some way. This is usuallyaccomplished using a coaxial cable. There are often interactions amongthe coaxial cables connecting the many antenna elements to the MR

system, and random coupling between coils and cables invariablydegrades SNR. Normally, MR systems with many channels require that allcoaxial cable shields be connected to a common RF ground point.However, as illustrated in Figure 2.1, connecting all of the antennas to acommon ground point can create many more “loops” of wire. These loopscan potentially become antennas resonating near 128 MHz, which mightthen pick up energy from the MR system body coil. When these current





loops interact with the antenna loops in the receive state, they maycause unwanted coupling, resulting in losses of SNR and uniformreception. In addition, the currents induced on coaxial cable shields canbecome very high during the transmit cycle of the MR system body coil,creating the potential for patient burns and arcing.

As mentioned, cable traps may be used to control currents on the cableshields. 5 Although these traps block current, they also may have largevoltages induced on them during the transmit period. The amplitude ofthis voltage is much higher at 3T than at 1.5T, and the patient must beprotected from this voltage (and the associated electric field), and theelectrical components of the cable traps must be capable of routinelysurviving these voltages. As with coil housings for 1.5T systems, only a

small rise in temperature is acceptable. Important design features for cables for a 3T versus a 1.5 system are:

Cables should be significantly shorter if possible.

More cable traps should be employed.

More insulation should be used to protect patients.

More care should be taken with the cable locations with respect to

FIGURE 2.1 ● A 2D coil array, four antennas wide by four antennaslong. The red lines represent the shields of the coaxial cables thatmust be attached to a common ground point.





patients.

As previously discussed, a basic requirement for coils is that during theMR system body coil transmit period, the antennas must be “turned off.”Because the frequency of the B 1 field is twice as high at 3T as at 1.5T,

similarly sized coils have twice as much voltage induced in them during

the MR system body coil transmit period. As a result, the decoupling trapvoltages are twice as high and the power dissipated in the decouplingtrap is four times as high unless the impedance of the trap is increased.Because decoupling traps are the locations for high voltages andcurrents, additional care must be taken at 3T to ensure that componentsdo not fail, that arcing through the housing cannot occur, and that localelectric fields associated with the trap cannot induce excessive power inthe patient—s tissue.

Although 3T coil design requires precision and care, many of the technicalchallenges are being overcome, and coil arrays for 3T clinical scannersare rapidly becoming available.

Dedicated Coils

Many of the design requirements imposed upon RF coils arise from thedesign of the electrical interface of the MR system, requirements imposedby MR system manufacturers, flammability requirements, insulation

requirements, laboratory standards, and requirements of local andinternational bodies such as the FDA, Canadian Standards Association,and the European requirements of IEC 601-1 and EN 60601-1. All ofthese design requirements are necessary, but not sufficient, to deliver agood image. Ultimately, the goal in designing new RF coils is to deliverimaging performance superior to that of existing coils or to address newimaging applications.

Only a few years ago, the lack of dedicated coils resulted in the use ofhead or knee coils to image the wrist, head coils to image the feet, orgeneral-purpose coils to image the shoulder. Demand for superiorimaging capability, however, has spawned an entire RF coil industry,resulting in the availability of many more RF coils dedicated to aparticular application. As these new RF coils found their way into clinicalpractice, they set a new standard for imaging performance.

The most important factor determining the SNR performance of a coil isthe volume of the RF coil. Since the SNR of an RF coil is almost directly





proportional to its volume, imaging the wrist, for example, in a dedicatedRF coil with a 10-cm diameter results in an image that is vastly superiorto a wrist image acquired in a 16-cm-diameter knee coil.

Intuitively, it makes sense to place the receiving antenna (the RF coil) asclose as possible to the source of the radio signals (the patient—s

tissue). Figure 2.2, for example, illustrates how the knee coil housing iscontoured to the shape of the knee. This results in SNR performancesuperior to that of a cylindrical RF coil. Of course, maximizing the numberof patients for whom an RF coil will be suitable is also important: as theRF coil gets smaller, so does the patient population for whom it issuitable.

T r a n s m i t / R e c e i v e V e r s u s R e c e i v e - O n l y C o i l s

Typically, the MR system body coil is used to transmit RF energy into the

patient. Because the MR system body coil is cylindrical, large, and faraway from the patient, the resultant B 1 RF transmit field is relatively

uniform—that is, if the pulse sequence calls for a 90° proton tip angle,most of the hydrogen protons inside the patient that are within the MRsystem body coil are “tipped” about 90°. If the protons are not tippedapproximately the same amount, the signal amplitudes received aredifferent. As a result, similar tissues would exhibit different signalintensities depending upon where they are located. Contrast betweentissues and even within the same tissue would be affected and could bemisleading. For this reason, it is convenient to use the MR system bodycoil to excite the tissue of interest.





However, sometimes it is advantageous to use a local transmit coil. Sucha coil transmits “locally,” into the knee for example, and also receives thesignal back from the tissue. These types of coils are called local transmitcoils or transmit/receive coils . For many years, the knee coils provided byseveral MR system manufacturers were transmit/receive. At first thesecoils were linear saddle coils. Later, quadrature birdcage or quadraturesaddle pair coils were introduced. These coils (because they werecylindrical and could produce a relatively uniform B 1 excitation field and

because they were single-channel coils) lent themselves to transmitting

RF energy into the knee through the coil and receiving the RF signal backfrom the knee through the same coil.

There are several benefits provided by a transmit/receive coil. First, onlythe tissue of interest, in this case the knee, is excited. This means thatthere is no unwanted signal returned to the coil from the rest of thebody. This is particularly advantageous in the knee, where foldover(“wrap”) artifacts can emanate from the contralateral knee, or fromtissue superior or inferior to the knee of interest. Second, RF power isdeposited only into the patient—s knee, rather than into whatever part ofthe body is within the MR system body coil. Since the amount of RFenergy required to “tip” the protons in just the knee is much less thanthe amount of RF energy required to tip the protons in the knee as wellas all other tissues inside the MR system body coil, a transmit/receiveknee coil deposits much less power (measured in Watts) into the patient.The FDA allows much higher power deposition (measured in Watts perkilogram of tissue) on a local basis than a whole-body basis. In addition,the patient is much better able to compensate for power that is deposited

locally, through respiration, perspiration, and circulation. Pulse sequences are now being developed to allow ultra-high-resolutionimaging of cartilage at 1.5T. These pulse sequences are likely to exceedthe allowable power deposition into the patient, known as the specificabsorption rate (SAR), however, if the MR system body coil is used totransmit RF energy to the patient. Therefore, transmit/receive coilsdeveloped to address the SAR limitations will be needed for such high-

FIGURE 2.2 ● Cutaway view of a knee array coil housing showing acontoured fit for optimal SNR.





resolution imaging.

At the 128-MHz operating frequency of 3T systems, the problem ismagnified. To achieve the same tip angle in a 3T system, the RF powermust increase by a factor of 4.

Transmit/Receive Knee Array Coil

A novel eight-channel transmit/receive knee array coil (Fig. 2.3) isavailable for most MR systems, at both 1.5T and 3T field strengths. Thearray comprises a cylindrical birdcage transmit antenna, with eightindividual receive antennas inside of it that are contoured to the shape ofthe knee to minimize the volume. Keeping the birdcage transmit coilcylindrical produces a more uniform B 1 transmit RF field. The “twist” inthe birdcage has been shown to make the B 1 field flatter in thesuperior/inferior direction, and to cause the B 1 field to fall off more

sharply at the ends of the coil. Since less tissue outside of the coil isbeing excited, there is less wraparound artifact and less unwanted signalentering the eight receive antennas.

M u l t i c h a n n e l C o i l s

Most MR systems manufactured today have at least eight receivers or

“c han ne ls.” Soon , howev er, 32-c han ne l MR sy ste ms wil l be the norm.These multichannel systems were developed to take advantage of theprinciple of the phased array coil. 6 The term “phased array,” borrowedfrom old RADAR terminology, is a misnomer because in MR imaging thearray of coils receives multiple independent signals simultaneously asopposed to a single signal created from multiple sources in a “phased”manner.

The ability of the MR system to receive RF signals on many channelssimultaneously provides a variety of benefits, including:

Higher SNR

Extended field of view

Ability to use parallel imaging techniques (see discussion below).

A typical modern RF coil consists of eight antenna elements mapped toeight receivers of the MR system. Such an array of antennas inside asingle coil housing is known as an array coil . In an array coil, each





antenna acts as a separate RF coil. As can be seen in Figure 2.4, theeight antenna elements are arrayed around a cylindrical phantom. Eachantenna acts as an independent receiver of RF energy. The MR systemcombines all eight signals into a composite image. Signal intensity at theperiphery of the phantom, close to the antennas, is markedly increased.

This is sometimes called the surface coil effect .

FIGURE 2.3 ● Eight-channel transmit/receive knee array coil. ( A )Coil housing with base plate. ( B ) Antenna configuration inside coilhousing. ( C ) Cylindrical twisted birdcage transmit antenna, foruniform B 1 transmit field. ( D ) Eight individual receive antennas close

to the knee, for optimal reception of signal.





SNR

SNR is affected by the size and number of antennas. If 16 antenna

elements were arrayed around the cylinder shown in Figure 2.4, forexample, they would not deliver higher SNR in the center of the phantom.However, because each of the 16 antennas would be about half as largeas the 8 antennas illustrated, the SNR close to each antenna would bemuch higher. In other words, more channels result in higher “bulk” SNR.Often the region of interest is not precisely at the center of the coil. Inthe knee, for example, the patella, and substantial portions of thehumeral head, and even the meniscus may not lie precisely in the centerof the RF array coil. In this situation, more receiving antennas, whichresult in higher SNR, would produce more clinically relevant information.

It is important to remember that many small receiving antennas result inpoor image uniformity, and the more antennas that are used, the smallerthey must be. This results in high signal intensities near the antennas,regardless of the tissue being imaged. It imaging joints, a high degree ofimage homogeneity is important, and MR system manufacturers areadding image uniformity correction algorithms to their software to

FIGURE 2.4 ● Typical eight-channel array coil. ( A ) One antennaturned on, with seven antennas decoupled. Note that they are notcompletely “off.” ( B ) A composite image with all eight antennasturned on.





address this problem. These algorithms use the image intensity profile ofthe RF coil to remove intensity variations in the image. This can beaccomplished during a calibration scan that compares an image acquiredwith the MR system body coil to the image acquired using the RF coilarray.

As mentioned, to fit eight antennas around the knee, each antenna needsto be relatively small. The smaller the antenna, the higher the SNR underthe antenna. This is because a small antenna receives both signal andnoise from a small amount of tissue. Since noise is not encoded by theMRI process, the noise associated with a given image pixel is actually acoil sensitivity-weighted superposition of noise originating fromeverywhere around the coil. RF noise is produced by the random motionof charged particles within any resistive sample. Since resistive material(e.g., blood, muscle) is found inside the patient—s tissue and noise isamplified by the coil sensitivity in a particular region, the noise in an MRimage comes from the area of the patient—s body near the coil wheresensitivity is highest.

Isolating Adjacent Antennas

If two antennas, each resonating at 64 MHz (1.5T operating frequency),are brought toward each other, they start to have two combined

resonance modes (a common mode and a counterrotating mode), andneither antenna will be an efficient receiver of RF signals at 64 MHz. Asthe antennas are brought even closer, the problem gets worse. However,as the antennas overlap one another, the problem is reduced. At someprecise degree of overlap, the antennas cease to affect one another, andeach resonates at 64 MHz once again. These antennas are said to be

“i sol ated” fr om eac h ot he r. Th e degree of isol at ion is mea su red indecibels. Antennas that are poorly isolated are “coupled” or have “mutualcoupling.”

In a flat antenna array, antenna elements can easily be overlapped withtheir nearest neighboring antenna elements to achieve isolation.However, isolation between one antenna and its next nearest neighborcannot generally be achieved in this way in dedicated extremity coils. Ifantenna elements are poorly isolated, or coupled to one another, theantennas tend to “talk” with one another (i.e., share signal and noise).As a result, the array advantage of having many independent coil





elements is lost and the antenna array acts like one large antenna withlower SNR.

In a linear array of eight elements, for example, each element isoverlapped with its nearest neighbors and is well isolated. If the array isbent into a cylinder, however, some antenna elements face one another,

and the degree of coupling between antenna elements that are notoverlapped increases dramatically. Isolation is complicated even in afour-element antenna array: antenna 1 must be isolated from antennas2, 3, and 4; antenna 2 must be additionally isolated from antennas 3 and4; and so on. In total, 3 + 2 + 1 = 6 individual isolations must beachieved. In an ideal 32-channel antenna array, 31 + 30 + … 1 = 496isolations are necessary for optimal coil performance.

Although designing and manufacturing many-channel coil arrays is a hugetask compared to the last generation of four-channel coil arrays, thereare a variety of techniques and trade secrets for improving isolation ofthe antenna elements of an array coil. One commonly employed methodis to use a low-impedance preamplifier on each antenna element. Thisapproach allows each coil impedance to be high enough to minimize coilcurrents and thus reduce coil coupling effects. Most MR systems beingmanufactured today allow for the use of such preamplifiers.

Parallel Imaging Techniques Parallel imaging techniques are innovative methods now available onnearly all new MR systems. These methods generally use the RF coilsensitivity profile to reduce the number of phase encode steps required togenerate an image 7 and work only with array coils. Reducing the numberof phase encode steps allows an image to be acquired much more quickly(called acceleration ) and has been shown to be very useful when imagingflow or organs subject to motion, such as the heart or lungs. The reducedacquisition times result in higher-quality images of these moving tissues,even though SNR is significantly reduced when acceleration factors areemployed.

Modern RF coils are being designed to take advantage of these methods.Antenna array elements must be laid out in a specific direction to permitan acceleration in that direction. For example, if an antenna array hastwo elements in the coronal plane, the acquisition of a coronal image canbe accelerated by a factor of 2. Imaging time can be cut in half. The





caveat is that the resulting SNR is reduced by at least 1/ √ 2, or 0.707, ofthe original SNR. Since this has the same result as reducing the numberof averages in half, parallel imaging was thought to be of little use unlessthe number of averages was not already equal to 1.

Orthopaedic images are generally acquired with two or four averages,

since one average rarely provides the necessary SNR to produce a goodhigh-resolution, small-field of view image. Since acceleration results in asignificant reduction in SNR, parallel imaging was initially though to haveno application in joint imaging (other than dynamic joint imaging).Although some MR systems today are capable of using parallel imagingaccelerations while acquiring multiple averages, the application of parallelimaging techniques in orthopaedic imaging is still in its infancy. Someusers are very enthusiastic, stating that acquisition of images in lesstime, and then averaging them together to regain SNR, results in imageswith significantly reduced motion artifact. Although this remains to beclearly demonstrated, there is an important use of parallel imaging inevaluation of joints. Using a parallel imaging technique with anacceleration factor of 1 (i.e., no acceleration), the array coil sensitivityprofile can be applied to correct any nonuniformities in the image. This isan excellent technique for the correction of image nonuniformities causedby many small antennas producing high signal intensities, and MR systemmanufacturers have begun making it available.

Because parallel imaging reduces the number of phase encode stepsneeded to create an image, using parallel imaging can reduce the SARassociated with a particular acquisition. 7 This may prove useful for 3Tsequences where body coil excitation is used (e.g., shoulder and hipimaging).

RF Coils for Orthopaedic Imaging

Dedicated eight-channel RF array coils for orthopaedic imaging arebecoming available for 1.5T MR systems. Clinical prototypes havedemonstrated that these coils can deliver SNR increases that result inincreased diagnostic confidence and, in some cases, increased diagnosticcapability. As array coil designs become available for 3T MR systems,faster, higher-resolution joint imaging will become possible. Orthopaedicimaging at 3T has proved to be less challenging than body imaging at 3T,since the dielectric resonance effect at 3T is not as severe in joints.





Knee coils were discussed earlier in the section on the Transmit/ReceiveKnee Array Coil.

S h o u l d e r A r r a y

Designing a shoulder array is particularly challenging because shouldersvary so much in size and the coil needs to be able to accommodate bothleft and right extremities. In addition, the chest size and shape vary(barrel-chested to flat-chested), and the pectoral muscles may beprominent (as in body builders). Imaging female patients with breastimplants can also be problematic, since the implant may hinder properplacement of the coil. Deep penetration is required to image the anteriorlabrum, whereas the rotator cuff and the acromion are very shallow. Toomuch penetration induces motion artifact from the lung. Anothercomplication stems from the fact that patients tend to unknowingly shrugtheir shoulders while breathing. Fixing the coil to a base plate is helpfulin reducing patient motion because the patient can feel resistance tomovement of the shoulder and is more aware of moving.

Because of cost considerations, it is not feasible to have multiple coils ofvarying size for each shoulder, and the solution was to create anadjustable hinged coil that can be used for both the right and leftshoulders (Fig. 2.5). The hinges allow the “wings” of the coil to move in

the anterior/posterior (A/P) direction, allowing the spread of the wings tobe adjusted to the patient—s body habitus. The coil can open wide toimage large shoulders or can be closed down to a 14-cm aperture forsmall shoulders. The hinges lock to provide resistance to patient motion.The coil is mechanically fixed to a base plate in the A/P direction tominimize motion-induced artifacts. The coil must be flipped over to imagethe contralateral shoulder; therefore, fixing the coil to the base platemust be simple and intuitive yet robust.

Although the coil housing has hinges, the housing is rigid. This allowscontrol of overlap of the antenna elements to maintain isolation.Preamplifier decoupling assists in maintaining isolation between thoseantenna elements that face each other and change orientation as thewings are repositioned. The preamplifiers are located in the box attachedto the coil, since the limited space between the patient and the body coilprecludes placing the preamplifiers directly on the antenna elements.





FIGURE 2.5 ● A dedicated eight-channel shoulder array. ( A )Shoulder array shown fixed to the base plate. ( B ) Antenna geometryfor the eight-channel shoulder array.

FIGURE 2.6 ● Image of the shoulder acquired at 3T using an eight-





The coil supports a parallel image acceleration factor of three in eachdirection (A/P, left/right, superior/inferior [S/I]) (Fig. 2.6), considerablyimproving imaging time; Figure 2.6 was acquired in less than 90 seconds.

H i p A r r a y

The coil for the dedicated hip array (Fig. 2.7) is flexible, since there issuch a wide variation in body habitus. In some patients, posterior fatcauses the buttocks to protrude laterally in the supine position. The hip

array is designed so that it wraps around the patient. For small patientsit may close completely, with a controlled overlap of antenna elementswhere they meet at the patient—s midline. For large patients, there is acontrolled separation of antenna elements at the patient—s midline toensure that the antenna elements do not detune each other.

As with similar shoulder imaging, hip imaging requires small fields ofview with high resolution. The individual antenna elements need to belarge enough to exhibit good depth of penetration to the acetabulum, butnot so large that they cover unwanted tissue superior or inferior to thearea of interest. Bilateral imaging is desirable, but unilateral imaging isrequired to achieve the small field of view and high resolution necessary.

To meet these requirements, the antenna elements are positioned toimage the hip joint from the anterior, lateral, and posterior approaches.Sixteen antennas are used, eight targeted at each hip. The coil supportsthree modes of operation: left hip with eight channels, right hip witheight channels, or bilateral imaging with all channels. The coil antennasthat are active may be chosen from the MR console. MR systems

equipped with 16 or more receivers can use all 16 channels. For MRsystems with eight channels, the 16 antenna elements are combined intoeight channels for bilateral imaging.

The coil supports a parallel imaging acceleration factor of at least 2 in thecoronal and sagittal planes and 4 in the axial plane. Bilateral acquisitionscan theoretically be acquired with an acceleration factor of 8, but theneed for SNR will dictate what acceleration factors are practical.

channel shoulder array. Slice thickness is 3 mm with 0.47 × 0.47 mmin-plane resolution.





W r i s t A r r a y

The wrist array coil (Fig. 2.8) is an eight-channel receive-only design.

This coil is contoured to the wrist and hand to minimize the volume of thecoil. The S/I coverage is 11 cm, with coverage extending distally to themetacarpals.

The mechanical design of the coil incorporates a hinged cover that allowsaccurate visual centering of the wrist inside the coil. Once the wrist isproperly positioned, pads can be added inside the coil, and the cover canbe closed and locked. One of the pads applies pressure on the palm,which presses the back of the hand into a single plane. The objective isto align the carpal ligaments so they are visible in the same imaging

plane. The coil can be used in several positions. The most comfortable is at thepatient—s side (see Fig. 2.8C). Because magnet homogeneity cansometimes degrade off isocenter near the body coil, resulting in poor fatsaturation, it is desirable to position the coil as far medially as possible.The coil can also be used over the patient—s head in the “Superman”position (see Fig. 2.8D). This allows positioning the coil at the most

FIGURE 2.7 ● Dedicated hip array. ( A ) 16-channel hip array. Thereare 8 channels for each hip. ( B ) Antenna configuration for the 16antennas.





homogeneous location in the magnet. This wrist array coil allows imagesto be acquired with very small fields of view and at very high resolutions(Fig. 2.9).

F o o t a n d A n k l e A r r a y C o i l

The foot and ankle array coil is an eight-channel coil designed for high-resolution imaging (Fig. 2.10). The patient—s foot is strapped into the

FIGURE 2.8 ● An eight-channel receive-only wrist array coil. ( A )Wrist array coil with a window and a hinged cover. ( B ) Eight elementssurround the wrist. ( C ) The wrist array positioned at the patient—sside. At such a distance from the isocenter, magnet homogeneity cansometimes be problematic, resulting in poor fat saturation. ( D ) The

“S up erman” pos it io n loc at es th e co il near the magn et isocenter,where B 0 field homogeneity is better. This leads to significant

improvements in fat saturation.





foot support and pads are applied under the heel. Although the preferredimaging position is dorsal flexion, many patients cannot tolerate thatposition after injury. The coil may be tilted 15° from the dorsal flexposition for patient comfort.

The coil has three modes of operation: ankle, forefoot, and entire foot

and ankle (Fig. 2.11). The mode of operation is chosen from the MRsystem console. Different modes are necessary because of variations inthe length of the foot. The small fields of view sometimes prescribed forthe ankle may exhibit wraparound artifact if the forefoot antennaelements were left active, and vice versa. The entire foot and ankle modeis useful for MR angiography studies and whole-foot studies.

FIGURE 2.9 ● High-resolution image acquired with a 3T version ofthe wrist array coil. Note the nerve bundle.





An array of eight antennas surrounds the foot and ankle (Fig. 2.12). Theforefoot is surrounded by three coils. The yellow coil is a two-turn

FIGURE 2.10 ● Foot and ankle array coil. ( A ) RF coil with base platepositioner. ( B ) The base plate provides up to 15° of dorsal flex forpatient comfort.

FIGURE 2.11 ● Modes of operation of the foot and ankle array coil.( A ) In the ankle mode of operation, six of the eight antennas are

active. The forefoot solenoid and saddle pair are not active. ( B ) In theforefoot mode of operation, three antennas are active: a single-turnsolenoid, a 2-turn solenoid, and a saddle pair.





solenoid, the blue coil is a single-turn solenoid, and the green coil is asaddle pair. Solenoids have been shown to be remarkably efficient coils.In fact, the ultimate coil is a solenoid that has the tissue of interest inthe middle of the loop. Solenoid coils must be positioned so that theplane of the solenoid is parallel to the B 0 field of the magnet. If the

solenoid is positioned with the B 0 perpendicular to the plane of the loop,the coil receives very little signal.

E l b o w C o i l

An eight-channel upper extremity coil (Fig. 2.13) is typically used forimaging the elbow, although it can be used for other small structures for

which a dedicated coil is not available. Wrist images acquired with thiscoil, while of reasonable quality, do not rival those acquired with adedicated wrist array.

FIGURE 2.12 ● Antenna configuration for foot and ankle array.





Since the coil is quite easily slid up the arm, it does not split in thecenter, avoiding the expense and reliability issues associated with themany RF feedthroughs that would be needed to connect the antennas ifthe coil did split. Windows in the coil allow for visual centering of theelbow within the coil.

References

1. Foster MA. Magnetic resonance in medicine and biology. New York:Pergamon Press, 1984.

2. Roemer PB, Edelstein WA, Hayes CE, et al. The NMR phased array.Magn Reson Med 1990;16:192-225.

3. Buchli R, Saner M, Meier D, et al. Increased RF power absorption in MRimaging due to RF coupling between body coil and surface coil. MagnReson Med 1989;9(1):105-112.

4. Chen CN, Hoult DI. Signal and noise. In: Biomedical magneticresonance technology. New York: Adam Hilger, 1989:118.

5. Peterson D, Beck BL, Duensing GR. Proceedings of the InternationalSociety of Magnetic Resonance in Medicine, 2002:850.

6. Pruessmann KP. Parallel imaging at high field strength: synergies andoint potential. Topics MRI 2004;15:237-244.

FIGURE 2.13 ● Eight-channel upper extremity coil.
