Synopsis

Keywords: Tumors, Tumor

In this exhibit, we outline the effect of TE shortening on the diffusion-weighted imaging (DWI) in clinical practice and the associated shortening of diffusion time on recent clinical MRI scanner for brain neoplasms diagnosis. In general, shortening the diffusion time reduces the contrast of lesions that show an abnormally high signal on DWI, and the apparent diffusion coefficient values also changes toward a larger value, which may lead the radiologist to err in differential diagnosis or grading of the neoplastic lesions. Therefore, it is important for radiologists to be aware of these effects when diagnosing brain neoplasms.

Purpose:

With advances in hardware and software on clinical MRI scanners, especially 3Tesla, the echo time (TE) of diffusion-weighted (DW) imaging using single-shot echo-planar imaging are being set up with shorter than before. Of course, in principle this is true in terms of improved image quality, but at the same time diffusion times are often automatically shortened. Also, clinicians are often unaware that changes in TE and diffusion time can change the contrast of the DW imaging and its quantitative value. The purpose of this exhibit is to present the effects of diffusion time and TE changes of diffusion MR imaging on clinical brain neoplasms diagnosis.

Outline of contents:

1. First, the representative case images are shown when the TE time is set shorter on a 3T MRI system in clinical settings. Usually, the diffusion time is shortened along with the TE shortening. As a result, both the contrast of the brain parenchyma and the contrast of neoplastic lesions in the brain change and the corresponding apparent diffusion coefficient (ADC) values also change (Figure 1). In the past, 1.5T MRI systems had a TE of approximately 100-120 ms for DW imaging, whereas the current state-of-the-art 3T MRI systems have a TE = 60 ms is set as a standard value in the latest clinical 3T MRI systems. Therefore, when referring to past images, it is necessary to consider not only differences in static magnetic field strength but also differences in TE and diffusion time.
2. In DW images, if the diffusion time is fixed and TE is varied, the contrast of the b=0 image is defined by the value of TE, and the contrast of the trace image, which is used for clinical diagnosis, also changes (Figure 2). In general, diffusion-weighted images are known to be TE-dependent1. Therefore, the radiologist should be aware of the TE value and its effect on the DW images, or the differential diagnosis may be incorrect.
3. Next, we show what can happen in DW images when TE is fixed and diffusion time is varied. The radiologist must first recognize that with newer MRI systems, the diffusion time is shorter than before as the TE is shortened. Shorter diffusion times reduce the contrast of brain neoplasms that show abnormally high signal on DW images. In some cases, there is a risk of unrecognizability^{2, 3} (Figure 3). In addition, changes in image contrast, i.e., various brain tumors known to show abnormally high signal on DW images and reduced ADC (e.g., germinoma, CNS lymphoma, glioblastoma, etc.) do not show typical imaging findings, which may lead the radiologist to err in differential diagnosis or grading of the neoplastic lesions⁴ (Figure 4 and 5).

Summary:

In this exhibit, we outline the effect of TE shortening on the DW imaging in clinical practice and the associated shortening of diffusion time on recent clinical MRI scanner for brain neoplasms diagnosis. In general, shortening the diffusion time reduces the contrast of lesions that show an abnormally high signal on DWI, and the ADC value also changes toward a larger value. It is important for radiologists to be aware of these effects when diagnosing brain neoplasms.

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 22K15850.