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Chapter 6

Composition of MR Images

Localization of Spins with Field Gradients

Excitation of Selected Spins

Spin Echo
Gradient Echo
Spatial Encoding

Frequency Encoding
Phase Encoding
Tomographic (2D) Slices

Slice Definition
Slice Selection
Multiple Slices

The Complete Imaging Experiment

Frequency Encoding
2D Fourier Transform
Partial Fourier Imaging

Three-Dimensional Fourier Imaging

Parallel Imaging

06-10 Parallel Imaging

An advanced approach for image acquisition uses multiple RF receiver coils with independent RF pathways, known as phased array or synergy surface coil ar­rays (see Chapter 3).

In these arrays, each individual coil produces a signal of its own. Several independent image data sets can be acquired at a time and, after post­pro­ces­sing, synthetized into a single image.

Commonly, a combination of such overlapping multiple receiver coil elements is utilized to improve the signal-to-noise ratio (Figure 06- 23).

In the sensitivity profiles of the in­di­vi­du­al receiver coil elements there is spatial information con­tain­ed, and multiple phase-encoded data can be derived at the same time. Thus, the number of gra­dient- based spatial encoding steps can be decreased and conventional Fourier encoding reduced.

Figure 06-23:
Parallel imaging with an array of two coils: each coil provides half of the field-of-view of the final image; thus, only half of the phase-encoding steps have to be acquired.

This procedure is based upon dedicated adjuvant reconstruction algorithms, among them SENSE (Table 06-01) [⇒ Pruessmann], and SMASH. These al­go­rithms must not be confused with pulse sequences although their acronyms sound similar. In principle, they can be applied to any imaging sequence; con­trast behavior does not change.

Table 06-01:
Adjuvant parallel imaging reconstruction algorithms. The names are different, but the approach is the same.

Both SMASH and SENSE algorithms reconstruct missing data to obtain an image without backfolding artifacts. The way that this is actually done re­pre­sents the main difference between the two techniques.

SMASH will perform this calculation on the raw data, before the Fourier trans­form, while SENSE will work with the images obtained from the respective coils. SENSE and SMASH are based on knowledge of the sensitivity profiles of the individual coil elements. This knowledge is acquired either through a very low resolution 3D volume scan (64×64×64 matrix) which then can be used for all consequent scans independent of orientation, or through adding some additional phase encoding steps to each reduced acquisition. Therefore, from a practical point of view, the first solution will be time effective if more scans are per­for­med, the latter is to be preferred if only one scan is wanted.

SENSE, PILS, and ASSET reconstruct the final image from the sub-images produced by each coil after the Fourier transformation in the image domain, whereas GRAPPA reconstructs the Fourier plane of the image from the fre­quen­cy signals of each coil before the Fourier transformation.

SENSE and its relatives work with most pulse sequences and clinical ap­pli­ca­tions. Moreover, they also allow the user to choose between either increased spa­tial or temporal resolution [Review articles: ⇒ Blaimer; ⇒ Larman].

The downside of parallel imaging is that the signal-to-noise ratio is reduced compared to phased-array imaging. The reduction in signal-to-noise does no longer follow the common square-root dependence because of the non-Cartesian sampling and the noise correlation between pixels.

Critical Remarks. Since with GRE and TSE sequences one can acquire an image with a full matrix in 2-3 minutes, there is often no real need for parallel imaging and therefore a loss of signal-to-noise. It is used in dynamic imaging and 3D imaging where saving time is important and one can afford to trade off some signal.

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