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High Contrast Cartilaginous Endplate Imaging Using Dual-Inversion Recovery Prepared Ultrashort Echo Time (DIR-UTE) Sequence

Jiyo Srinivasan Athertya¹, James Lo^1,2, Alicia Ji¹, Charles Ding¹, Xiaojun Chen¹, Soo Hyun Shin¹, Bhavsimran Singh Malhi ¹, Saeed Jerban¹, Micael Carl³, Monica Guma^4,5, Eric Y Chang^1,5, Jiang Du^1,2,5, and Yajun Ma¹
¹Radiology, UCSD, San Diego, CA, United States, ²Bioengineering, UCSD, San Diego, CA, United States, ³GE Healthcare, San Diego, CA, United States, ⁴Medicine, UCSD, San Diego, CA, United States, ⁵Radiology Service, VA, San Diego, CA, United States

Synopsis

Keywords: MSK, Contrast Mechanisms, Cartilaginous endplate, Spine

The cartilaginous endplate (CEP) plays a key role in maintaining the normal function of the intervertebral disc (IVD) by acting as a bridge for the transport of nutrients into the IVD cells. In this study, we developed a 3D dual inversion recovery prepared ultrashort echo time (DIR-UTE) sequence for high contrast CEP imaging and compared its performance with previously developed techniques on a clinical 3T scanner. We found that the proposed DIR-UTE sequence demonstrated the best image contrast for CEP imaging, which is highly promising for future clinical use.

Introduction

Intervertebral disc (IVD) degeneration has been recognized as the main cause of chronic low back pain (1). The cartilaginous endplate (CEP) is a thin layer of hyaline cartilage that acts as the nutrient transport bridge in the disc cells (2). Because the CEP has a relatively short T2 relaxation time (i.e., ~15ms) (3), conventional clinical sequences cannot detect sufficient CEP signals for direct imaging or quantification. Ultrashort echo time (UTE) sequences with echo times shorter than 100µs can overcome this limitation and is able to handle imaging tissues with very short T2s (4,5).

Recently, several UTE techniques have been developed for long T2 suppression and selective imaging of short T2 tissues, such as dual-echo UTE with subtraction(6), T1 weighted fat saturated UTE (T1w-FS-UTE) (7), inversion recovery prepared and fat saturated UTE (IR-FS-UTE) (8) and dual inversion recovery prepared UTE (DIR-UTE) (9) sequences. The UTE sequences incorporating adiabatic full passage (AFP) pulses are very efficient in generating high contrast amongst short and long T2 tissues (8,9,11–13). Implementing such techniques from the literature, such as IR-FS-UTE, that have previously been used for high contrast imaging of the OCJ and CEP (8,14). In CEP imaging, the IR-FS-UTE sequence can produce a very high contrast between CEP and NP. However, the contrast between CEP and BM is limited because of the utilized FatSat module that prohibits efficient fat suppression if more spokes are used in a TR. This inefficient fat suppression can be overcome by replacing the FatSat module with another AFP pulse that is centered on the fat frequency, like the DIR-UTE technique. The DIR-UTE sequence has been shown to generate a very high image contrast for the OCJ region with efficient suppression of signals from both the superficial cartilage and marrow fat (9).

Given its advantages in high contrast imaging of short T2 tissues, in this study, we propose to further optimize the DIR-UTE sequence for high contrast imaging of CEP in the human lumbar spine, and compare its performance with other established UTE techniques, namely T1w-FS-UTE and IR-FS-UTE.

Methods

This study was approved by the institutional review board. Four healthy subjects and one patient with low back pain were recruited for lumbar spine imaging using both clinical (i.e., T2w-FSE) and UTE (DIR-UTE, IR-FS-UTE, T1w-FS-UTE, and FS-UTE) sequences on a 3T GE scanner.

Figure 1 shows the sequence diagram for different UTE techniques. The DIR-UTE sequence utilizes two AFP pulses to invert long T2 water (e.g., NP) and fat with center frequencies of 0 and -440Hz respectively (Figure 1A). The IR-FS-UTE sequence employs an AFP pulse for inverting long NP while the FatSat module is utilized to improve CEP contrast against BM (Figure 1B). Only the FatSat module was applied for fat suppression in both T1w-FS-UTE and FS-UTE sequences (Figure 1C). The multispoke acquisition strategy was employed in all UTE sequences to reduce the total scan time. For signal excitation in each spoke, a slab selective half pulse (Shinnar-Le Roux design, duration 1132μs and bandwidth 16 kHz) with variable-rate selective excitation (VERSE) design (15) was utilized (Figure 1D). The 3D Cones trajectory enables efficient k-space coverage for all UTE scans (Figure 1E). The detailed parameters for all the sequences are listed in Table 1.

To evaluate image contrast, CNRs between the CEP and BM (CNR_CEP-BM) and between the CEP and NP (CNR_CEP-NP) were calculated as the mean differences in signal between these tissues divided by the background noise. The noise was calculated as the standard deviation of signals measured from an ROI in an artifact-free background region. The CNRs were calculated for each disc for each subject.

Results and Discussion

Figure 2 shows the representative lumbar spine images from two healthy volunteers. The CEP signal cannot be efficiently captured in the clinical T2w-FSE sequence owing to its relatively short T2 relaxation time, while it is clearly seen on all UTE images. The DIR-UTE, IR-FS-UTE, andT1w-FS-UTE sequences all produce higher CEP contrast than the regular FS-UTE sequence. The DIR-UTE images show the best CEP contrast.

Figure 3 shows representative lumbar spine images from a patient with low back pain. Similar to healthy subjects, the clinical T2w-FSE sequence does not capture signals from the CEP region, while DIR-UTE, IR-FS-UTE, T1w-FS-UTE present better CEP contrast than regular FS-UTE. The NP regions of degenerated discs in DIR-UTE, IR-FS-UTE, and T1w-FS-UTE images show relatively higher signals than those in normal discs. This may be because of shortened T1 relaxation time in NP due to disc dehydration (16–18).

Table 2 summarizes CNR_CEP-BM and CNR_CEP-NP for all UTE sequences. Amongst these different sequences used, DIR-UTE presents the highest CEP contrast, followed by IR-FS-UTE, T1w-FS-UTE, and FS-UTE. The CEP region for the abnormal case records lower values than normal subjects.
These findings demonstrate that the 3D DIR-UTE sequence can achieve high contrast imaging of the CEP region with better CNRs compared to other sequences.

Conclusion

The optimized 3D DIR-UTE sequences proposed in this study showed the best CEP contrast of all the tested sequences, suggesting that the former may facilitate better evaluation of the vital CEP region in clinical practice.

Acknowledgements

The authors acknowledge grant support from the National Institutes of Health (R01AR062581, R01AR068987, R01AR075825, R01AR079484, RF1AG075717 and R21AR075851), VA Clinical Science and Rehabilitation Research and Development Services (Merit Awards I01CX001388, I01CX002211, and I01RX002604), and GE Healthcare.

References

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Figures

Figure 1. Sequence diagram. The DIR-UTE utilizes two AFP pulses to invert long T2 water (e.g., NP) and bone marrow (BM) with center frequencies of 0 and -440 Hz respectively (A). The IR-FS-UTE employs an AFP pulse for inverting magnetization of NP while the FatSat module is utilized to suppress BM signal to improve CEP contrast (B). Only the FatSat module is applied for fat suppression in both T1w-FS-UTE and FS-UTE sequences (C). Half slab selective UTE sequence is utilized for signal excitation (D). The 3D Cones trajectory is employed for efficient data sampling in UTE imaging (E).

Table 1. Sequence parameters of 3D DIR-UTE, IR-FS-UTE, T1w-FS-UTE, and FS-UTE for in vivo lumbar spine imaging.

Figure 2. Representative lumbar spine images of two healthy volunteers. The clinical T2w-FSE sequence shown in (A,F) fails to capture CEP signal owing to its relatively short T2 relaxation time. DIR-UTE (B,G) provides the best contrast amongst other UTE imaging techniques, especially between CEP and BM. IR-FS-UTE (C,H), and T1w-FS-UTE (D,I) generate better contrast in the CEP region than regular FS-UTE (E,J).

Figure 3. Representative lumbar spine images of a patient with low back pain. While the CEP region is invisible in clinical T2w-FSE (A), it is well highlighted in DIR-UTE (B), IR-FS-UTE (C), and T1w-FS-UTE (D). Regular FS-UTE (E) is able to capture the CEP signal but has relatively low contrast of the CEP region compared to DIR-UTE, IR-FS-UTE, and T1w-FS-UTE. The NP regions of degenerated discs (indicated by arrows) exhibit stronger signals than those in normal discs for DIR-UTE, IR-FS-UTE, and T1w-FS-UTE, which can be attributed to disc degeneration due to shortened T1 relaxation in NP.

Table 1. Summary of the mean CNR_CEP-BM and CNR_CEP-NP measurements from DIR-UTE, IR-FS-UTE, T1w-FS-UTE, and FS-UTE images for four healthy volunteers (i.e., “Normal”) and one patient with low back pain (i.e., “Abnormal”).

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)

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DOI: https://doi.org/10.58530/2023/1111