Local Linear Shimming for Cardiac SSFP Imaging at 3T

The steady state free precession (SSFP) sequence is the method of choice for the assessment of cardiac function at 1.5T. The higher Bo field inhomogeneities at 3T cause large artifacts near the posterior vein of the left ventricle. Therefore more attention has to he paid to the shimming procedure. A local shim tool based on Bo-maps was implemented to calculate lhe shimming parameters. The available linear shim sets at the 3T system were used to suppress artifacts for inviva imaging. Nearly artifact-free SSFP cardiac images are demonstrated. Introduction Steady state free precession (SSF") sequences, also called balanced fast field echo or true FISP [I], offer a high contrast between myocardium and blood. The signal-to-noise ratio is higher in comparison to standard spoiled gradient echo protocols. SSFP therefore is suitable to study the function of the heart [2]. Shorter T*2 values and the larger Bo inhomogeneities within the left ventricle (LV) make fast imaging of the heart more difficult at 3T compared to lower field strengths [3]. Especially adjacent to the posterior vein of the LV (PVLV) high Bo field inhomogeneities were measured. The purpose of this work was to develop a local shim tool to correct for the Bo field inhomogeneities in the LV in order to acquire artifact-free images. Methods We implemented a local shim tool using IDL (Research Systems, Inc., Boulder, CO) on a PC. The shimming procedure is based on a Bo-map. The Bil-map is the difference of two phase-images measured at different echo times (TE) and it therefore reflects the field inhomogeneities. The two phase-images were measured with a multi-slice dual echo gradient echo sequence. On the modulus image the region of interest (ROI) is selected. Possibly the Bo-map has to he unwrapped. Using a linear shim the field inhomogeneities can be accounted for with U I Therefore the gradient offsets G,, G,, and G, have to be determined. The measured Bo-map is the phase @ accumulated during At = TE2 TE, as given in P I In the 12-bit phase images a scale of 4096 is equivalent to 271. With the gyromagnetic ratio of protons y / (2n) = 42.577 M H f l an array B for all points r within the ROI is defined as AB0(x, Y , Z ) = G, . x + G , . y + G, . Z. @(F) = y . At , ABo ( F ) The shimming parameters are calculated solving the equation AX = B, with A = [[x, y, z, 11. I, an array for every point in the ROI, X the parameters to be evaluated (the shim gradients with the offset frequency Af.: [Gx, Gy, Gz, AfJ), and B as defined above. The equation is solved using a constraint Levenberg-Marquardt least-squares minimization (MINPACK-1) provided by C.B. Markwardt (NASNGSFC Code 662, Greenbelt, MD 20770). Measurements were performed on a 3T Intera MR whole body system (Philips Medical Systems, Best, The Netherlands) equipped with a transmitlreceive body coil and a 6-element cardiac coil. For the acquisition of the BO-map one cardiac coil element was used, for the SSFP scans a five element coil array was positioned around the subject's chest. The images were acquired in healthy volunteers. After the survey scans the Bo-map was measured at the position of the planned SSFP function scans: 3 overcontiguous (5mm) slices with a thickness of lOmm and a field of view of 300mm by 225mm. The acquisition was vector ECGtriggered with a trigger delay of 500ms. Respiratory motion was compensated using a navigator echo based gating and tracking technique using a gating window of 5mm. The navigator was placed in feet-head direction on the dome of the right hemi-diaphragm. The data of the Bo-map was evaluated on the PC as described above. At the same position two single slice, breath-hold SSFP series were performed with a field of view of 330mm by 264mm (scan resolution 214x160) and a slice thickness of 8mm. In the 12s breath-holds 19 heart phases were acquired. The other parameters were T R 3.4ms, TE: TW2, and flip angle 40". The first scan was performed with a standard shim and the second scan with the local linear shim settings. Results In Figure 1 the procedure for shimming is shown for one slice of the Bomap. ROI selection on the modulus image is shown in Figure 1 A). Only the posterior part of the LV, where the highest gradients are seen (Figure 1 B), was selected. The calculated linear shim corrections for this position are plotted in 1 C). Figure 1 D) shows the expected BoFigure I : A ) The selected ROI in the modulus image. B ) The Bo-map showing a strong gradient near the PVLV (arrow). C) The ralculnted linear shim corrections. D) The corrected Bo-map (Bo-map + corrections). In Figure 2 phases 8 (B, C) and 13 (A, C) out of the 19 phases are shown. In the upper row are the images acquired with the standard shim settings and in the lower row are the ones with the local linear shim. In the images without the local shim large artifacts are visible near the PVLV. Using the local shim settings these artifacts are suppressed. A small artifact remains in the imaze at svstole (arrow in Figure 2 D). Figure 2: Short axis SSFP function images at end-diastole (A, C) and at end-systole (B, D) obtained in a 12s breath-hold. Images A and B were acquired without the local shim and show artifacts near the PVLV (arrows). Images C and D show the same heart phases as A and B measured with the local linear shim. Only a small artifact remains in image D (arrow).