Synopsis

Radial turbo spin-echo (RADTSE) based methods have been proposed for quantitative T2 mapping. RADTSE yields high spatio-temporal resolution and allows the reconstruction of co-registered images at multiple TE times from a short acquisition (i.e. a breath hold). We investigate the clinical utility of RADTSE for the quantitative characterization of abdominal neoplasms.

Introduction

T2-weighted imaging is widely used for the diagnosis of pathologies such as focal liver lesions. These are typically diagnosed as benign or malignant based on the qualitative assessment of signal intensity relative to liver [1-4]. T2 relaxation has also been proposed for the quantitative characterization of abdominal neoplasms. Previous studies have found significant differences in T2 relaxation times between different types of lesions, providing a range of classification for cysts, hemangiomas and malignancies [5-9].

Most methods for T2 mapping of the abdomen suffer from low spatial and/or temporal resolution, mis-registered TE images, or long acquisition times. Radial turbo spin-echo (RADTSE) based methods have been proposed for quantitative T2 mapping [10]. RADTSE yields high spatio-temporal resolution and allows the reconstruction of images at multiple TE times (which are registered by the nature of the acquisition) from a short acquisition (i.e. a breath hold). The use of a radial trajectory makes the technique robust to motion [11]. Recent developments in RADTSE incorporate model-based reconstructions. These take into account the presence of indirect echoes in T2 estimation and partial volume effects for small lesion (diameter < 1.5 cm) characterization.

In this work, we investigate the clinical utility of RADTSE for the quantitative characterization of abdominal neoplasms using data acquired on 56 subjects.

Methods

In vivo Imaging:

Informed consent was obtained from the subjects prior to imaging in compliance with the Institutional Review Board requirements. Breath-held abdominal imaging was performed using RADTSE on a 1.5T Siemens Aera scanner using torso and spine phased array coils. The pulse sequence parameters are shown in Table 1. The acquired data have high temporal sampling (every 6 ms) and covers TEs from 6 ms to 190 ms.

Image Reconstruction

In RADTSE all radial views sample the center of k-space and this enables the reconstruction of images at each individual TEs from highly under-sampled data using model based iterative algorithms [12-13] (Figure 1). These methods reconstruct the echo images by imposing principal component subspace constraints. The subspace basis is generated based on the underlying signal model. In order to account for the indirect echoes arising from imperfections in the refocusing pulse, the slice-resolved extended phase graph (SEPG) model was used to describe the T2 signal evolution. A T2 map is generated by fitting the echo images to the signal model using a pattern matching approach [14].

In multi-slice 2D imaging, partial volume effects introduce error in the estimated T2 due to contamination from the liver parenchyma. A joint estimation algorithm for bi-exponential fitting was previously proposed [15] to improve the T2 estimation in the presence of partial volume. This algorithm was adapted to work with the EPG model and used to generate more accurate T2 maps for the analysis.

Data from 56 subjects were analyzed, resulting in a set of 90 neoplasms (48 malignancies, 10 hemangiomas and 32 cysts).

Results

Figure 2 shows T2-weighted images and T2 maps for a subject with liver metastases and a subject with hepatocellular carcinoma. The malignancies appear mildly hyper-intense at longer TEs. Figure 3 shows representative images from subjects with benign lesions.

The mean T2 values for the abdominal neoplasms were extracted using a manual ROI and used for statistical evaluation. The plot in Figure 4(A) shows the T2 values of the malignant and the most common benign lesions. Separation between cysts and malignancies is excellent. Separation between malignancies and hemangiomas is excellent for larger lesions (diameter > 2 cm). For small hemangiomas (as the one shown in Figure 4B) T2s are underestimated due to contamination from liver, misclassifying them as malignancies (false positive). Complete separation between malignancies and all hemangiomas was achieved when partial volume correction was applied (as shown in Figure 4C). The mean T2 value of the lesions across the subjects were: malignancies = 90.70 ± 13.85 ms, hemangiomas = 228.15 ± 61.62 ms and cysts=525 ± 100.04 ms. A two-tailed t-test of the means showed significant difference (p-value < 0.01 and $$$\alpha$$$ = 0.05) in the mean T2 values of the three types of neoplasms.

Conclusion

Acknowledgements

References

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