3D-multi-spectral T2 mapping near metal implants

Abstract

Purpose: Due to host-mediated adverse reaction to metallic debris, there is an increasing need for noninvasive assessment of the soft tissue surrounding large joint arthroplasties. Quantitative $T_2$ mapping can be beneficial for tissue characterization and early diagnosis of tissue pathology but current $T_2$ mapping techniques lack the capability to image near metal hardware. A novel multi-spectral $T_2$ mapping technique is proposed to address this unmet need.

Methods: A $T_2$ mapping pulse sequence based on routinely implemented 3D multi-spectral imaging (3D-MSI) pulse sequences is described and demonstrated. The 3D-MSI pulse sequence is altered to acquire images at 2 echo times. Phantom and knee experiments were performed to assess the quantitative capabilities of the sequence in comparison to a commercially available $T_2$ mapping sequence. The technique was demonstrated for use within a clinical protocol in 2 total hip arthroplasty (THA) cases to assess $T_2$ variations within the periprosthetic joint space.

Results: The proposed multi-spectral $T_2$ mapping technique agreed, within experimental errors, with $T_2$ values derived from a commercially available clinical standard of care $T_2$ mapping sequence. The same level of agreement was observed in quantitative phantoms and in vivo experiments. In THA cases, the method was able to assess variations of $T_2$ within the synovial envelope immediately adjacent to implant interfaces.

Conclusions: The proposed 3D-MSI $T_2$ mapping sequence was successfully demonstrated in assessing tissue $T_2$ variations near metal implants.

Keywords: ${\text{T}}_2$ mapping; metal implants; multi-spectral imaging.

FIGURE 1

Pulse sequence diagram for the Multi-series 3D-MSI(a) and the modified dual-echo 3D-MSI(b) techniques: Constant flip angle train, and data acquisition for 2 echo times are the main differences between the 3D-MSI and the modified dual-echo 3D-MSI sequence. The encoding and readout gradients timings are adjusted to account for the desired TEs as shown in (b).

FIGURE 2

Phantom experiment results: The signal decay as function of echo time is shown in the plots for CARTIGRAM, Multi-series 3D-MSI and Dual-echo 3D-MSI sequences for each vial. The signal decay is similar for all the three sequences in all the three vials. The T₂ values for this experiment are given in Table 1. Note: The 2%, 3% and 4% agarose gel concentrations provide different T₂ values for quantitative mapping comparisons.

FIGURE 3

In-vivo evaluation of the modified dual-echo 3D MSI sequence in knee: The top row shows a T₂ weighted image for CARTIGRAM with and without the presence of metal and modified 3D-MSI sequence in the presence of metal in knee as seen in (a), (b) and (c) respectively. The T₂ map for the trochlear cartilage obtained using CARTIGRAM (d), Modified dual-echo 3D-MSI without (e) and with (f) metal are seen in the bottom row. The trochlear cartilage is completely obscured in CARTIGRAM near metal (b) due to the metal induced susceptibility as seen by the white arrow.

FIGURE 4

In-vivo application of the modified dual-echo 3D-MSI sequence in patients with total hip replacement: A T₂ -weighted image (a, c) and the corresponding T₂ maps (b, d) for the two patients are shown. The mean and the standard deviation of the T₂ values for four ROIs shown above (1,2,3,4) are 55 ± 13ms, 104 ± 11ms, 54 ± 9ms and 104 ± 10ms respectively.