Interstitial fluid flow is vital for drug delivery and distribution, particularly in treating tumors. Unfortunately, there are limited non-invasive methods to measure slow biological fluid movement. In a recent publication in Magnetic Resonance Letters, Chinese researchers introduced a new MRI technology tailored for precisely measuring interstitial fluid flow without invasive procedures.

“Interstitial fluid flow measuring has always been a challenge for understanding of the tumour microenvironment, which drives the development of an applicable characterization method for quantitative description,” says the study’s corresponding author Dong Han, PhD, a professor at the National Center for Nanoscience and Technology. “Phase-contrast MRI (PC-MRI) is widely used technology for measuring the velocity of rapid flow in biological tissues, such as blood”.

Interstitial fluid moves much slower than blood. When using PC-MRI for slow flow measurements, strong and prolonged gradients are needed, but these can lead to motion artifacts. Additionally, slow flow measurements involve large encoding gradients and longer echo times, resulting in significant signal-to-noise ratio (SNR) loss due to T2* relaxation decay. This limits the practicality of PC-MRI for such applications.

To address this issue, the researchers combined PC-MRI with an improved stimulation echo sequence (ISTE). Co-corresponding author Wentao Liu, PhD, a professor at the same center, explains: “Conventional PC-MRI usually uses gradient echo, spin echo (SE), and stimulated echo (STE). 

Compared to the gradient echo, SE uses a 180-degree focusing pulse to focus the signal on the transverse plane, and its signal is affected by T2 relaxation, which decays more slowly and has a slightly higher image SNR. STE excites a part of the signal to the longitudinal plane and mitigates part of the T2 relaxation decay.”

STE doesn’t outperform SE at any TE condition. Researchers introduced ISTE to realign magnetic moments in the longitudinal plane, leading to better SNRs compared to STE or SE. This innovation extends the velocity encoding gradient interval, reducing diffusion-related signal loss and improving accuracy in detecting slow-flow conditions. 

The goal is to enhance understanding of interstitial fluid flow using this method.