To study primarily the influence of MRI scanning position on ultrasound and MRI fusion imaging accuracy prospectively in elderly volunteers. Secondly to explore the feasibility of using gadolinium doped anesthetics for studying the distribution of local anesthetic injectate following block of the lumbosacral plexus.

MR imaging in different positions
Under the hypothesis that the lumbosacral plexus remains in the same position during change from the supine position normally used during MRI to the lateral decubitus position normally used for ultrasound guided lumbosacral plexus blocks, we scanned 25 elderly healthy volunteers in the supine and in a lateral decubitus position with a 70 cm bore 1.5T Philips Ingenia scanner. The dS flex coverage posterior coil and a flex coverage anterior coil with 16 elements was used for signal reception. A 3D T2-TSE sequence (VISTA) with an isotropic scanned resolution of 1.2 x 1.2 x 1.2 mm3 (overcontiguos 2.4 mm slices with 1.2 mm spacing), TE 60 ms and TR 1200 ms was used. Coronal images of the spine and its surrounding structures from L1 to S4 using feet-head phase encoding for minimizing artefacts due to respiration and peristaltic (Fig. 1) were obtained with a scan time of 6:26 min.

MR imaging of local anesthetics
Physical compatibility between the gadolinium contrast agent Dotarem® (27.9% gadoterate meglumine) and the local anesthetics 2% lidocaine with 0.0005% epinephrine, 0.75% ropivacaine hydrochloride, and 0.5% bupivacaine hydrochloride was investigated because no documented compatibility of the combination could be found despite previously use.¹ After examination for visible color change and particle formation, subvisible particle formation, and pH change, we confirmed that Dotarem® is physical compatible with all three local anesthetics, ^2,3 and established a MR imaging protocol for visualizing the spread of local anesthetics.

Image analysis
The images obtained in the supine and in the lateral decubitus positions were rigidly registered (OsiriX, Pixmeo, Switzerland) using the lateral part of the transverse process of L4 and L5, respectively, as reference because this is a key reference point in the ultrasound guided procedure. The distances between the targets of the local anesthetic, spinal nerves L3 and L4, in both positions were measured (Fig. 2). The mean (±SD) or the median (IQR) distance between the spinal nerves in the two positions was calculated depending on the normality of data.

T1 weighted 3D gradient echo breathheld mDixon water scans, visualizing the gadolinium doped injectate (Fig. 3), were combined with DWI based visualization (B-values: 0 and 500 s/mm2) of the target nerves. To discriminate between the injectate and the nerves, ADC maps were used. The spread of the injectate could hereafter be compared to sensory blockade after the injection.⁴ This approach was used in 26 healthy volunteers in a recent prospective randomized controlled crossover trial aiming to compare the effectiveness of the lumbosacral plexus block guided by ultrasound vs. guidance with MRI/ultrasound fused images.

Table 1 (Fig. 5) displays the distances in mm between the spinal nerves in the supine and in the lateral decubitus position recorded on MRI. The initial results of the randomized controlled trial, comparing ultrasound guidance of lumbosacral plexus block with MRI/ultrasound fused images guidance, demonstrated that it was possible to obtain adequate MRI image quality to identify the nervous structures and to perform meaningful MRI/ultrasound fusion guided blocks of the lumbosacral plexus in all volunteers.Only slight displacement were seen of the L3 and L4 spinal nerves in different MRI scanning positions – most likely because the lumbosacral plexus is situated in the posterior part of the major psoas muscle, juxtapositioned to the rigid lumbar vertebral column. Considering fused images can be adjusted in all three planes, we consider these minor displacements to be clinically acceptable. In perspective, elderly patients with a damaged hip can be MRI scanned in a realistic supine position and have a MRI/ultrasound fused images guided lumbosacral plexus block at the L4-L5 level in the lateral decubitus position with a reduced risk of possible detrimental cardiovascular and respiratory side effects. A randomized controlled trial will further explore the possible benefits in effectiveness and safety of lumbosacral plexus blocks guided by MRI/ultrasound fused images.MRI to be used for MRI/ultrasound fusion based regional anesthesia of the lumbosacral plexus can be performed in a realistic supine position for fusion with ultrasound used in the lateral decubitus position. Detailed analysis of a recently performed prospective randomized controlled crossover study will reveal if MRI/ultrasound imaging guidance are superior to ultrasound imaging guidance for securing effective regional anesthesia of the lumbosacral plexus.