MR cell size imaging with temporal diffusion spectroscopy

Abstract

Cytological features such as cell size and intracellular morphology provide fundamental information on cell status and hence may provide specific information on changes that arise within biological tissues. Such information is usually obtained by invasive biopsy in current clinical practice, which suffers several well-known disadvantages. Recently, novel MRI methods such as IMPULSED (imaging microstructural parameters using limited spectrally edited diffusion) have been developed for direct measurements of mean cell size non-invasively. The IMPULSED protocol is based on using temporal diffusion spectroscopy (TDS) to combine measurements of water diffusion over a wide range of diffusion times to probe cellular microstructure over varying length scales. IMPULSED has been shown to provide rapid, robust, and reliable mapping of mean cell size and is suitable for clinical imaging. More recently, cell size distributions have also been derived by appropriate analyses of data acquired with IMPULSED or similar sequences, which thus provides MRI-cytometry. This review summarizes the basic principles, practical implementations, validations, and example applications of MR cell size imaging based on TDS and demonstrates how cytometric information can be used in various applications. In addition, the limitations and potential future directions of MR cytometry are identified including the diagnosis of nonalcoholic steatohepatitis of the liver and the assessment of treatment response of cancers.

Keywords: Cell size; Cytometry; Diffusion; IMPULSED; MRI; Oscillating gradient; dMRI.

Figure 1

Typical Diffusion-weighted signals for voxels from a single slice of healthy human liver. (A) raw DW signals; (B) Corrected DW signals with the removal of the IVIM effect.

Figure 2

Pulse sequences used in the IMPULSED data acquisitions.

Figure 3

(A) Correlation between fitted diameters and preset diameters in simulations. Fitted diameters generated from three different combinations of OGSE and PGSE signals, mean ± std (n=3, with three different intracellular volume fractions: 43%, 51%, and 62%) vs. preset diameters. The dot line represents the identity line. (B) Fitted intracellular volume fraction generated from three different combinations of OGSE and PGSE signals, mean ± std (n=6, with cell size ranging from 2.5 to 25 μm) vs. preset intracellular volume fractions. The dot line represents the identity line.

Figure 4

Microscopic pictures of K562, MEL, and MDA-MB-231 before and after anti-mitotic treatment. (B) IMPULSED-derived cell sizes for K562 and MEL at three different cell densities and for MDA-MB-231 before and after anti-mitotic treatment. Noted that the green bands represent the microscopy-derived cell sizes.

Figure 5

H&E stained histological image, IMPULSED-derived cell size and cell density maps of a representative slice through tumor, overlaid on T2-weighted MR images. (B). Box-and-whisker plot of the fitted cell size for three types of tumors. The 25th-75th percentiles are blocked by the box, the black and red bands inside the box are the median and mean, respectively, and the whiskers mark the SD. C. Correlation between histological-derived cellularities and IMPULSED-derived apparent cellularities for all the tumors.

Figure 6

Summary of IMPULSED fitted microstructural parameters versus intracellular water lifetime τ_in and cell membrane permeability P_m. Error bars in each subfigure denote across-sample STD. The $\overline{d_{vw}}$ range indicates histology-derived mean cell diameter ± STD of all cells, and mean d is the volume-weighted cell diameter.

Figure 7

Diagram of various cell morphology changes responding to anti-cancer treatment. All listed therapy approaches and corresponding TDS MRI results are reported previously: barasertib (7), paclitaxel (40), cetuximab (42), anit-PD-L1/anti-CTLA4 (4), and radiation (60)

Figure 8

(A) Tumors from ICB responders had a significantly higher percent of CD3+ T cells as measured by immunohistochemistry compared to ICB non-responders and control IgG-treated mice; (B) ICB responding tumors had a significantly smaller mean cell size as measured by IMPULSE than tumors from control or ICB non-responders; (C). Examples of Immunohistochemical analyses of MC38 tumors treated with either control IgG (top) or checkpoint inhibitors (bottom); (D). Validation of IMPULSED-derived cell sizes using histology.

Figure 9

Representative IMPULSED-derived maps of mean cell size d (left), intracellular volume fraction v_in (middle), and extracellular diffusion coefficient D_ex (rigth) overlaid on a high-resolution fat-suppressed anatomical image of a breast cancer patient.

Figure 10

Representative cell size distributions and MRI-Cytometry derived parametric maps of a breast tumor. (b) is T2-weighted b = 0 image of the tumor and (f) is an enlarged view. (a,c,e,g) are four examples of cell size distributions. (d) and (h) are MRI-Cytometry derived v_in and $\overline{d}$ maps of the same tumor.

Figure 11

IMPULSED-derived maps of mean cell size d, intracellular diffusion coefficient D_in, apparent intracellular volume fraction v_in, and extracellular diffusion coefficient D_ex overlaid on a high-resolution fat-suppressed anatomical image for three continuous slices of liver from a healthy human subject.