Synopsis

This lecture focuses on three pulse sequence strategies to increase spatial resolution, accelerate acquisition, and improve image quality while reducing artifacts. First, strategies for reducing the field-of-view (FOV) are described using examples of spatial saturation, multi-dimensional RF excitation, and selective RF refocusing. Second, reference scans are presented for measuring errors in k-space and enabling various phase corrections in echo-train pulse sequences. Third, gradient pre-emphasis is discussed as an effective method to reduce the adverse effects caused by eddy currents in a variety of pulse sequences. Although these three topics may appear isolated, together they reflect a central theme of how to improve image quality and/or speed while avoiding artifacts.

Highlights

• Reduction in FOV without aliasing can be achieved by selective saturation, selective excitation, and/or selective focusing of the magnetization using various RF pulses and pulse sequences.
• Reducing FOV is an effective approach to obtain high spatial resolution and/or decrease spatial distortion.
• Reference scans are often required to measure errors in k-space data acquired in echo-train pulse sequences.
• A reference scan is typically acquired by turning off the phase-encoding gradient, but other approaches for reference scans are also available.
• Gradient pre-emphasis is necessary to compensate for eddy currents, particularly in fast imaging pulse sequences.
• Different pulse sequences have different “critical eddy current time constants” that are the most problematic in producing image artifacts.
  

Target Audience

Objectives

Introduction

Methods

Imaging with reduced FOV

In both Cartesian and non-Cartesian sampling, it is well known that signals outside the FOV can alias, producing characteristic artifacts depending on the specific k-space sampling strategy¹. To avoid aliasing artifacts, the FOV is typically chosen to accommodate the entire spatial extent of a subject, leading to a limited spatial resolution. Several techniques, however, have been developed to restrict the FOV to a fraction of a regular FOV without being subject to aliasing. This allows high spatial resolution be achieved over a zoomed region. A straight-forward solution is to employ radiofrequency (RF) saturation pulses to eliminate the signals away from the targeted region^1,2. Alternatively, multiple RF pulses can be used with their spatial selection gradients along different directions. For example, a combination of 90° and 180° RF pulses with non-parallel slice-selection gradients can selection a slice with a reduced FOV^3,4. Obviously, these sequence designs interrupt the spins away from the selected regions, compromising multi-slice imaging capability. This problem can be effectively addressed using 2D RF excitation^1,5, together with slice tilting^6–9. The principals involved in the 2D RF pulse design will be described and practical implementations presented.

Reference Scans

Reference scans are typically performed prior to the actual scan for measuring errors in k-space. The measured errors are corrected in the subsequent image acquisition and/or reconstruction. Although phase errors are of primary interest, imperfections in signal amplitude can also be measured using reference scans¹⁰. A common source of phase errors is eddy currents which are particularly detrimental in echo-train pulse sequences that rely on coherence of transverse magnetization. Even on an MRI system with well compensated eddy currents, reference scans are often necessary to measure errors caused by a minute amount of residual eddy currents that can result in non-negligible artifacts¹.

Reference scans are often accomplished by disabling the phase-encoding gradient in an echo-train pulse sequence^11,12. The underlying assumption is that the phase of each echo would be identical in the absence of inconsistent phase errors. Any deviation from this ideal situation can be measured by comparing the phases among the echo signals. Although the phase difference can be decomposed into multiple spatial dependencies, the vast majority of reference scans focus on only two phase components: (a) a spatially constant component or zeroth-order phase, and (b) a spatially linear component or first-order phase. In fast imaging pulse sequences, these two components account for the vast majority of the artifacts, although higher-order components have also been demonstrated important in diffusion-weighted pulse sequences¹³.

Reference scans can alternatively be acquired without disabling the phase-encoding gradient. Examples of these strategy include PLACE¹⁴, phase-encoded reference scan¹⁵, and two-frame phase labeling¹⁶. k-Space errors can be also estimated without acquiring a reference scan^17–19.

Gradient Pre-emphasis

While effects caused by a small amount of residual eddy currents can be corrected using reference scans, the majority of eddy currents are compensated by gradient coil shielding and waveform pre-emphasis. The idea behind gradient waveform pre-emphasis is to intentionally distort the current waveform that is input to the gradient coil, such that the pre-emphasis distortion cancels the predicted eddy current distortion. The effectiveness of waveform pre-emphasis depends strongly on the accuracy of eddy current measurements. With an accurate eddy current measurement, gradient pre-emphasis can reduce eddy current levels by one to two orders of magnitude.

Characterization of eddy currents requires a quantitative model to describe their spatiotemporal features. The spatial dependence is best described using a spherical harmonic decomposition or a Taylor series expansion. The spatially independent term is often called the “B₀ eddy currents”, while the spatially linear terms are referred to as the “linear eddy currents”. The B₀ eddy currents can be compensated by using a separate B₀ coil or by modifying the receiver frequency or phase. The linear eddy currents are most effectively addressed by gradient waveform pre-emphasis. The majority of image artifacts are caused by the B₀ and linear eddy currents.

The temporal dependence of eddy currents is typically analyzed using a linear model, such as the one proposed by Jehenson et al²⁰. This model uses a series of amplitudes and time constants to describe eddy currents with any spatial dependence. It is important to recognize that the sensitivity of different pulse sequences to time constant is drastically different. For example, single-shot echo planar imaging is most sensitive to eddy currents with short time constants (e.g., 400-800 µs), whereas diffusion-weighted pulse sequences with a Stejskal-Tanner gradient pair are sensitive to much longer time constants (e.g., 30-100 ms). For a given eddy current amplitude, the time constant that produces the worst adverse effects is referred to as critical time constant²¹. Several pulse sequences for characterizing eddy currents over a broad range of time constants (e.g., 10 µs – 2 s) will be described.

Acknowledgements

References

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