Musculoskeletal MRI at 3.0T and 7.0T: A Comparison of Relaxation Times and Image Contrast

Abstract

OBJECTIVE

The purpose of this study was to measure and compare the relaxation times of musculoskeletal tissues at 3.0T and 7.0T, and to use these measurements to select appropriate parameters for musculoskeletal protocols at 7.0T.

MATERIALS AND METHODS

We measured the T₁ and T₂ relaxation times of cartilage, muscle, synovial fluid, bone marrow and subcutaneous fat at both 3.0T and 7.0T in the knees of five healthy volunteers. The T₁ relaxation times were measured using a spin-echo inversion recovery sequence with six inversion times. The T₂ relaxation times were measured using a spin-echo sequence with seven echo times. The accuracy of both the T₁ and T₂ measurement techniques was verified in phantoms at both magnetic field strengths. We used the measured relaxation times to help design 7.0T musculoskeletal protocols that preserve the favorable contrast characteristics of our 3.0T protocols, while achieving significantly higher resolution at higher SNR efficiency.

RESULTS

The T₁ relaxation times in all tissues at 7.0T were consistently higher than those measured at 3.0T, while the T₂ relaxation times at 7.0T were consistently lower than those measured at 3.0T. The measured relaxation times were used to help develop high resolution 7.0T protocols that had similar fluid-to-cartilage contrast to that of the standard clinical 3.0T protocols for the following sequences: proton-density-weighted fast spin-echo (FSE), T2-weighted FSE, and 3D-FSE-Cube.

CONCLUSION

The T₁ and T₂ changes were within the expected ranges. Parameters for musculoskeletal protocols at 7.0T can be optimized based on these values, yielding improved resolution in musculoskeletal imaging with similar contrast to that of standard 3.0T clinical protocols.

Introduction

Magnetic resonance imaging (MRI) at higher magnetic field strengths offers several advantages over imaging at lower magnetic field strengths. Signal-to-noise ratio (SNR) is expected to increase with increasing magnetic field strength due to the increased spin polarization. The increased SNR can often be traded for higher spatial resolution or decreased scan time^1–3. Certain cartilage imaging techniques, such as in vivo sodium imaging, may be improved at 7.0T since in vivo sodium imaging has the challenges of lower SNR, limited spatial resolution, and long acquisition times³. Tissue contrast is dependent on the repetition time (TR) and the echo time (TE) of the sequence. Knowledge of the T₁ and T₂ relaxation times of musculoskeletal tissues at 7.0T is important for obtaining image contrast similar to that of existing methods at lower magnetic field strengths. Relaxation times are also important to know for their biochemical significance.

However, there are also distinct disadvantages to scanning at higher magnetic field strengths. Currently, 7.0T MRI is not FDA-approved for clinical use. Additional disadvantages include increased specific absorption ratio (SAR) effects from the increased radiofrequency (RF) power deposition, greater B₀ and B₁ magnetic field inhomogeneities, and increased chemical shift artifacts from fat^{2, 3}.

Previous studies of ¹H NMR relaxation times indicate that T₁ is expected to increase with increasing field strength according to a general curve of the form T₁=Aν^B, where ν is the Larmor frequency, and A and B are constants that have been fit for the specific tissue⁴.

For example, this previous work calculated A = 0.000455 and B = 0.4203 for skeletal tissue, and at 7.0T, ν is about 298 MHz. Although these studies indicate that the observed T₂ is independent of magnetic field strength, more recent work in musculoskeletal tissues has found that T₂ typically decreases with increasing magnetic field strength^4–7. Thus in our experiment, we expect T₁ to increase substantially and T₂ to decrease slightly at 7.0T compared to the relaxation times at 3.0T^{5, 7}.

There have been a number of studies measuring T₁ and T₂ relaxation times in musculoskeletal tissues at different magnetic field strengths. Several studies have previously measured and compared musculoskeletal relaxation measurements at 1.5T, 3.0T and 4.0T. One representative study found an average T₁ increase from 1.5T to 3.0T of about 20%, and a slight average T₂ decrease⁵. Another study found that T₁ values increased for all tissues from 1.5 T to 4.0T, while T₂ values decreased by 10–20% for all tissues except for bone marrow⁶. Several studies of cartilage tissue at 3.0T and 7.0T found that cartilage T₁ values increased by about 35%, while cartilage T₂ values decreased or remained about the same^{7, 8}. Knowledge of relaxation times is necessary to develop musculoskeletal protocols at 7.0T.

This study differs from previous work in that we measured and compared the T₁ and T₂ relaxation times of five human musculoskeletal tissues in the knee at both 3.0T and 7.0T in a single subject population, and then used these measurements to select appropriate parameters for musculoskeletal MRI methods at 7.0T that provide similar fluid-to-cartilage contrast to that of current 3.0T clinical protocols.

Materials and Methods

A total of twelve healthy volunteers were enrolled in this study (4 females, 8 males, ages 22–39, mean age 28.7). For the T₁ measurements, the right knees of a set of five healthy volunteers were imaged on a 3.0T GE Discovery whole body scanner, and on a 7.0T GE Magnex whole body scanner for the T₁ measurements (GE Healthcare, Waukesha, WI, USA). Since one volunteer who was scanned at 7.0T was not able to be scanned at 3.0T, a sixth volunteer was scanned as a replacement at 3.0T only. The right knees of another set of five healthy volunteers were imaged at both 3.0T and 7.0T for the T₂ measurements. All scans were done under a protocol approved by the local institutional review board. A Nova Medical quadrature birdcage transmit-receive knee coil was used at 7.0T for the volunteers and the phantoms (Nova Medical, Wilmington, MA, USA). At 3.0T, a GE eight-channel receive/quadrature transmit knee coil was used to scan volunteers, and a single-channel quadrature birdcage Invivo coil was used to scan a volunteer for the clinical standard 3.0T protocols and the peanut oil phantoms (Invivo, Gainsville, FL, USA).

T₁ Relaxation Time Measurement Protocol

The T₁ relaxation times were measured using a spin-echo inversion recovery (SE-IR) sequence with six different inversion times (TI) spaced logarithmically (approximately) along a hypothetical T₁ recovery curve. The T₁ relaxation times were calculated by performing a fit of the mean pixel signal intensities of a selected region of interest (ROI) at the different inversion times. The inversion pulse was an adiabatic Silver-Hoult pulse, with a pulse width of 8640 μs at both 3.0T and 7.0T. Table 1 shows the inversion times, and the sequence parameters for 3.0T and 7.0T. Taking into consideration the effects of incomplete T₁ recovery due to the 8000ms repetition time, we used a nonlinear least squares regression fit to find M₀, T₁, and ε in MATLAB using the following equation,

Table 1.

Spin-echo inversion recovery and spin-echo protocols at both 3.0T and 7.0T

Parameter	SE-IR	SE
Matrix	256×128	256×128
TR (ms)	8000	4500
TE (ms)	15	[10, 20, 40, 80, 160, 320, 640]
TI	[50, 150, 450, 900, 1800, 3600]	N/A
BW (kHz)	31.25	62.5
FOV (cm)	16	16
Scan Plane	Axial	Axial
NEX	0.75	1

$${\text{M}}_{\text{t}}={\text{M}}_0{\text{e}}^{-\text{TE}/\text{T}2}\{1-(2+\epsilon ){\text{e}}^{-\text{TI}/\text{T}1}+(1+\epsilon ){\text{e}}^{-(\text{TR}-\text{TE}/2)/\text{T}1}-(1+\epsilon ){\text{e}}^{-\text{TR}/\text{T}1}\},$$ (1)

where the small ε term accounts for an imperfect inversion pulse (MathWorks, Natick, MA, USA) ⁹. M₀ and e^−TE/T2 are both independent of TI, and so we combined their fit in the calculation. We then calculated the regression coefficient R² to determine the goodness of fit.

T₂ Relaxation Time Measurement Protocol

The T₂ relaxation times were measured using a spin-echo (SE) sequence by calculating an exponential fit of the mean pixel signal intensities of a selected ROI at seven echo times. Table 1 shows the logarithmically spaced echo times and the scan sequence parameters for both 3.0T and 7.0T. Again taking into consideration the effects of incomplete T₁ recovery due to the 4500ms repetition time, we used a nonlinear least squares regression fit to find M₀ and T₂ in MATLAB, using the following equation,

$${\text{M}}_{\text{t}}={\text{M}}_0{\text{e}}^{-\text{TE}/\text{T}2}\{1-2{\text{e}}^{-(\text{TR}-\text{TE}/2)/\text{T}1}+{\text{e}}^{-\text{TR}/\text{T}1}\},$$ (2)

where T₁ is the averaged measured T₁ value from the previous measurements⁹. The regression coefficient, R², was calculated for each fit to determine the goodness of fit.

We used only the last five echo time points to fit the synovial fluid curve, since the echo time spacing of these points was more appropriate for the expected longer T₂ value. We used only the first five echo time points for the other four tissues with expected shorter T₂ values, as the last two echo time points had very low SNR for these tissues. To maintain the accuracy of the relaxation curve t, points below or near the noise oor with extremely low SNR were discarded to avoid biasing the fit.

For both sets of measurements, relaxation times were averaged across all subjects and the standard deviations of the measurements were calculated. The statistical significance of differences between the 3.0T and 7.0T measurements was calculated using a paired Student’s t-test, and the p-values were recorded.

Phantom Validation

T₁ and T₂ measurements were performed twice on different days for reproducibility on a vial of peanut oil at both 3.0T and 7.0T. These measurements were used to assess repeatability of the fitting methods. We used the same T₁ and T₂ fitting protocols for fat as described above and calculated the regression coefficient R² for each fit.

Data Processing

For each scan, a single axial slice with a 16cm field-of-view (FOV) and slice thickness of 3mm was acquired through a region in the knee that included five musculoskeletal tissue types: patellar cartilage, lateral gastrocnemius muscle, femoral bone marrow, patellofemoral joint uid, and medial subcutaneous fat. A region of interest (ROI) was drawn within each tissue type, and the mean signal intensity was recorded for each signal curve fitting. ROIs in the synovial uid and cartilage were relatively small because of the smaller volumes of tissue present. The ROI of the patellar cartilage was chosen such that it was directly posterior to the cortical bone of the patella.

7.0T Protocol Design

Based on the measured T₁ and T₂ relaxation times, the fluid-to-cartilage image contrast ratio was estimated for the following 3.0T clinical sequences: proton-density (PD) weighted fast spin-echo (FSE), T₂-weighted FSE, and 3D-FSE-Cube with isotropic resolution^10–12. Figure 1A shows how we found the pixel with the closest signal to the simulated fluid-cartilage contrast ratio at 3.0T, and then chose the corresponding TE and TR for each sequence at 7.0T. We imaged the left knee of a healthy 29-year-old male volunteer at 3.0T, using the standard clinical protocols, and at 7.0T, using our optimized 7.0T protocols on different days. The measured fluid-to-cartilage contrast ratio was estimated by calculating the SNR in a region of interest in each tissue, and taking the ratio of the mean SNRs. The flip angle train of the 3D-FSE-Cube was optimized at 7.0T for minimizing the SAR.

Figure 1.

This figure demonstrates the approximate difference in signal levels at varying relaxation times, and the importance of choosing appropriate TEs and TRs at different magnetic field strengths. In A) we show simulated fluid-to-cartilage contrast at 7.0T for a PD-weighted SE sequence as a function of TE and TR. This graph was used to match the calculated contrast at 3.0T. Theoretical tissue signal levels for a spin-echo sequence (equation 2) at 3.0T B) and 7.0T C) are plotted at a fixed TE (15ms), over a range of repetition times using the measured relaxation times.

Results

Phantom results

Table 2 lists the measured T₁ and T₂ relaxation times of peanut oil at both 3.0T and 7.0T from the repeated scans of the peanut oil phantom, the percent change in relaxation times between the repeated scans, the percent increase or decrease in relaxation times between magnetic field strengths and the average regression coefficient R². The phantom measurements demonstrate that the T₁ and T₂ measurement method is repeatable over time, as the percent changes between the repeated scans are small. We measured a 55% increase in the T₁ of peanut oil from 3.0T to 7.0T, similar to our measured average increase in T₁ of 51% for musculoskeletal tissues in vivo (described below). We measured a 24% decrease in the T₂ of peanut oil from 3.0T to 7.0T, similar to our measured average decrease in T₂ of 21% for musculoskeletal tissues in vivo (also described below).

Table 2.

Repeated peanut-oil phantom measurements of T₁ and T₂ relaxation times at 3.0T and 7.0T

	3.0T Scan 1	3.0T Scan 2	% change between scans	7.0T Scan 1	7.0T Scan 2	% change between scans	% change between 3.0T and 7.0T	Average R2
T1 (ms)	281	280	0.3%	433	435	0.5%	55% increase	0.99
T2 (ms)	53.9	58.8	8.3%	43.4	42.7	1.6%	24% decrease	0.99

In vivo results

Table 3 shows the measured T₁ relaxation times at 3.0T and 7.0T, the percent increase, and the p-value, demonstrating a statistically significant increase from 3.0T to 7.0T for all tissues. The regression coefficient R² is greater than or equal to 0.99 for all fits. The average increase in T₁ relaxation times was 51%. Table 4 shows the measured T₂ relaxation times at both 3.0T and 7.0T, the percent decrease, and the p-value demonstrating significant decrease for all tissues. The regression coefficient R² is greater than or equal to 0.99 for all fits. The average decrease in T₂ relaxation times was 21%.

Table 3.

In vivoT₁ Relaxation times at 3.0T and 7.0T

Tissue	T1 (ms) at 3.0T	T1 (ms) at 7.0T	% Increase	p-value
Patellar Cartilage	1015.6 ± 71.1	1568.1 ± 335.0	54.4	0.028
Subcutaneous Fat	403.8 ± 17.7	583.1 ± 22.1	44.4	2.4e-5
Femoral Bone Marrow	381.2 ± 8.0	548.7 ± 11.8	43.9	1.4e-5
Lateral Gastrocnemius Muscle	1255.9 ± 57.9	1552.5 ± 97.3	23.6	0.007
Synovial Fluid	2564.7 ± 269.7	4813.2 ± 701.4	87.7	0.007

Table 4.

In vivoT₂ relaxation times at 3.0T and 7.0T

Tissue	T2 (ms) at 3.0T	T2 (ms) at 7.0T	% Decrease	p-value
Cartilage	39.1 ± 1.3	31.7 ± 3.4	18.9	0.020
Subcutaneous Fat	48.3 ± 0.8	46.1 ± 1.7	4.5	0.019
Femoral Bone Marrow	51.5 ± 1.2	46.7 ± 0.8	9.1	0.001
Lateral Gastrocnemius Muscle	29.3 ± 1.1	23.0 ± 1.0	21.6	0.003
Synovial Fluid	652.9 ± 113.4	324.5 ± 60.9	50.2	0.016

7.0T protocol design results

The relaxation times were used to develop examples of very high-resolution 7.0T protocols for a PD-weighted FSE sequence, a T2-weighted FSE sequence, and a 3D-FSE-Cube sequence. The 7.0T sequence parameters were chosen to mimic as closely as possible the fluid-to-cartilage contrast obtained by the standard clinical protocols at 3.0T, while achieving significantly higher resolution. Figure 1 demonstrates the importance of choosing the appropriate TE and TR at different magnetic field strengths. Figure 1B and 1C demonstrate the theoretical signals from musculoskeletal tissues as a function of TR for a spin-echo sequence at 3.0T and 7.0T using the measured T₁ and T₂ values, given a TE of 15ms.

Figure 2 shows the images from the three sequences used in our standard clinical protocol at 3.0T and the three sequences comprising our example protocol at 7.0T. The 7.0T protocol achieves a significant increase in the in-plane image resolution relative to the 3.0T protocol for each of the sequences while yielding similar image contrast. Table 5 shows the scan parameters for all three sequences.

Figure 2.

Sagittal knee protocol images acquired at 3.0T and 7.0T. The 7.0T images have higher resolution, while maintaining similar fluid-to-cartilage contrast at each magnetic field strength with in-plane resolution of A) 0.29mm × 0.47mm, B) 0.15mm × 0.20 mm, (a 4.8x reduction in voxel volume at 7.0T). C) 0.39mm × 0.47mm, D) 0.20mm × 0.29mm, (a 3.2x reduction in voxel volume at 7.0T). E) 0.50mm × 0.59mm, F) 0.39mm × 0.59mm, (a 1.3x reduction in voxel volume at 7.0T).

Table 5.

Sequence parameters of the clinical 3.0T and designed 7.0T protocols

	PD-FSE		T2-FSE		3D-FSE-Cube
Parameter	3.0T	7.0T	3.0T	7.0T	3.0T	7.0T
TE	29ms	24ms	80ms	74ms	81ms	84ms
TR	3000ms	4500ms	4200ms	5000ms	2800ms	3000ms
BW	31.25kHz	31.25kHz	31.25kHz	31.25kHz	50kHz	50kHz
Matrix	512×320	1024×768	384×320	768×512	288×256×124	384×256×124
Slices	21	21	15	15	-	-
FOV	15cm	15cm	15cm	15cm	15cm	15cm
Slice Thickness	2.5mm	2.5mm	2.5mm	2.5mm	0.6mm	0.6mm
Spacing	0.5mm	0.5mm	0.5mm	0.5mm	-	-
NEX	1	1	2	2	0.5	0.75
Freq. Dir.	S/I	S/I	S/I	S/I	S/I	S/I
Flip Angle	90	90	90	90	90	90
ETL	6	6	8	8	80	80
In-plane	0.29mm x	0.15mm x	0.39mm x	0.20mm x	0.50mm x	0.39mm x
resolution	0.47mm	0.20mm	0.47mm	0.29mm	0.59mm	0.59mm
Scan time	2:48	9:40	5:40	10:45	8:28	6:45

Figure 2A shows PD-weighted FSE images with an in-plane resolution of 0.29mm × 0.47mm at 3.0T, while in Figure 2B, the 7.0T sequence achieves an in-plane resolution of 0.15mm × 0.20 mm (a 4.8x reduction in voxel volume at 7.0T). The measured fluid-to-cartilage contrast ratios for the PD-weighted FSE images were 1.9 and 2.4 at 3.0T and 7.0T respectively.

Figure 2C shows T₂-weighted FSE images at 3.0T with an in-plane resolution of 0.39mm × 0.47mm, while in Figure 2D, the 7.0T sequence achieves an in-plane resolution of 0.20mm × 0.29mm (a 3.2x reduction in voxel volume at 7.0T vs. 3.0T). The measured fluid-to-cartilage contrast ratios for the T₂ FSE sequence were 4.2 and 3.9 at 3.0T and 7.0T respectively.

Figure 2E shows 3D-FSE-Cube images at 3.0T with an in-plane resolution of 0.50mm × 0.59mm, while in Figure 2F, the 7.0T sequence achieves an in-plane resolution of 0.39mm × 0.59mm (a 1.3x reduction in voxel volume at 7.0T). The measured fluid-to-cartilage contrast ratios for the 3D-FSE-Cube sequence were 4.2 and 2.6 at 3.0T and 7.0T respectively.

Figure 3 demonstrates higher SNR efficiency at 7.0T than at 3.0T for all tissues for the three sequences. The SNR measurements at 7.0T are scaled relative to the in-plane voxel size and the square root of total readout time.

Figure 3.

SNR efficiency is higher at 7.0T for all tissues for the three protocols. SNR measurements at 7.0T are scaled by the relative voxel size and the square root of total readout time. The asterisks indicate significantly higher SNR efficiency for the following sequences: A) proton density-weighted fast spin-echo (FSE) B) T2-weighted FSE and C) 3D-FSE-Cube.

Discussion

Our T₁ measurements at 3.0T are in reasonable agreement with values reported in the literature. Our cartilage T₁ measurements at 3.0T and 7.0T are close to literature values, although may be slightly lower due to the ROI chosen in the deeper region of the patellar cartilage^{8, 13}. Our muscle and synovial fluid T₁ measurements at 7.0T were lower than the T₁s measured at 4.0T. This could be due to different measurement biases in our spin-echo inversion-recovery sequence relative to the Look-Locker sequence⁶. The lower repetition time used to keep scan times reasonable may affect the T₁ and T₂ measurements for synovial fluid, due to incomplete signal recovery. Our synovial fluid T₁ measurements at 3.0T are also lower than a similar study’s measurements at 3.0T, again perhaps due to our use of the spin-echo inversion-recovery sequence, instead of a Look-Locker sequence⁵. A previous study found the 3D fast Look Locker sequence obtained accurate T₁ relaxation times, although they noted that it did tend to slightly over-estimate T₁ values¹⁴. A detailed study of the measurement bias of each of these techniques is needed, but is beyond the scope of the present study.

Our T₂ measurements are also in good agreement with values reported in the literature. Our cartilage T₂ measurements at 7.0T were consistent with a previous study’s measurement of cartilage in the deep cartilage layer⁷. Our fat and bone marrow T₂ values at 3.0T are lower than those of a previous study, perhaps due to our use of a spin-echo sequence instead of a spiral T2 preparation sequence⁵. The choice of T₁ was important to finding accurate nonlinear T₂ fits for synovial fluid, so we used the average measured T₁ value from this study.

A 256×128 matrix size was used for the T₁ and T₂ measurements to keep scan times reasonable; however, this limited the number of pixels in the ROIs. The reduced resolution may lend itself to partial-volume effects from the surrounding tissue and may reduce the T₁ value, particularly in small ROIs, such as the ROIs for cartilage and fluid. For the T₁ measurements, only six inversion times were used, ranging from 50ms to 3600ms, so that exam time would be kept reasonable. A longer inversion time might have been helpful for the synovial fluid T₁ fit.

We designed three 7.0T musculoskeletal protocols based on fast spin-echo sequences. However, the signal equation we used to theoretically estimate the fluid-to-cartilage image contrast was based on the spin-echo signal equation (2). The choice of parameters for the FSE images at 7.0T might improve the sequence and its contrast if a fast spin-echo algorithm is used. A recent study has optimized clinical ankle sequences and demonstrated higher SNR and contrast-to-noise ratio (CNR) at 7.0T over 3.0T, using a QED 28 channel dedicated knee coil at 7.0T (QED, Mayfield Village, OH, USA) ¹⁵.

The primary goal of developing these musculoskeletal methods at 7.0T was to obtain ultra high spatial resolution images, and a similar fluid-to-cartilage contrast to that of the standard clinical protocols at 3.0T. Further studies are needed to do a more detailed comparison between the achievable SNR and SNR efficiency of the optimized protocols at 7.0T vs. 3.0T.

Conclusion

In summary, we measured the T₁ and T₂ relaxation times in five musculoskeletal tissues at both 3.0T and 7.0T in a single subject population, and verified our methods in phantoms. For all five tissues at 7.0T, the T₁ relaxation times increased by an average of 51%, whereas the T₂ relaxation times decreased by an average of 21%. We used these measurements to help optimize parameters for several musculoskeletal sequences at 7.0T that achieved both high spatial resolution and similar fluid-to-cartilage image contrast as that of standard 3.0T clinical protocols. Knowledge of these relaxation parameters will enable further parameter optimization for musculoskeletal imaging protocols at 7.0T.