High-Resolution Diffusion-Weighted Breast MRI Acquisition

EPI Parameter selection

Single-shot EPI is one of the fastest approaches used for image acquisition in MRI, offering images in well under 100 ms and often much faster. Typically, the EPI trajectory is designed to maximize imaging speed. The readout gradient amplitude is maximized given limits on gradient amplitude and sample spacing (1/FOV _x ). In many cases, sampling occurs on both the readout ramps and plateaus that are shown in Fig. 12.1 , with the readout gradient duration sufficient to achieve the desired k _x extent and spatial resolution in x . Multiple repeated readout lobes enable sampling of more k-space lines in both directions, which maximizes speed. A phase-encode gradient initially sets the starting k _y location, while “blip” gradients between each readout lobe skip by 1/FOV between k _y to satisfy Nyquist sampling. The number of k _y lines provides sufficient extent in k _y to achieve desired phase-encode resolution. The alternating sampling typically results in ghosts, known as “FOV/2” ghosts or Nyquist ghosts, due to delays or phase differences between the readout gradients of opposite polarity induced by eddy currents or other imperfections. Typically, a reference scan is built into protocols, and enables very good correction of these ghosts, by measuring any phase and delay variation between all readout lines and correcting data for these before Fourier reconstruction.

Limitations of Epi: Readout Duration, Distortion, and Blurring

Single-shot EPI acquires all data for a slice following one excitation. The primary limitations of this approach are that the signal changes in both magnitude and phase during the acquisition. The magnitude change is due to T2* decay and leads to a loss of resolution or blurring in the phase-encode direction. Often half-Fourier acquisitions are used to sample k _y from near the center out to the edge. This reduces the EPI duration and the amount of signal decay, at a cost of minor blurring from the partial-k-space reconstruction. The resulting shortened TE also reduces the inherent T2-weighted contrast that leads to T2 “shine-through” effects. Any off-resonance, caused by B0 inhomogeneity or chemical shift, causes linear phase accrual between k _y lines that depends on the echo spacing (T _esp ) . B0 inhomogeneity leads to spatially varying distortion, especially around the nipple, whereas the chemical shift frequency difference between water and fat causes a constant displacement of the fat with respect to water signal. Neither the off-resonance-induced distortion nor the fat/water displacement is affected by the use of half-Fourier sampling, because the k _y velocity described in the next section is unchanged. Phase and amplitude differences along the readout (k _x ) are smaller and usually can be ignored.

Fov Relationship

To better understand the distortion effects, it is useful to point out the fundamental relationship between displacement, the off-resonance effects, and the FOV. To support a given FOV in the phase-encode direction (FOV _y ) requires a k-space line separation of ∆ k _y = 1 / FOV _{y .} If the echo spacing is T _esp , the k _y velocity is ∆ k _y /T _esp = 1 / (FOV _y T _esp ), and the displacement ∆ y as a function of off-resonance ∆ f is ∆ y = FOV _y ∆ fT _esp . This is a fundamental relationship with EPI, that off-resonance-induced distortion is proportional to the frequency offset FOV _y and echo spacing T _esp .

For typical parameters of T _esp = 1 ms, FOV = 20 cm, and a 50-Hz frequency offset, the displacement is 1 cm. At 1.5 T and 3 T, respectively, the fat/water displacement is approximately 4.4 and 8.8 cm, which suggests the importance of fat suppression. Methods to reduce this displacement must reduce FOV _y or T _esp and are one of the main advances in breast DWI, as discussed later. Note that many of the approaches to reduce distortion will also reduce blurring, as they tend to shorten the readout duration and thus the amount of T2* decay.

Distortion Correction

Image distortion in EPI results primarily from B0 field variations from sources such as air-tissue boundaries or metallic biopsy markers. When the field variation is sufficiently smooth and measurable, the resulting distortion can be corrected by an image “warping” that remaps signal to original pixel locations. More rapid changes in B0 lead to “pile-up” effects, where the signal from multiple locations is mapped to one voxel, and require more advanced correction. Currently a common approach is to repeat the EPI acquisition with the phase encode gradients flipped in reverse polarity directions. Either using the data itself or in combination with a field map, the distortions can then be corrected, and the repeated acquisitions can recover more signal than a single acquisition with a field map-based correction, due to averaging. Distortion correction is particularly important when pixel-wise comparisons are desired between different sequences (e.g., to calculate the ADC maps).

Navigator Acquisitions

Navigator acquisitions are a general approach to acquire additional data to measure and possibly correct for motion during a scan. The large area of diffusion-encoding gradients, on the order of tens of degrees of rotation per micron, means that bulk motion of a few microns between the two gradients can impart substantial phase. In multishot imaging, this phase will modulate the k-space data resulting in different artifacts depending on the order in which k-space lines are acquired. Even in single-shot imaging, if a complex average of images is used (to reduce noise bias), the phase variations must be addressed. One approach to this is to add a second spin echo to the sequence and acquire a low-resolution, full-FOV image that ideally has the same motion-induced phase as the data acquired on the first spin echo. The phase of the navigator can then be used to reduce or remove phase variations in the data from the first spin echo of different excitations so that the overall image artifact is reduced or removed. Navigators have been used extensively for different multishot EPI approaches (described shortly) and for non-EPI approaches. Although they can lengthen the scan time per slice, resulting in fewer interleaved slices, they can additionally offer some correction for respiratory motion and other motion that can degrade images.

Summary

Fat-suppressed single-shot EPI is very commonly used for DWI, because the rapid readout avoids motion artifacts that would result in multishot techniques, and most importantly the bulk motion–induced phase can usually be ignored. This sequence is widely available on most MRI systems, with careful corrections built in. The main limitations of distortion and blurring have been studied extensively and are described in detail in the remainder of this chapter. Specifically for breast MRI, the choice of fat suppression is important, and different approaches are described next.

Chemically Selective Fat Suppression

Chemically selective fat suppression, often simply “fat saturation,” uses a spectrally selective pulse to selectively excite only the fat signal, as shown at the start of the sequence in Fig. 12.1 . An applied gradient then dephases the fat spins so that they contribute no signal. The standard excitation and imaging follow the fat saturation block, with timing unaffected by the fat saturation. Fat saturation is simple and fast but can suffer from sensitivity to B0 and B1 variations. In the breast, susceptibility variations from proximity to the lungs or air-tissue boundaries near the chest wall, axillae, nipple, or the inferior portion of the breast often cause fat suppression to fail. This occurs when the broad spectral fat peak shifts outside the suppression bandwidth of the pulse. Fat saturation can use a simple 90-degree excitation or an inversion followed by an inversion time. Adiabatic spectral inversion recovery (e.g., ASPIR or SPAIR on different systems) is sometimes used to reduce sensitivity to off-resonance at a cost of increased RF power.

Stir

Short TI inversion recovery (STIR) suppresses fat based on the shorter T1 (typically 250 ms at 1.5 T and 350 ms at 3.0 T) of fat compared with that of other tissues (typically over 1 second) in the breast. As the name suggests, STIR uses in inversion pulse, which is usually not spatially selective, to invert all signal. After an inversion time (TI) of 150 to 180 ms, normal excitation and imaging proceeds. This time is carefully chosen to null the signal from fat (i.e., based on e ^{− TI/T 1} ≈ 0.5). A strength of STIR is the robustness to static magnetic field inhomogeneities, and with the commonly used adiabatic inversion pulses, additional robustness to transmit B1 field variations. The main drawback of STIR is that the inversion-recovery causes a loss in signal of about 30% to 40%, which translates to a need to double the number of averages to achieve adequate SNR. Additionally, it is challenging to interleave other slices within the inversion time, so the number of slices that can be interleaved within one repetition is reduced, which reduces efficiency. Finally, in cases where DWI is performed after DCE, the presence of contrast can result in nulling of other tissue or tumor because of its shortened T1, which can be similar to that of fat.