Quantitative T2 mapping of the carotid artery has been demonstrated as a promising technique for characterising atherosclerotic plaque¹. The method uses a multiple spin-echo (MSE) sequence to acquire data at 14 echo-times for fitting of T2. In the current multi-slice implementation, phase-encode lines are acquired sequentially with a ~2s repetition time (TR) between lines. This acquisition scheme is sensitive to motion during and between phase-encode lines, which generates inconsistencies in k-space leading to ghosting and blurring in the reconstructed images. The majority of artifacts observed in acute patient scans are believed to be related to swallowing or coughing.

Previous approaches to mitigating swallowing artifacts include 1D tracking of the epiglottis, self-gating, and reacquisition with free induction decay navigators^2-4. In this study, a navigator acquisition, consisting of a single phase-encode line at $$$k_y=0$$$, was added after the MSE acquisition to identify echo-trains affected by swallowing or coughing movements^5,6.

The duration of each echo-train was increased by 9.1ms to accommodate the navigator (see Fig. 1) with no increase in TR required for an acquisition with 5 slices.

Reacquisition decision: the sum of the magnitude signal across all channels in each navigator was used as a “slice score” $$$S_{sl}(k_y)$$$ for each echo-train:

$$S_{sl}(k_y)=\sum_c\sum_{k_x} |N_{sl}(k_x,k_y,c)|\tag{1}$$

where $$$sl$$$ refers to the slice number and the complex navigator signal for slice $$$sl$$$ and line $$$k_y$$$ at k-space location $$$k_x$$$ in channel $$$c$$$ is given by $$$N_{sl}(k_x,k_y,c)$$$. TR periods were ranked and identified for reacquisition based on the sum of slice scores within each TR block (5 slices in this study); we call this the “TR score” $$$S_{TR}(k_y)=\sum_{sl}S_{sl}(k_y)$$$. The lowest TR scores were reacquired at the end of the scan with the number of reacquisitions specified by the scanner operator in advance.

Replacement decision: replacement for each $$$k_y$$$ line was determined on a slice-specific basis using the “slice score” $$$S_{sl}(k_y)$$$. The originally-acquired data were replaced with the reacquisitions when the original slice score was lower than the corresponding reacquired slice score. This approach is intended to reacquire a block of slices with the same $$$k_y$$$ line index by detecting a drop in TR score, which can be affected by only one slice, but then only replace the affected slice by judging replacement using slice scores.

Five healthy volunteers were scanned under a technical development ethics agreement on a Siemens Verio 3T scanner with a 4-channel surface coil. The following acquisition parameters were used: 14 echo-times ranging from 9.1 to 127.4ms at 9.1ms intervals, TR=2s, FOV=128×128mm², matrix size=192×192, 5/8 partial Fourier and five 2mm slices (100% slice gap). A 60ms DANTE preparation module was used for flowing spin suppression⁷ with the following parameters: slice gradient amplitude 18mT/m, 120 pulses, flip angle 8°, 500μs between RF pulses, 340μs gradient duration.

Three volunteers were scanned with 15 reacquisitions and two were scanned with 30 reacquisitions. Scan times with 15 and 30 reacquisition TR periods were 4:36 and 5:06min, respectively (additional 30 and 60s).

Images with reacquisition replacement described above were presented on the scanner. The “without reacquisitions” versus “with reacquisitions” images shown in comparisons below were reconstructed offline in Matlab (Mathworks).

Figure 2 shows example plots of the TR scores (used to judge reacquisition) for each phase-encode line. The drops in TR scores correspond well to periods when the subject was asked to move intentionally (swallowing).

Figure 3 compares the images without and with reacquisitions in a scan with 15 reacquisitions. Figure 4 demonstrates the differences in ghosting and blurring around the carotid wall in scans with 30 reacquisitions. Figure 5 shows the effect of motion during the acquisition of the central k-space lines and the improvement possible with reacquisition.

The addition of a navigator line at the centre of k-space allows identification and reacquisition of data affected by motion on a slice-specific basis. Images using reacquired data show reduced blurring and ghosting compared to the original images without reacquisition.

Investigating the reliability of the TR score and testing the reacquisition scheme in patients will be the subject of future work. The reacquisition technique is expected to reduce artifacts caused by occasional motion under the assumption that the subject returns to the original position after moving. Prospective motion techniques that can be used for neuroimaging are challenging in this setting due to the non-rigid body motion of the neck.