Measurement of Coil Sensitivity Proﬁ les 8

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M. Nittka, PhD

Siemens AG Medical Solutions, MREA, Karl-Schall-Str. 6, 91052 Erlangen, Germany

C O N T E N T S

8.1 Introduction 107

8.2 Two Basic Calibration Methods:

Pre-scan vs Autocalibration 108 8.2.1 The Pre-scan Method 108 8.2.2 The Autocalibration Method 108 8.3 Tailored Calibration Strategies 110 8.3.1 The Sequence-Based Pre-scan Method 110 8.3.2 Self-calibration Methods 110

8.4 Conclusion 112 References 112

Measurement of Coil Sensitivity Proﬁ les 8

Mathias Nittka

8.1

Introduction

The concept of parallel imaging is based on local sensitivity variations of the receiving coil array (cf.

chaps. 2 and 3). These so-called coil sensitivity pro- ﬁ les are incorporated into the spatial signal encoding.

In turn, information about the sensitivity proﬁ le of each receiving coil element is a fundamental input parameter for image reconstruction.

The process of acquiring the coil sensitivity data is commonly called coil calibration. Precise coil calibration is a crucial step within parallel imaging and deviations can easily lead to a degradation in image quality. This chapter outlines the basic approaches and practical considerations of current calibration techniques.

The most favourable coil calibration method would be to perform a single setup procedure once for each available receive coil array, e.g. by means of a phantom scan, and to apply this data set as a preset to any following parallel imaging scan.

There are two main reasons why such a “preset”

calibration will not work in practice:

The sensitivity profi le of a coil is not static but is subject to changes induced by the interaction of the coils electromagnetic fi eld and the dielectric properties of matter in its surrounding; thus, the coil profi le varies when it gets close to a dielectric sample such as the human body. Even if the same coil is applied to the same body region, the variations from patient to patient may be not neg- ligible.

Many routinely used coils are ﬂ exible to adapt to different body shapes. If the coil bends, its receive characteristic will change, too.

Therefore, some kind of coil calibration has to be incorporated into every examination after patient and coils are positioned inside the scanner. For clinical routine use, the ideal calibration procedure should be fast, robust and reliable, ideally running unnoticed in the background without any need for user interaction.

In practice, the most challenging task is to achieve a trade-off between additional calibration scan time and precision: high-precision calibration data, which might even be reacquired several times during an examination, may easily turn parallel imaging inefﬁ cient in terms of total scan time. Several different techniques for coil calibration found its way to clinical routine use.

Each of them provides speciﬁ c advantages and disadvantages for certain applications.

Before discussing some basic approaches, it should be noted that it is in general not required to determine absolute coil proﬁ les for each coil. Most reconstruction methods applied with parallel imaging just require the relative sensitivity differences between the individual receive coils. This fact is signiﬁ cant, since of course the signal of any calibration scan performed on the patient

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will consist of the superposition of the coils’ sensitivity proﬁ les with anatomical structures of different tissue types. One common method to calculate absolute sensitivity maps performs two interleaved acquisitions: the second acquisition uses the receive signal of the volume coil, which is usually only applied for RF transmission, instead that of the coil array. Taking the homogeneous receive proﬁ le of the volume coil as a reference, pure coil sensitivity maps can be determined by dividing out the underlying anatomy.

8.2

Two Basic Calibration Methods:

Pre-scan vs Autocalibration

It is instructive to ﬁ rst look at two quite comple- mentary calibration techniques, that both have found widespread use on commercial scanner software plat- forms: we refer to them here as the pre-scan method and the autocalibration method.

8.2.1

The Pre-scan Method

When faced with the problem of determining coil sensitivity proﬁ les, one may consider this as the most straightforward approach: a coil calibration scan is performed right before the actual imaging scans are performed, i.e. when patient and coils are in position.

Typically, a 3D scan covering the maximum possible volume is performed. Any following parallel imaging scan may extract the coil calibration data from this data set by interpolation to the actual slice geometry (Fig. 8.1). In order to restrict the scan time, a fast gradient-echo sequence is applied with modest spatial resolution, resulting in a scan time well below a minute.

8.2.2

The Autocalibration Method

The autocalibration method integrates the acquisition of coil calibration data into the imaging scan instead of performing it as a separate calibration step. It was originally introduced as a SMASH variant (Jakob et al. 1998, Heidemann et al. 2001) but has meanwhile been applied with different reconstruction types (e.g.

Griswold et al. 2002).

A few additional phase-encoding lines are added to the k-space sampling scheme of the accelerated sequence (Fig. 8.2). These additional calibration lines ﬁ ll in the “gaps” in the central k-space region, which otherwise would be left out by the accelerated acquisition.

Thus, the central region of k-space is fully sampled to some extent and may supply the coil calibration data for the image reconstruction process. The k-space-based reconstructions make direct use of the additional lines to determine the reconstruction coef- ﬁ cients (upper row of Fig. 8.2). Image domain related methods Fourier-transform the fully sampled inner k-space region to a low-resolution image for each coil

Fig. 8.1. A 3D pre-scan method. The whole imaging volume is covered by a 3D calibration scan.

The coil proﬁ les for any slice orientation may be extracted before image reconstruction.

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element and extract the sensitivity information from them (lower row of Fig. 8.2).

In the ideal case, the pre-scan method offers the advantage that the calibration scan has to be performed only once for an examination. But there are cases that may turn this property into a disadvantage: any time that the coil proﬁ les change, e.g. by an unwanted move- ment of the patient, the resulting mismatch between pre-scan and imaging scan may lead to artefacts. If the coil setup is changed intentionally, e.g. if the patient table is moved to another scan region position, the calibration pre-scan has to be repeated. The risk of unin- tended coil displacement is low for rigid, non-ﬂ exible coils. But even for rigid coils the problem might occur that parts of the patients body move to regions that did not contain any structures at the time the calibration took place: a priori, no coil sensitivity informa-

tion is available within such regions. Such may occur, for example, for breath-hold exams, if a deep breath- hold moves the anterior body surface into regions that were not covered during the pre-scan. In such cases, an extrapolation of the pre-scan data is required, which may not be sufﬁ ciently accurate.

The intrinsic property of autocalibration that each slice carries its own calibration data is very appealing.

Neither an interpolation to the scanned slice geometry is necessary, nor is there a problem if the coil position changes during the examination (due to patient move- ments or due to imaging protocols that scan at different table positions). Further on, the calibration lines may not only be used for the calibration but can also be incorporated into the ﬁ nal image, thus increasing the signal-to-noise ratio; however, autocalibration is also accompanied by a number of drawbacks.

Fig. 8.2. Autocalibration method. Additional lines are added to the undersampled acquisition (left) resulting in a fully sampled, low-resolution image for each slice (centre), that may be used to reconstruct the ﬁ nal high-resolution image (right). Upper row:

acquisition scheme in k-space. Lower row: corresponding data sets of upper row after Fourier transform.

undersampled acquisition (with calibration lines)

Low resolution calibration data reconstructed image

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110 ^{M. Nittka}

Firstly, since the calibration data are part of the actual imaging scan, they do also carry the image contrast properties determined by the type and timing of the imaging sequence. This is not so much an issue for k-space based reconstruction algorithms (these beneﬁ t from the fact that the calibration lines are closely related to the image data). But this may cause complications for reconstructions that take place in the image domain:

after the data set is Fourier transformed, the resulting calibration images may be superimposed by strong contrast or phase variations (e.g. gradient-echo sequences with opposed phase for water and fat) making it difﬁ cult to extract valid coil proﬁ les from such images.

A more severe disadvantage may come along with the additional scan time required by the additional calibration lines. If the additional calibration lines make up for just a small fraction of the scanned matrix size, as it is the case for high-resolution protocols, the scan time is increased only by a few percent which can be regarded as well tolerable. If, however, the matrix size decreases and the acceleration factor increases, a high fraction of calibration lines may consume a considerable part of the acquisition time, turning the autocalibration approach ineffective.

Common to all imaging approaches that have to acquire coil calibration data is the question of which spatial resolution is required, since lower resolutions enable much faster calibration procedures. Fortunately, the resolution of the sensitivity map may be considerably lower than that of commonly used imaging protocols, since in general the sensitivity proﬁ les of coils change smoothly; however, care has to be taken, if the coil wires are located close to the skin or if the diameter of the coil elements becomes very small: both effects result in high local ﬁ eld variations leading to reconstruction artefacts if not properly resolved by the calibration scan. Since many surface coil arrays are designed to be as close to the body surface as possible, some vendors suggest to locate additional distance pads between coil and patient, when these coils are used for parallel imaging.

8.3

Tailored Calibration Strategies

The broad range of applications that beneﬁ t from parallel imaging has led to a variety of sophisticated calibration approaches tailored for speciﬁ c applications. The following two methods shall exemplify this for a number of practical aspects.

8.3.1

The Sequence-Based Pre-scan Method

The sequence-based pre-scan method combines the advantage of a fast, separately acquired calibration scan with that of an up-to-date calibration dataset of an autocalibration approach: a calibration scan is run directly prior (or after) each imaging scan (Fig. 8.3).

Thus, each image is supplied with an up-to-date calibration data set.

This approach allows to tailor the calibration scan to the speciﬁ c needs of different applications:

Since the calibration scan is independent from the actual sequence, it may circumvent the problems of autocalibration discussed above (image contrast in calibration data), but keeping the advantage of constantly updated calibration data.

Even though the calibration time has to be spent for each imaging sequence, it can be generally acquired much faster than a single 3D pre-scan, since the acquisition can be restricted to the current slice geometry instead of having to cover the whole volume.

For certain pulse sequence types, the autocalibration approach might not be favourable. For example, single-shot sampling schemes, such as echo- planar imaging (EPI) or single-shot turbo spin echo sequences, strongly beneﬁ t from parallel imaging techniques, because the echo-train length is considerably reduced. Autocalibration would counteract this advantage by inserting additional echoes.

The EPI provides a further example of how a dedicated calibration pre-scan may be adapted to sequence-speciﬁ c calibration problems: EPI is often accompanied with strong susceptibility-induced distortions that might be difﬁ cult to reconstruct on the basis of a non-distorted calibration scan. The reconstruction may be facilitated, if an EPI sequence is employed for the calibration scan as well, thus intrinsically matching the geometric properties of the calibration data to the image data.

8.3.2

Self-calibration Methods

Some applications offer the advantageous situation that they do not require a separate calibration scan at all: the coil calibration data can directly be extracted from the imaging data. Two examples are radial sampling schemes and certain dynamic studies:

If dynamic processes are scanned by succeeding image series, such as commonly performed for

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Fig. 8.3. “Sequence-based pre-scan”

method. A calibration scan is performed immediately before (or after) each imaging sequence. In contrast to a single 3D (whole volume) pre-scan, the calibration scan only has to cover the slice geometry of the current sequence.

Fig. 8.4. Self-calibration scheme for time series. Succeeding undersampled frames of a time series are sampled in an interleaved fashion. The diagram shows a factor-2 accelerated acquisition scheme, where each frame either scans even or odd lines. An non-aliased calibration data set can be extracted for each pair of succeeding frames.

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cardiac cine imaging, self-calibrating parallel imaging can be easily achieved (cf. chap. 12; Kellman et al. 2001). A simple line-sharing scheme from succeeding time frames provides a non-undersampled image data for coil calibration, if the phase-encoding lines are sampled in an interleaved fashion (Fig. 8.4).

The reduced temporal resolution of the calibration data subsets is not a problem, unless the coil proﬁ les change considerably from frame to frame.

It is the very nature of radial sampling schemes that the region towards k-space centre is sampled more densely than outer regions (Fig. 8.5); thus, an inner k-space region with dense sampling may provide the required calibration data to reconstruct missing k-space data in the subsampled outer k-space region.

8.4

Conclusion

The described methods for the measurement of coil sensitivity proﬁ les have advantages and disadvantages in terms of time efﬁ ciency, user comfort, or robust-

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Fig. 8.5. Self-calibration method for radial scanning. The central k-space region of a radial acquisition is sampled with higher density than the outer region; thus, a non-undersampled region for coil calibration may be extracted (grey region).

ness of reconstruction. Ideally, the optimum method should be selected depending on the application, pulse sequence, and anatomical region being imaged; however, in many cases, a certain method will be used because of the development history of the applied software or the general philosophy of the manufacturer who might prefer one of the described approaches.

As a consequence of such differences, commercially available imaging systems still exhibit weaknesses and strengths in different areas of MRI examination.

References

Griswold MA, Jakob PM, Heidemann RM, Nittka M, Jellus V, Wang J, Kiefer B, Haase A (2002) Generalized autocalibrat- ing partially parallel acquisitions (GRAPPA). Magn Reson Med 47:1202–1210

Heidemann RM, Griswold MA, Haase A, Jakob PM (2001) VD- AUTO-SMASH imaging. Magn Reson Med 45:1066–1074 Jakob PM, Griswold MA, Edelman RR, Sodickson DK (1998)

AUTO-SMASH, a self-calibrating technique for SMASH imaging. MAGMA 7:42–54

Kellman P, Epstein FH, McVeigh ER (2001) Adaptive sensitivity encoding incorporating temporal ﬁ ltering (TSENSE).

Magn Reson Med 45: 846–852