) simulations were performed prior to experiments in order to check the efficiency of magnetization transfer of the TOCY mixing using GOIA-W(16,4) pulses and search for the low power conditions. The minimum time-bandwidth product of R = 20 for which pulses still behave adiabatically was used. In particular, three conditions were investigated: i) low power – Tp = 2 ms, BW = 10 kHz, γB1,max
= 0.76 kHz, ii) medium power – Tp = 1.5 ms, BW = 13.34 kHz, γB1,max
= 1.02 kHz, and iii) high power – Tp = 1.25 ms, BW = 16 kHz, γB1,max
= 1.22 kHz. Typically a maximum γB1,max
= 1.0 kHz is imposed on clinical scanners, hence we searched for values that are within 25% below and above this value (lower values render the transfer too inefficient, higher values would produce an SAR that is not feasible in-vivo).
In the buildup curves for longitudinal mixing are shown. Compared to the ideal magnetization transfer in the absence of chemical shifts (black curve, only the J-coupling in the case of high power RF condition), the buildup curves in the presence of the chemical shifts become progressively slower for decreasing B1,max fields. The first maximum of the buildup that corresponds to 72 ms (1/2J) in the ideal curve, shifts to 87 ms for γB1,max = 1.22 kHz (red curve), 96 ms for γB1,max = 1.02 kHz (green curve), and 139 ms for γB1,max = 0.76 kHz (blue curve). In practice limitations in SAR impose an upper limit of the mixing time. On our hardware 64 ms mixing is still possible within the limits of brain’s SAR for reasonable values of the TR in the range of 2–3 s (for other organs which have a lower SAR threshold longer mixing is possible). For 64 ms mixing the predicted transfer (in the absence of relaxation) amounts to 82% for γB1,max = 1.22 kHz, 73% for γB1,max = 1.02 kHz, and 44% for γB1,max = 0.76 kHz from the ideal case (black curve). Thus, under the most favorable conditions (γB1,max = 1.0 kHz) a transfer efficiency of 73% is possible on clinical scanners. Compared to the γB1,max = 1.0 kHz, the low power condition of γB1,max = 0.76 kHz achieves 60% transfer efficiency.
Figure 4 Simulations of magnetization transfer buildups curves for longitudinal TOCSY mixing (panel A) and transverse TOCSY mixing (panel B). The lactate spin system is considered at 3T and the transfer is performed with MLEV-16 scheme employing GOIA-W(16,4) pulses. (more ...)
In the case of transverse mixing is considered for the same situations: the ideal case with no chemical shifts (black curve, only the J-coupling in the case of high power RF condition), and including the chemical shifts for high- (γB1,max
= 1.22 kHz, red curve), medium- (γB1,max
= 1.02 kHz, green curve), and low-power (γB1,max
= 0.76 kHz, blue curve) mixing. The transverse mixing buildup curves show two important differences compared to longitudinal mixing: i) faster oscillations that superimpose on the magnetization transfer curve driven by the slower scalar coupling, and ii) the maximum possible transfer is five times lower than in the case of longitudinal mixing. The faster oscillations are due to the fact that transverse magnetization is not aligned with the direction of the effective field (along z) at the beginning of the adiabatic pulse, hence the magnetization starts to precess around the effective field during mixing. The oscillations becomes faster as γB1,max
and the effective field increase. By comparison the longitudinal magnetization is aligned with the effective field at the beginning of the adiabatic pulse and becomes spin locked during mixing. In addition to this, the transverse magnetization is also subjected to the faster R2ρ
relaxation. Measurements of R2ρ
relaxation rates at 4T in the case of HS4 and HS1 pulses (53
) indicate that R2ρ
can be more than four times faster than R1ρ
. Simulations or measurements have not been attempted for R2ρ
relaxation under GOIA-W(16,4) pulses, however similar results are expected (the difference seems even to increase for pulses with larger BW as indicated in Ref. (53
)). The combination of faster oscillations around the effective field and damping of the transfer due R2ρ
relaxation may result experimentally in an earlier maximum of an apparent faster buildup curve for transverse magnetization than would be expected from the scalar coupling (maximum at 1/2J) as observed previously in Ref. (29
). The net result of all these factors is a reduced efficiency of the transverse mixing as shown also experimentally in .
Figure 5 Selective magnetization transfer from the CH3 group to the CH group in the lactate spin system measured experimentally at 3T on a phantom containing equimolar (50 mM) mixture of Lactate and GABA using the 1D edited sequences: A) Z-TOCSY-LASER with longitudinal (more ...)
Simultaneous localization and transverse mixing can be performed when using GOIA-W(16,4) pulses according to the LT-TOCSY pulse sequence of . Simulations are performed for the slice profile of a refocusing MLEV-16 train using GOIA-W(16,4) pulses of 2 ms duration, 10 kHz bandwidth, and 0.76 kHz maximum RF amplitude. shows the comparison between the MLEV-16 slice profile (red curve) and the slice profile obtained with a double adiabatic spin echo sequence (60
) (black curve) using the same pulses. Accurate localization is obtained in both cases. The advantage of the 2D LT-TOCSY sequence is the considerable reduction of SAR by eliminating the LASER block. However, the behavior of transverse mixing shown in , the faster R2ρ
relaxation, and the need to introduce small gaps (200–400 μs) in the MLEV-16 train after each group of 4 or 8 pulses for the ramp-up/down of the gradients make this sequence less efficient for magnetization transfer.
Before attempting the 2D experiments, efficiency of our TOCSY mixing was verified by 1D edited selective magnetization transfer in a phantom containing an equimolar (50 mM) mixture of Lactate and GABA. show the results of the 1D editing selective TOCSY with longitudinal mixing and transverse mixing, respectively. Magnetization is transferred from the methyl group of Lactate, initially excited by a selective Gaussian pulse, to the methine group using a 64 ms mixing time for TOCSY with GOIA-W(16,4) pulses of 2 ms duration, 10 kHz bandwidth, and 0.76 kHz maximum amplitude. The results show that approximately five times more transfer is obtained with longitudinal mixing (1.45 maximum signal, ) compared to transverse mixing (0.3 maximum signal, ). Similar results are obtained for shorter mixing times of 32 ms. Indicative of the transfer is also the relative ratio of the CH to CH3 signal that can be seen in the insets shown in the upper right corners of (0.21 ratio for longitudinal mixing and 0.06 for transverse mixing). The sequences employed are based on the diagram shown in (Z-TOCSY-LASER), for which we replaced the first 90° BIR-4 pulse with a 90° selective Gaussian pulse (BW = 20 Hz), and in the case of transverse mixing the z-filter was removed according to the design proposed in Ref. (29
). The same LASER localization (GOIA-W(16,4) pulses of 3.5 ms duration, 20 kHz bandwidth, and 0.82 kHz maximum amplitude, other acquisition parameters were identical TR = 1.8 s, NA = 8, 0 ms t1 evolution) was used for both transverse and longitudinal mixing. Comparison with COSY transfer is made employing the COSY-LASER sequence from which has the first 90° BIR-4 pulse replaced with 90° selective Gaussian pulse (BW = 20 Hz) for selective excitation of the Lactate methyl group (the same LASER localization and acquisition parameters as in the case of TOCSY are employed). In the COSY transfer to the methine group obtained for 64 ms (t1 was set to 64 ms) is shown, which indicates that 50% more signal can be obtained compared to the longitudinal TOCSY mixing. However, an important aspect can be noticed for COSY: a much wider multiplet structure with a wider baseline is obtained due to the transfer of both in-phase and anti-phase magnetization components. TOCSY transfers in-phase magnetization, giving rise to narrower peaks (this is true for the longitudinal mixing from , the transverse mixing needs special purge pulses (61
) to remove the anti-phase magnetization). Phase distorted multiplets can result in lower resolution of the 2D COSY compared to 2D TOCSY spectra, and this is important given the low resolution of in-vivo spectra. The increased resolution and the fact that long range (relayed) correlations can be obtained in TOCSY might represent an attractive counterbalance for less sensitivity compared to COSY. Examples that support this will be shown from both phantom and in-vivo data.
Accuracy of localization has been checked for all the sequences in a double layer phantom that contains and outer shell of oil and an inner core of brain metabolites. A voxel was placed inside the inner core close to the boundary with the oil shell, and only the first t1 increment (0 ms t1 evolution, 1D spectra) was acquired. In results are shown for 1D Z-TOCSY-LASER (), 1D Z-TOCSY-STEAM (), 1D Z-TOCSY-PRESS (), 1D COSY-LASER (), and 1D L-COSY (), for the voxel positioned according to . The fully adiabatic sequences Z-TOCSY-LASER and COSY-LASER show excellent localization and no signs of lipid contamination (the lactate peak from the inner core can be easily seen), while both semi-adiabatic sequences Z-TOCSY-STEAM and Z-TOCSY-PRESS show a large lipid signal (obscuring lactate), which is larger for the PRESS localization. The largest lipid contamination is shown by the L-COSY experiment. The lipid contamination in the non-adiabatic sequences is due to CSDE and wider slice profiles for localization pulses. In reality, a bigger voxel than the one prescribed is selected by these sequences. For the 3×3×3 cm3
voxel shown in , the selected voxel is approximately 5 mm larger in each direction, yielding a real voxel size of 4×4×4 cm3
(double volume of the voxel). For the LASER localization a much sharper excitation selects a voxel of the same size as the one prescribed. The difference in the size of the real voxel selected by the localization schemes explains also the SNR difference observed (vide infra) between the 2D spectra obtained with adiabatic and non-adiabatic localization. In principle, saturation bands can be placed around the voxel to reduce the lipid contamination for non-adiabatic localization. However, the typical saturation bands existing on our clinical scanner are not optimized to have very sharp edges, hence they do not suppress completely the lipids outside the voxel and in addition they might suppress slightly some of the metabolite signal inside the voxel (this can be checked in a homogeneous phantom with and without saturation bands). Optimized saturation bands (62
) and their automatic placement (63
) can eliminate errors and make this approach more robust.
Figure 6 Accuracy of voxel localization at 3T for Z-TOCSY-LASER (A), Z-TOCSY-STEAM (B), Z-TOCSY-PRESS (C), COSY-LASER (D), and L-COSY (E) sequences in a double layer phantom containing an outer shell of oil and an inner core of brain metabolites at physiologic (more ...)
2D spectra have been acquired with all the sequences on the phantom containing lactate and GABA. GABA is in particular an important metabolite which is hard to observe in 1D spectra, and represents a good test case to show that long range cross-peaks can be obtained in 2D TOCSY. In results are compared for the localized 2D TOCSY obtained with the fully adiabatic 2D Z-TOCSY-LASER (), and the semi-adiabatic 2D Z-TOCSY-PRESS () and 2D Z-TOCSY-STEAM () sequences. As can be seen in all spectra the entire correlation network is obtained for each spin system. The intensity of the crosspeaks is greatest in 2D Z-TOCSY-STEAM, less in 2D Z-TOCSY-LASER and the lowest in 2D Z-TOCSY-PRESS (the same contour levels are chosen, the minimum contour level is 10 times the noise level, the crosspeaks for each metabolite are labeled according to the protons involved). However, the line-width in both dimensions is larger for the semi-adiabatic sequences compared to the fully adiabatic sequence. These can be explained by the larger selected voxel experiencing a greater B0 inhomogeneity, and by the fact that line shape modulation of coupled spins during the echo time is more pronounced for non-adiabatic localization (PRESS and STEAM).
Figure 7 Example of localized 2D TOCSY spectra obtained at 3T with fully adiabatic and semi-adiabatic sequences in a phantom containing an equimolar (50 mM) mixture of lactate and GABA. The same TOCSY mixing as described in was used in all three experiments. (more ...)
Localized 2D TOCSY (Z-TOCSY-LASER) and 2D COSY (COSY-LASER and L-COSY) spectra are compared on the lactate and GABA phantom in . It can be easily noticed that the pair of crosspeaks corresponding to the long range correlations between Hα and Hγ protons of GABA are not present in the 2D L-COSY spectra. However, one of the crosspeaks is present in the 2D COSY-LASER. The possible mechanism, which is verifiable by simulations, can be given by the fact that the train of LASER pulses may act, to a limited degree, also as mixing for magnetization transfer between strongly coupled spins with small chemical shift offsets such as GABA (the carrier was placed in the middle of the GABA spectrum, yielding offsets in the range of ±50 Hz, this effect of LASER is not seen for lactate protons that have more chemical shift dispersion). The difference in relaxation for the spins originating the magnetization could explain why only one of the crosspeaks is obtained (water suppression is unlikely to play a role since lactate crosspeaks are symmetric and the GABA crosspeaks are further away from water). Similar to , several observations can be made: i) the crosspeaks have the highest intensity for L-COSY, lower for COSY-LASER and the lowest for Z-TOCSY-LASER, and ii) the lines are sharper in the case of Z-TOCSY-LASER. These can be explained by larger voxel selected by L-COSY and transfer of both in-phase and anti-phase magnetization for L-COSY and COSY-LASER.
Figure 8 Example of localized 2D TOCSY and COSY spectra obtained at 3T in a phantom containing an equimolar (50 mM) mixture of lactate and GABA using Z-TOCSY-LASER, COSY-LASER, and L-COSY sequences. The same TOCSY, COSY and LASER blocks as described in (more ...)
Representative in-vivo localized 2D TOCSY and COSY brain spectra are presented next. In the comparison is made between 2D Z-TOCSY-LASER () and 2D L-COSY () obtained from a healthy volunteer (voxel position shown in ). The 1D spectra corresponding to the first t1 increment (0 ms t1 evolution) are shown in for 2D Z-TOCSY-LASER and in for 2D L-COSY. It is immediately apparent that large lipid contamination from subcutaneous fat is present in the L-COSY spectra due to a larger real voxel size that explains also the intensity difference. The crosspeaks of several metabolites can be easily identified in both 2D spectra (NAA, ASP-aspartate, Glx-glutamate/glutamine, Cho-choline, Myo-myoinositol, and tentatively for GABA and Lys-lysine). Some crosspeaks that seem to be present only in 2D Z-TOCSY-LASER have chemical shifts suggestive of glycerophospocholine (GPC) and gluthatione (GSH) which were found previously also in L-COSY (64
) (albeit double the number of averages was used). On the other hand, threonine (Thr) appears to be present only in the 2D L-COSY (Thr was observed in 1D edited TOCSY (30
), however in our low power implementation slower buildup might reduce the crosspeak intensity, this could be recovered potentially for the stronger mixing conditions). In addition, in the 2D L-COSY the crosspeaks corresponding to the contaminating lipids (Lip) are also present. Overall the crosspeaks in the 2D L-COSY spectrum are more intense, while the crosspeaks in the 2D Z-TOCSY-LASER are sharper. As explained and shown in phantoms sharper crosspeaks are not simply a scaling effect of the contour levels due lower intensity but the contribution of in-phase and anti-phase magnetization transfer in 2D L-COSY, while in 2D Z-TOCSY-LASER only in-phase magnetization transfer is ensured. In an overlay between the 2D Z-TOCSY-LASER spectrum (red) and the 2D L-COSY (blue) is shown. Long range crosspeaks (marked by the black crosses) are present in the 2D Z-TOCSY-LASER for Glx and GABA. Crosspeaks that seem to be observed only in the 2D Z-TOCSY-LASER are indicated by black arrowheads.
Figure 9 In-vivo brain spectra obtained at 3T with the 2D Z-TOCSY-LASER (D) and 2D L-COSY (E) sequences from a healthy volunteer (the voxel of 4×4×3 AP-RL-FH cm3 positioned in the occipital lobe is shown in C). 1D spectra corresponding to the first (more ...)
In the 2D Z-TOCSY-LASER data obtained from a patient with glioblastoma is shown. The voxel position and size is chosen to include most of the FLAIR abnormality seen in . In the 1D spectrum corresponding to the first t1 increment (0 ms t1 evolution) is shown. Reduced NAA, increased Choline and the presence of Lactate can be noticed. contains the 2D spectrum which shows crosspeaks for lactate (Lac), glutamate/glutamine (Glx), choline (Cho), glycerophospocholine (GPC), ethanolamine (Etn) and phosphoethanolamine (PE), and myoinositol (Myo). Notice that glutathione is absent, while important additional metabolites (Etn, PE) involved in the phospholipids turnover are present. Tentatively, some crosspeaks are assigned to the aminoacids proline (Pro) and Isoleucine (Ile). Note, that more robust methods (65
) for automatic assignment and fitting of the 2D spectra could be employed. An added benefit (two for the price of one) of the 2D MRS methods is that a conventional 1D spectrum corresponding to the first t1 increment (0 ms t1 evolution) is also obtained, which can be analyzed and fitted with advanced routines (9
) developed so far for in-vivo 1D MRS. A summary of the main experimental results obtained in-vivo and phantoms are given in .
Figure 10 In-vivo 2D Z-TOCSY-LASER brain spectrum obtained at 3T from a patient with glioblastoma. A voxel of 5×3×4 AP-RL-FH cm3 is positioned to include most of the FLAIR abnormality (shown in A). The 1D spectrum corresponding to the first t1 experiment (more ...)