Dedicated Coil Systems from Head to Toe 14

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R. Duensing, PhD

Department of Diagnostic Imaging, Invivo Corporation, 3545 SW 47th Avenue, Gainesville, FL 32608, USA

C O N T E N T S

14.1 Introduction 161

14.2 Coil Sensitivities and Parallel Imaging 162 14.3 SENSE vs GRAPPA 163

14.4 Reconstruction Time vs Number of Channels 163

14.5 Uniformity Correction 163

14.6 Very-High-Field Sensitivity Proﬁ les 163 14.7 Available Products 164

14.7.1 Orthopedic Coils 164 14.7.2 Breast Coils 164 14.7.3 Head Coils 164

14.7.4 Cardiovascular and Torso Coils 165 14.8 Limitations of Coil-Design Improvements 166 14.9 Transmit/Receive Arrays and

Transmit SENSE 166

14.10 Vertical Low-Field Parallel Imaging 167 14.11 Conclusion 167

References 167

Dedicated Coil Systems from Head to Toe 14

R andy Duensing

14.1 Introduction

Parallel-imaging algorithms have driven rapid changes to the RF sub-systems in clinical MRI scan- ners. The typical high-end scanner in use during 2003 had 4–8 RF receiver channels. These channels were used primarily for RF coil arrays designed to increase the signal-to-noise ratio (SNR) over a certain anatomical region. Because of the profound impact of parallel imaging on clinical practice, the state-of- the-art scanner in 2005 has 16, 18, or 32 receiver channels available for simultaneous acquisition. As is demonstrated in Chap. 3, while an asymptotic limit is reached, the more independent receiver coils employed, the higher the permissible reduction rates and the lower the resulting g-factors; therefore, the rapid demand for receiver channels has been matched by the demand for coil arrays that can be utilized on these systems. There are currently two dis- parate uses for arrays and parallel-imaging systems:

the fi rst utilizes specialized high-density coil arrays for optimizing the image quality in a given time for a specifi c anatomical or functional region; the second utilizes arrays with large or complete body coverage to allow rapid whole-body scans, including moving- table applications. These products are mostly available at 1.5 T on horizontal fi eld systems, with growing availability at 3 T. The vertical fi eld products for open low-fi eld MRI systems lag behind horizontal fi eld but are becoming available.

The state-of-the-art MRI system in 2006 comes with many array coils available. All of the major manufacturers have ﬁ elded systems with 32 receiver channels.

Arrays with from 8 to 32 channels are either currently or soon will be available for orthopedic, body, head, breast, and vascular imaging. The “total imaging matrix” (“Tim”) system introduced by Siemens Medi- cal Solutions, Erlangen, Germany, utilizes 32 channels from 76 selectable array elements to cover most of the body, although specialty coils can also be employed for certain applications (cf. Chap. 13).

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Table 14.1 presents URL list of some coil manufacturers and available coils.

Table 14.1. A URL list of some coil manufacturers and avail- able coils

http://www.gehealthcare.com/usen/mr/mr_coils_allprod/index.

html

http://www.medical.siemens.com/

http://www.medical.philips.com/main/products/mri/products/

achieva15t/coils/

http://www.hitachimed.com/gallery.asp?ID=5

http://medical.toshiba.com/clinical/radiology/vantage-524-631-455.

htm

http://www.invivomde.com/products/CoilProductsAll.aspx http://www.medrad.com/products/mr/mr-coils/index.html http://www.usainstruments.com/

http://scanmed.com/table.htm#table http://www.novamedical.com/products.html http://www.midwestrf.com/products.htm http://www.temcoils.com/

http://www.rapidbiomed.de/humancoilsindex_27_26_0_f.htm http://www.dotynmr.com/mri/mri_mainpg.htm

http://www.xlres.com/

A clinical RF coil array must be safe, ergonomi- cally useable, reliable, and must provide high performance (SNR, uniformity, etc.). Parallel-imaging capability in arrays does not eliminate these requirements in traditional imaging procedures; instead, the additional requirements of parallel-imaging capability must be added without damaging performance.

The RF coil sets the SNR for the study and no hardware can subsequently increase it. A drop of SNR of around 1015% is visually noticeable, unless the SNR is extremely high at the start. This critical emphasis on SNR and parallel-imaging capability leads to new challenges discussed in this chapter.

14.2 Coil Sensitivities and Parallel Imaging

Parallel-imaging techniques, as have been described in previous chapters, rely upon the spatial distribution of coil elements to provide enough information to replace some of the standard acquisition points. Each phase- encoded step represents one coefﬁ cient of the Fourier series describing the image. If the coefﬁ cients of certain basis elements are not acquired, then these must be con- structed from the other samples and the coil information.

The coil sensitivity patterns, when described in the frequency domain, i.e. in k-space, generally have low spatial frequencies, and thus, multiplication by the sensitivity patterns (i.e., receiving with a given coil set) will produce components in the missing frequency locations. By acquiring with multiple coils with different sensitivity patterns, the missing Fourier coefﬁ cients can be approxi- mated. This is simply solving a set of linear equations with enough equations to enable the unknowns to be calculated. The equations, in general, have complex coef- ﬁ cients. Images are usually observed in magnitude form, so it is easy to overlook the fact that two receiver channels may have similar amplitude patterns, but different phase behavior, thus enabling them to provide independent information (cf. Chap. 3).

Coil design for a give anatomy and application currently requires compatibility with parallel imaging. Vir- tually all new array coils built presently must include this capability. The reduction times, in a practical sense, are typically around a factor of 23. It is also common for certain studies to be performed with phase-encoding oriented in certain preferred directions generally due to motion of some kind (such as heart motion, respira- tion, and blood ﬂ ow in vessels). In many cases, therefore, the ability to perform parallel imaging is only required in a particular plane. In such cases fewer channels are required than might otherwise be the case.

The g-factor equation is dependent upon fi eld of view (FOV), location, reduction factor, and phase- encoded direction. This makes it extremely diffi cult to guide array design from g-factor analysis. From a practical standpoint, a design of array elements is proposed using the generic requirement that the mag- netic fi eld profi les are considerably different over the range of FOVs to be employed. The elements should be arranged such that composite image profi le covers that FOV with adequate SNR. Generally, the accessible surface is covered with surface elements that cover all of the accessible surface area. The number of elements is often chosen on the basis of the number of channels utilized by the system to which the array will be attached.

When a basic design can be described in software, then a g-factor analysis is carried out. A variety of reduction factors (R) should be tested to ensure that there are no aberrantly high g-factors for a range of, for example, R=1 to R=3. The results of this analysis may result in small changes to the layout of the array, or dismissal of the basic geometry. In some cases, it may be required to produce reduction-factor capability in two directions simultaneously. Although not equivalent to 2D, it may be acceptable to perform separate 1D analyses for coil design. As long as the total channel count “seeing” the

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volume of interest is substantially higher than the total 2D reduction factor, then the separate reduction-factor analyses should provide the necessary information as to the acceptability of the array layout.

Several publications on the analysis of the g-factors of various coil layouts have been performed (Weiger et al. 2001; Zwart et al. 2002). These publications have demonstrated that two loops should be slightly sepa- rated instead of overlapped (for zero mutual induct- ance) to have the best g-factors and therefore the best parallel imaging capability (cf. Chap. 3); however, it has furthermore been shown (Griswold 2005) that the total SNR with moderate speed-up factors is similar between the two approaches. For large channel count arrays, the losses due to many coupling partners can signiﬁ cantly reduce the SNR; thus, it may be advanta- geous to design the coils for low coupling. Most exist- ing clinical coil designs are made in this way.

14.3 SENSE vs GRAPPA

Generally, coils can be designed based on performance for either k-space-based techniques (such as GRAPPA) or image-space-based techniques. While there are differences in the spatial location of noise enhancement between the various techniques the overall performance is similar for a given array. This means that a coil that works well for SENSE works well for GRAPPA. (For detailed discussions of these issues see Chaps. 2 and 3.)

14.4 Reconstruction Time vs Number of Channels

High reduction factors in two dimensions, as some- times employed for certain cardiac studies, result in a large numerical problem for the image reconstruction. The result is that reconstruction time can be quite long, e.g., a 3D volume with reduction factors of 3×3 acquired using a 32-channel array. If one com- pares this to a reduction factor of 2 for an 8-channel array, one would expect roughly a factor of 300 times longer reconstruction time; thus, a 2-s reconstruction turns into a 10-min reconstruction. Combiners (King et al. 2003; Reykowski and Blasche 2004)

can be used to optimize the data density while still permitting the required ﬁ eld patterns in the speed- up directions (see Chap. 13). An N-fold reduction in the number of total channels results in at least a N² reduction in reconstruction time. Additionally, software algorithms can also be employed to reduce the effective channel count prior to reconstruction.

These algorithms put as much energy into as few channels as possible and enable large improvements in speed while preserving nearly all of the SNR and parallel-imaging capability (Huang et al. 2005).

14.5 Uniformity Correction

Because parallel-imaging capability is driven by the relatively rapidly changing sensitivity patterns of many coils, the uncorrected uniformity of a standard phased-array image (root of sum of squares) may be poor. One method for correcting this intensity variation is to acquire a reference image with the body coil, so that the sensitivity maps are referenced to a con- stant sensitivity. Another method is to use ﬁ ltering that seeks to remove low-frequency variation in the overall intensity without losing contrast in the image.

Both of these have limitations and it appears that further advancements of software will be required to provide adequate uniformity for multi-channel arrays (Cheng and Huang 2005).

14.6 Very-High-Field Sensitivity Proﬁ les

Considerable work has been performed to analyze the behavior of RF coils for very-high-frequency MRI (Wies- inger et al. 2004). This work has demonstrated that because of the rapid phase changes within the sample, higher reduction factors can be supported at higher frequencies (cf. Chap. 44). This is consistent with other technologies in which the wavelength is the limiting resolution factor. In conventional MRI, this wavelength shortening has no value and is unrelated to resolution.

In a pure tomographic approach (i.e., no encoding), the resolution depends greatly upon wavelength. Reduction factors in the range of six or more (per dimension) appear to be plausible at 7 T and above.

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14.7 Available Products

New coils are being developed rapidly for parallel- imaging capability in all application areas of the body. Generally, nearly all coils developed are or will be available for 1.5- and 3.0-T systems. In the follow- ing, each application area is related to the current state-of-the-art arrays and their capabilities.

14.7.1

Orthopedic Coils

Orthopedic arrays include specialized coils for the hand and/or wrist, the elbow, the shoulder, the knee, and the foot and/or ankle. Imaging of the hip is an emerging growth application as well. Currently, 8- channel arrays with parallel-imaging capability are available for the knee, wrist, and foot/ankle. Eight- channel prototypes, by several companies, have been demonstrated for the shoulder and hip. The wrist and knee coil permits transverse speed up directions, whereas the remainder can be used for any direction of acceleration. Figures 14.1–14.3 demonstrate available coils for foot-and-ankle MRI (Fig. 14.1), knee MRI (Fig. 14.2), and wrist MRI (Fig. 14.3).

14.7.2 Breast Coils

Breast imaging is a growing application area, and new coils are available with 7 and 8 channels. These arrays permit transverse acceleration directions. Prototypes with 12 and 16 channels have been ﬁ elded and it is anticipated that the next generation will permit accelerations in any direction. Figures 14.4 and 14.5 demonstrate available coils for breast-biopsy MRI applications.

14.7.3 Head Coils

State-of-the-art brain imaging is currently performed with either an 8-channel array or 12-channel array.

The 8-channel array is designed for transverse speed- up factors, whereas the 12-channel version permits speed-up factors in any direction. Prototypes with 16–

90 channels have been demonstrated (cf. Chap. 44), Fig. 14.3. A GE 8-channel wrist coil

Fig. 14.1. A GE 1.5-T 8-channel foot/ankle array

Fig. 14.2. A Philips 3-T 6- to 8-channel high-resolution knee array

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and it is likely that the next generation of head arrays will include at least 16 channels. Figures 14.6–14.9 demonstrate available coils for the head.

14.7.4

Cardiovascular and Torso Coils

Cardiovascular and torso arrays are currently available in conﬁ gurations from 4 to 32 channels. The 32-channel versions permit very high two-direction speed-up capability, which may prove beneﬁ cial for permitting very fast cardiac imaging. Torso applications are particularly well suited to improvement from parallel imaging because of the rapid motion of both the lungs and heart. Figures 14.10–14.12 demonstrate available coils for cardiovascular and torso MRI.

Fig. 14.4. A GE 1.5-T 7-channel biopsy breast array

Fig. 14.5. A 16-channel prototype biopsy breast array

Fig. 14.6. In vivo 8-channel high-resolution head array

Fig. 14.7. A 16-channel prototype head coil

Fig. 14.8. A 32-channel MGH prototype head coil. (Courtesy of G. Wiggins, Massachusetts General Hospital)

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Fig. 14.12. A GE 1.5-T 8-channel torso array Fig. 14.9. A 90-channel MGH prototype head coil. (Courtesy of

G. Wiggins, Massachusetts General Hospital)

Fig. 14.10. In vivo 32-channel cardiac array

Fig. 14.11. Siemens “Panoramic“ body arrays

smaller elements, whereas the SNR in the center of the body remains comparable with the one achievable with optimized volume coils.

14.9 Transmit/Receive Arrays and Transmit SENSE

There are several transmit/receive (T/R) arrays currently in clinical use; these all employ a single transmit source with power splitters to direct the power to each element of the array. The goal for these coils has

14.8 Limitations of Coil-Design Improvements

Without a change to a pulse sequence, parallel imaging always produces a loss of SNR that is at least related to the root of the time of the acquisition (see Chaps. 24); therefore, one requirement for parallel imaging that impacts the coil design is that the SNR must be such that a reduction by R still results in an image of adequate SNR to be diagnostically valuable.

Generally, for structures deep within the body, new optimized array designs are not capable of producing SNR that has such relatively large gains over exist- ing arrays. The SNR for shallower structures can be substantially improved by utilizing arrays with more,

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been to create as uniform an excitation profi le as possible, over a limited FOV. There are several reasons why one would want to employ a T/R array. The fi rst reason would be to excite a local area because of wrap artefacts that occur when whole-body excitation is employed. Another reason is that very high peak B₁ fi elds may be diffi cult to achieve with the body coil, but are achievable with a local coil. Clinical T/R arrays may either use the same elements for transmit and receive, with power being distributed amongst the channels to produce the best uniform excitation, or separate elements may be used with the receive channels being essentially the same as a standard receive-only array. If transmit SENSE (Katscher et al. 2003) becomes clinically relevant (cf. Chap. 45), such coils will be relatively easily converted to multi- port arrays.

14.10 Vertical Low-Field Parallel Imaging

Parallel-imaging techniques have recently been demonstrated on vertical low-fi eld systems. (Warntjes et al. 2005) Generally, twofold or lower reduction factors are typically employed due to the coil design constraints placed upon vertical fi eld receiving elements, and the intrinsically lower SNR available at lower fi eld strengths.

14.11 Conclusion

In the current state of the art, coil arrays must be compatible with parallel-imaging methods. The speed-up factor and directions required are speciﬁ c

for the application. Dynamic studies have the great- est need for parallel imaging, as these studies can result in signiﬁ cantly improved results because of the shorter acquisition times. Virtually all manufacturers provide up to 32 channels on their high-end systems, and coil arrays to utilize these abilities are rapidly becoming available. These technological improvements are driven by the need for faster, better exams and improved patient throughput.

References

Cheng H, Huang F (2005) Proc 27th Annual International Con- ference of the IEEE EMBS, #642

Griswold M (2005) RF coils for parallel imaging. ISMRM 13th Scientiﬁ c Meeting and Exhibition, Miami, Fla. (Morning categorical course “New Developments in MRI Hardware”, 13 May 2005)

Huang F et al. (2005) Proc 27th Annual International Confer- ence of the IEEE EMBS, # 651

Huang F et al. (2005) Proc 27th Annual International Confer- ence of the IEEE EMBS, #1190

Katscher U, Bornert P, Leussler C, van den Brink JS (2003) Transmit SENSE. Magn Reson Med 49:144150

King SB, Varosi SM, Huang F, Duensing GR (2003) The MRI eigencoil: 2N-channel SNR with N receivers. Proc Int Soc Magn Reson Med 11:712

Reykowski A, Blasche M (2004) Mode matrix: a generalized signal combiner for parallel-imaging arrays. Proc Int Soc Magn Reson Med 12:1587

Wiesinger F, Boesiger P, Pruessmann KP (2004) Electrodynam- ics and ultimate SNR in parallel MR imaging. Magn Reson Med 52:376390

Warntjes M, Ylihautala M, Kuipers E, Bloemers K (2005) SENSE performance of the 1.0-T vertical-ﬁ eld MRI. Proc Int Soc Mag Reson Med 13:914

Weiger M, Pruessmann KP, Leussler C, Roschmann P, Boesiger P (2001) Speciﬁ c coil design for SENSE: a six-element cardiac array. Magn Reson Med 45:495504

Zwart JA de, Ledden PJ, Kellman P, van Gelderen P, Duyn JH (2002) Design of a SENSE-optimized high-sensitivity MRI receive coil for brain imaging. Magn Reson Med 47:12181227