Investigation of the Saturation Pulse Artifact in Non-Enhanced MR Angiography of the Lower Extremity Arteries at 7 Tesla

<div><p>When performing non-enhanced time-of-flight MR angiography of the lower extremity arteries at 7 T with cardiac triggering, the acquisition time is a crucial consideration. Therefore, in previous studies, saturation RF pulses were applied only every second TR. In the axial source images a slight artifact with an appearance similar to aliasing could be observed. The purpose of this study was to investigate the origin of this artifact. The reason for the artifact is supposed to be related to the two effective TRs during acquisition caused by the sparsely applied saturation RF pulse. Several sequence variants were simulated and implemented within the sequence source code to examine this hypothesis. An adaptation of the excitation flip angles for each TR as well as a correction factor for the k-space data was calculated. Additionally, a different ordering of the k-space data during acquisition was implemented as well as the combination of the latter with the k-space correction factor. The observations from the simulations were verified using both a static and a flow phantom and, finally, in a healthy volunteer using the same measurement setup as in previous volunteer and patient studies. Of all implemented techniques, only the reordering of the k-space was capable of suppressing the artifact almost completely at the cost of creating a ringing artifact. The phantom measurements showed the same results as the simulations and could thus confirm the hypothesis regarding the origin of the artifact. This was additionally verified in the healthy volunteer. The origin of the artifact could be confirmed to be the periodic signal variation caused by two effective TRs during acquisition.</p></div

Sequence comparison for non-enhanced MRA of the lower extremity arteries at 7 Tesla.

In this study three sequences for non-contrast-enhanced MRA of the lower extremity arteries at 7T were compared. Cardiac triggering was used with the aim to reduce signal variations in the arteries. Two fast single-shot 2D sequences, a modified Ultrafast Spoiled Gradient Echo (UGRE) sequence and a variant of the Quiescent-Interval Single-Shot (QISS) sequence were triggered via phonocardiogram and compared in volunteer examinations to a non-triggered 2D gradient echo (GRE) sequence. For image acquisition, a 16-channel transmit/receive coil and a manually positionable AngioSURF table were used. To tackle B1 inhomogeneities at 7T, Time-Interleaved Acquisition of Modes (TIAMO) was integrated in GRE and UGRE. To compare the three sequences quantitatively, a vessel-to-background ratio (VBR) was measured in all volunteers and stations. In conclusion, cardiac triggering was able to suppress flow artifacts satisfactorily. The modified UGRE showed only moderate image artifacts. Averaged over all volunteers and stations, GRE reached a VBR of 4.18±0.05, UGRE 5.20±0.06, and QISS 2.72±0.03. Using cardiac triggering and TIAMO imaging technique was essential to perform non-enhanced MRA of the lower extremities vessels at 7T. The modified UGRE performed best, as observed artifacts were only moderate and the highest average VBR was reached

Ringing artifact.

<p>Ringing artifact generated by the reordering of k-space (black line) compared to the uncorrected artifact image (blue line) and with additional k-space correction factor applied (orange line). On the left (a, c, e) profiles through the images of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119845#pone.0119845.g002" target="_blank">Fig. 2B, E</a> show the relative signal increase compared to the original artifact image within the phantom. On the right (b, d, f) the upper border of the phantom was zoomed in to show artifact signal outside. Note that the scaling of the axes is different.</p

Artifact and sequence diagram.

<p>Axial source image of MRA of the lower extremities at the level of the upper thighs (a). Imaging with two effective TRs leads to an artifact that appears similar to aliasing (arrows). A diagram of the TFL sequence implementation shows the variation in effective TR due to the saturation pulse block preceding every second excitation (b).</p

Flow phantom and volunteer measurements.

<p>Upper row: measurement results in the flow phantom. Only the tubes outside the phantom contained flowing water at constant velocity. The white arrows in (a) point to the aliasing artifact while in (b) the ringing artifact (arrow) is visible that originates from the reordering scheme. In the lower row, in-vivo images are shown with the aliasing artifact almost not visible ((c), arrows). Using a reordered acquisition led to slight ringing artifacts ((d), oblique grey arrow) and to multiple depiction of the arteries (white arrow). Image (c) and (d) are windowed identically.</p

Static phantom measurements.

<p>Results of phantom measurements showing uncorrected artifact image (a), image with k-space correction (b), image with reordered acquisition (c), and image with reordering and k-space correction factor combined (d). (e) shows an estimation of flip angle obtained by reconstructing both signals individually and calculating a flip angle map as described for the AFI method [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0119845#pone.0119845.ref024" target="_blank">24</a>]. In the lower row, profiles through the middle of the phantom (f) and on the right side of the phantom (g) are shown. Positions of the profiles are exemplarily shown in (a). Reordering and reordering combined with k-space correction are almost congruent (black and orange lines in f, g).</p

Improved cerebral time-of-flight magnetic resonance angiography at 7 Tesla--feasibility study and preliminary results using optimized venous saturation pulses.

PURPOSE: Conventional saturation pulses cannot be used for 7 Tesla ultra-high-resolution time-of-flight magnetic resonance angiography (TOF MRA) due to specific absorption rate (SAR) limitations. We overcome these limitations by utilizing low flip angle, variable rate selective excitation (VERSE) algorithm saturation pulses. MATERIAL AND METHODS: Twenty-five neurosurgical patients (male n = 8, female n = 17; average age 49.64 years; range 26-70 years) with different intracranial vascular pathologies were enrolled in this trial. All patients were examined with a 7 Tesla (Magnetom 7 T, Siemens) whole body scanner system utilizing a dedicated 32-channel head coil. For venous saturation pulses a 35° flip angle was applied. Two neuroradiologists evaluated the delineation of arterial vessels in the Circle of Willis, delineation of vascular pathologies, presence of artifacts, vessel-tissue contrast and overall image quality of TOF MRA scans in consensus on a five-point scale. Normalized signal intensities in the confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter were measured and vessel-tissue contrasts were calculated. RESULTS: Ratings for the majority of patients ranged between good and excellent for most of the evaluated features. Venous saturation was sufficient for all cases with minor artifacts in arteriovenous malformations and arteriovenous fistulas. Quantitative signal intensity measurements showed high vessel-tissue contrast for confluence of venous sinuses, M1 segment of left middle cerebral artery and adjacent gray matter. CONCLUSION: The use of novel low flip angle VERSE algorithm pulses for saturation of venous vessels can overcome SAR limitations in 7 Tesla ultra-high-resolution TOF MRA. Our protocol is suitable for clinical application with excellent image quality for delineation of various intracranial vascular pathologies

Non-enhanced MR imaging of cerebral aneurysms: 7 Tesla versus 1.5 Tesla.

PURPOSE: To prospectively evaluate 7 Tesla time-of-flight (TOF) magnetic resonance angiography (MRA) in comparison to 1.5 Tesla TOF MRA and 7 Tesla non-contrast enhanced magnetization-prepared rapid acquisition gradient-echo (MPRAGE) for delineation of unruptured intracranial aneurysms (UIA). MATERIAL AND METHODS: Sixteen neurosurgical patients (male n = 5, female n = 11) with single or multiple UIA were enrolled in this trial. All patients were accordingly examined at 7 Tesla and 1.5 Tesla MRI utilizing dedicated head coils. The following sequences were obtained: 7 Tesla TOF MRA, 1.5 Tesla TOF MRA and 7 Tesla non-contrast enhanced MPRAGE. Image analysis was performed by two radiologists with regard to delineation of aneurysm features (dome, neck, parent vessel), presence of artifacts, vessel-tissue-contrast and overall image quality. Interobserver accordance and intermethod comparisons were calculated by kappa coefficient and Lin's concordance correlation coefficient. RESULTS: A total of 20 intracranial aneurysms were detected in 16 patients, with two patients showing multiple aneurysms (n = 2, n = 4). Out of 20 intracranial aneurysms, 14 aneurysms were located in the anterior circulation and 6 aneurysms in the posterior circulation. 7 Tesla MPRAGE imaging was superior over 1.5 and 7 Tesla TOF MRA in the assessment of all considered aneurysm and image quality features (e.g. image quality: mean MPRAGE7T: 5.0; mean TOF7T: 4.3; mean TOF1.5T: 4.3). Ratings for 7 Tesla TOF MRA were equal or higher over 1.5 Tesla TOF MRA for all assessed features except for artifact delineation (mean TOF7T: 4.3; mean TOF1.5T 4.4). Interobserver accordance was good to excellent for most ratings. CONCLUSION: 7 Tesla MPRAGE imaging demonstrated its superiority in the detection and assessment of UIA as well as overall imaging features, offering excellent interobserver accordance and highest scores for all ratings. Hence, it may bear the potential to serve as a high-quality diagnostic tool for pretherapeutic assessment and follow-up of untreated UIA

Sequence diagrams of GRE, UGRE and QISS.

<p>In GRE, no cardiac triggering is used. Here, the TIAMO trigger changes the excitation mode directly after every excitation pulse, which means that both modes needed for a complete slice are acquired in an interleaved fashion. Saturation pulses are applied every TR. In UGRE, the cardiac trigger event starts the acquisition of a complete slice in a single shot. Due to TIAMO, this acquisition has to be repeated to acquire the same slice with the second mode. Venous saturation RF pulses are applied only sparsely. QISS uses three different saturation pulses to prepare the image contrast: In the imaging slice to suppress background tissue, a travelling venous saturation pulse below the imaging slice and a fat saturation pulse in the imaging slice. After the venous saturation, a time interval QI allows unsaturated arterial blood spins to enter the imaging slice. No saturation pulses are applied during the single-shot slice acquisition. The duration of the trigger events is shorter than pictured in these diagrams.</p