Slice profile effects on nCPMG SS‐FSE

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Excerpt

Magnetic resonance imaging (MRI) technology has evolved over the years as a robust imaging modality that enjoys widespread use in the clinic. Despite the advances in MRI, this technique has been fundamentally limited by slower acquisition times compared to other modalities such as CT. To reduce scan times, many protocols use a single‐shot acquisition, where the entire image signal is captured in one repetition time (TR). A common example is the single‐shot fast spin echo (SS‐FSE).
The SS‐FSE sequence is able to produce images near areas of strong off‐resonance without distortion. This is due to the fact that the signal is refocused at each echo, thereby reducing phase accrual due to off‐resonance and the resultant image distortion and signal loss due to intravoxel signal dephasing. A SS‐FSE sequence is generally designed to satisfy the Carr‐Purcell‐Meiboom‐Gill (CPMG) condition 1 to preserve high signal level throughout the echotrain as well as a smoothly varying signal from echo to echo that avoids introducing ghosting artifacts in the reconstructed image. In particular, the Meiboom‐Gill condition requires that the excitation radiofrequency (RF) pulse and subsequent refocusing RF pulses must be constructed as JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM1/v/2017-12-21T175206Z/r/image-png . Here, JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM2/v/2017-12-21T175206Z/r/image-png is the time between excitation and the first refocusing pulse and JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM3/v/2017-12-21T175206Z/r/image-png is the time between each refocusing pulses afterward. This JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM4/v/2017-12-21T175206Z/r/image-png spacing is commonly known as the echo spacing.
There are many factors that can prevent an SS‐FSE sequence from being able to satisfy the CPMG condition. For example, the introduction of a magnetization preparation module following the excitation can result in an unknown magnetization phase leading into the echotrain. As a result, the phase of the transverse magnetization at half of the echo spacing ( JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM5/v/2017-12-21T175206Z/r/image-png ) in advance of the first refocusing pulse cannot be guaranteed to be parallel or anti‐parallel to the phase of the first refocusing pulse. This violation of the MG condition has profound consequences on image quality, namely banding, distortion, and signal dropouts.
Several approaches have been proposed to phase modulate the refocusing train 3 to allow imaging using a SS‐FSE sequence that is immune to variations in starting phase while maintaining a signal comparable to CPMG SS‐FSE. Here we focus on the quadratic phase modulation scheme that has been proposed 3 as a non‐Carr‐Purcell‐Meiboom‐Gill (nCPMG) SS‐FSE sequence. Despite the promise that a nCPMG SS‐FSE sequence offers, some drawbacks exist. First, image reconstruction is made more complex as the quadrature component of the signal receives a ±1 modulation throughout the echotrain that leads to JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM6/v/2017-12-21T175206Z/r/image-png aliasing in the image space. This issue has been previously discussed at length in the original publication 3 and subsequent work 5. Second, the refocusing RF pulses must provide a consistent flip angle above JOURNAL/mrim/04.02/01445475-201801000-00043/math_43MM7/v/2017-12-21T175206Z/r/image-png from one echo to the next throughout the echotrain as reported in Ref. 6 in order for the sequence to achieve a stable signal. This precludes using variable low flip angle refocusing pulse schemes that are used to reduce the SAR for SS‐FSE sequences 7. This also implies that the excited signal must experience a uniform flip angle throughout the slice.
The focus of this work addresses how to best design the refocusing pulse given the nCPMG requirement of exceeding a minimum flip angle to maintain a stable echotrain signal. We investigate two options: first, a low time‐bandwidth RF pulse (low selectivity) with an effective slice width much larger than the excited slice, and, second, a higher time‐bandwidth RF pulse (high selectivity) whose sharper slice profile allows a reduction in the overspecification of the refocusing slice width.

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