The challenges of conducting in vivo WASPI are two folds: special requirements for the transmit and receive hardware (a strong B1 field and a very short receiver dead time) which unmodified clinical MR scanners do not provide, and suppression of fluid state signals.
Even with a high powered transmit amplifier providing the order of 35 kW of pulse power, the B1 field of the body RF coil is insufficient to properly excite the wide spectral bandwidth of solid substances with a single hard pulse in the range of 10 – 20 μs. A local transmit coil is therefore essential. The small diameter quadrature birdcage T/R coil utilized in this study is suitable for WASPI of human extremities, but imaging of the central skeleton will require improvements in technology.
Because clinical MR scanners use pulse sequences based almost exclusively on spin or gradient echoes, the long receiver recovery time is never a limitation. However, the recovery time is a critical element when imaging FIDs. The 40 μs of lost data (equivalent to 8 complex data points at a 5 μs dwell time) when using the standard PIN diode T/R switch resulted in unrecoverable distortion, even when supplemented with acquisition of additional projections at low gradient strength to recover the lost data points (34
). The improvement achieved by acquiring additional projections is limited when the T2
of the tissue is short and the number of missing data points is considerable (eight points in this case), because the measurement of these many missing central k-space data would occur far in time following the RF pulse where the loss in SNR due to T2
decay is considerable. The crossed diode T/R switch reduced the lost data points to two, resulting in a dramatic improvement in image quality.
Both projection pixel size Δxp
and intrinsic pixel size Δxi
determine the resolution of WASPI images. In general, with stronger projection gradients, Δxi
is smaller (Eq. 3
), and the image resolution is improved (). When the maximum available gradient strength is applied, Δxi
reaches its minimum, and any improvement of the image resolution would be limited by Δxp
, achieved either with smaller FOV or more projections. There is a trade-off between resolution, SNR, and total acquisition time. In this study, the maximum gradient strength is 30 mT/m, and the corresponding intrinsic Δxi
is 1.5 mm for a resonance linewidth of 2 kHz. Δxp
would be 1.6 mm if the FOV was 80 mm and the number of independent pixels was 51 (8148 projections). The total acquisition time would be 18 min with TR = 65 msec, and the number of averages = 2. If the number of independent pixels was 40 (5216 projections), Δxp
would be 2 mm, and the total imaging time, including the second acquisition for the missing initial FID data points, would be reduced to 12 min. Our study subjects found that it was easier for them to keep their wrists still inside the coil during a 12-min scan than during an 18-min scan, reducing discomfort and motion artifacts. The protocol of 5216 projections and a 12 min scanning time was considered a reasonable choice for human wrist WASPI scanning.
Theoretically, the best suppression of water and fat signal would be achieved by two consecutive low power long duration chemically selective 90° pulses each followed by spoiler gradient pulses (35
). In practice, it is not easy to set the exact flip angle inside bone tissue, particularly when part of the wrist is very close to the coil. It was found that the scanner-calculated 2 ms 90 pulse was not the most effective suppression pulse. The suppression pulse was empirically optimized by adjusting the pulse amplitude and duration on pig leg specimens. It was found that optimum suppression results in pulses that are larger in amplitude than what the scanner calculates to be a 90° pulse. were acquired with these manually optimized 3 ms pulses. However, these stronger pulses also reduced the solid bone matrix signal. In WASPI experiments on the polyethylene cylinder, it was found that the WASPI signal intensity of the polyethylene acquired with 2 ms scanner calculated 90° suppression pulses was reduced to 75% of the non-suppressed signal level, while the 3 ms optimized pulses (which had higher RF amplitude) reduced the WASPI intensity of the polyethylene to 50% of its non-suppressed signal level. This is another reason that the SNR of was lower than the SNR of . Longer suppression pulses, which are more spectrally selective for fluid state signals, will reduce but not eliminate the loss of solid state MR signals because the solid state resonances are homogeneously broadened and are significantly affected by off resonance radiation. It is easy to suppress narrow water resonances with long suppression pulses, but for fat resonances, which are typically on the order of 400 Hz in width, suppression pulses longer than 3 ms will not suppress the fat signal down to less than 5% of its initial signal intensity. We attempted to narrow the fat resonance line width with an image-based analytic shimming technique (40
), but the resulting line widths were no narrower than could be achieved with manual shimming. The trade-off between soft tissue suppression and matrix SNR therefore needs to be considered when optimizing the WASPI sequence, and the trade-off may be different for applications requiring clean visualization versus good quantitative matrix density. In the bone specimen experiments, a tube of bone marrow was used to serve as an indicator of the suppression (34
). For in vivo
human wrist imaging, the suppression of the water signal of the forearm muscle and subcutaneous fat can serve as intrinsic indicators of the suppression.
Concern over inflow of fresh blood into the FOV compromising the soft tissue suppression proved to be unfounded. Although the blood vessels are very bright in spin-echo images and caused artifacts in the phase encoding direction (), these vessels did not appear in WASPI images and did not create artifacts. This could be due to a combination of the absence of echoes in the WASPI sequence, the short time between the suppression pulses and the FID acquisition, and manner in which motion artifacts affect radially acquired k-space data.
It is striking how the WASPI images resemble plane film x-ray radiographs, with soft tissue dark and solid bone bright, but of course the sources of WASPI and x-ray signals are quite different. The WASPI signal arises from a portion of collagen protons, tightly bound water, and other motionally restricted molecules. The x-ray signal arises predominantly from the mineral content. Although these constituents have similar spatial distributions in bone, the distributions are not identical, which makes WASPI useful for measuring matrix density, which in combination with mineral density information yields the degree of mineralization. Note that the clear visibility of tendons in WASPI images may prove useful in evaluating these tissues by MRI.
In Conclusion, WASPI imaging of human wrist bone matrix with clinical MR scanners is feasible, provided that the transmit/receive system meets the requirements of strong B1 field and short receiver recovery time. A physical support and restraint system that is invisible in solid state MR images is required to stabilize the wrist during scanning to eliminate motion artifacts. WASPI images with resolution of 2 mm can be acquired in 12 min, showing only solid bone matrix and tendons. If calibration phantoms are imaged with the wrist, bone matrix density can be quantified.