UCLA

Purpose

To develop and investigate a self-gated MR imaging technique to improve motion compensation robustness for dynamic contrast enhanced MRI, to develop and evaluate a low-dose CT perfusion reconstruction technique, and to develop motion tracking for uterine motion and evaluate its characteristics in MRI. These all could potentially allow for more practical applications of quantitative MRI and CT in clinical workflow.

Methods

A 3D dual-echo golden angle stack-of-radial gradient echo sequence was developed such that a fat-only self-gated signal (SGSF) could be extracted using a two-point Dixon method. The SGSF was validated by i) numerical simulations of SGSF in the presence of B0 inhomogeneities and varying fat fractions, ii) comparing to conventional SGS and an external video for respiratory motion detection, and iii) comparing SGS's in four liver DCE MRI scans.

A previously developed MRI reconstruction technique called k-space weighted image contrast (KWIC) was used to reduce the number of x-ray projections per gantry rotation, therefore reducing the radiation dose for CT perfusion (CTP) imaging. KWIC reconstruction was evaluated on i) a numerically simulated FORBILD head phantom with numerically simulated time-varying objects using multiple projection undersampling amounts (50%, 25%, and 12.5% of the original dose) and compared to conventional CT reconstruction, and ii) three clinical CTP cases. Quantitative perfusion metrics were computed and compared between KWIC reconstructed CTP data and those of standard FBP reconstruction.

The 3D golden angle stack-of-radial gradient echo sequence and KWIC reconstruction were then used to scan sixty-one pregnant women. Forty-eight subjects were scanned between 14-18 weeks and 19-24 weeks gestational age (GA). Thirteen additional pregnant subjects underwent only a single MRI between 14-18 weeks GA. An image-based algorithm was used on 3D dynamic images to track uterine motion over time in the superior-inferior and left-right directions. Uterine contraction and maternal motion cases were separated and compared between GA, fetal sex, and placental location. Comparisons were done in relation to direction and duration of the uterine motion. An unpaired t-test and a paired t-test were performed between GA, fetal sex, and placental location in terms of direction and duration for uterine contraction and maternal related motion, respectively.

Results

Numerical simulation showed that the fat extraction for SGS has less than 15% error at ∆f = ±0-50 Hz. There is strong correlation between SGS and SGSF (mean correlation = 0.91), and video motion curve and SGSF (mean correlation coefficient = 0.87). SGSF(t) was not significantly affected by the contrast uptake in liver DCE MRI experiment.

For the CT perfusion the numerical simulations showed that KWIC were unaffected by the undersampling/dose reduction (down to 12.5% dose) with KWIC reconstruction compared to the fully sampled FBP reconstruction. The normalized root-mean-square-error (NRMSE) of the AUC in the FORBILD head phantom is 0.04, 0.05 and 0.07 for 50%, 25%, and 12.5% KWIC respectively as compared to FBP reconstruction. The cerebral blood flow (CBF) and cerebral blood volume had no significant difference between FBP and 50%, 25%, and 12.5% KWIC reconstructions (p>0.05).

In uterine motion we observed the mean duration of the contractions was significantly longer by 26.5 seconds during GA 14-18 weeks compared to GA 19-24 weeks (p=0.034) and significantly longer for male fetuses versus female fetuses (p=0.044; 141.7 ± 20.1s and 106.9 ± 43.2s, respectively). There was no significant difference between duration and direction in maternal-related motion by GA, fetal sex, or placental location (p > 0.05).

Conclusions

Respiratory motion correction in the liver can be achieved using fat-only SGS with minimal error in the fat-water separation. The proposed technique has potential implications for more robust motion correction for liver dynamic contrast enhanced MRI.

The low dose CTP study demonstrates that KWIC preserves perfusion metrics for CTP with substantially reduced dose. Clinical implementation will require further investigation into methods of rapid switching of a CT X-ray source.

There is a significant difference in the duration of uterine contractions between the two gestational ages examined and the fetal sex. No association in maternal motion during early gestation was seen between GA, fetus sex and placental position.