Improving the understanding and performance of clinical MRI using tissue property filters and the central contrast theorem, MASDIR pulse sequences and synergistic contrast MRI

Abstract

This paper updates and extends three previous papers on tissue property filters (TP-filters), Multiplied, Added, Divided and/or Subtracted Inversion Recovery (MASTIR) pulse sequences and synergistic contrast MRI (scMRI). It does this by firstly adding the central contrast theorem (CCT) to TPfilters, secondly including division with MASTIR sequences to make them MASDIR (Multiplied, Added, Subtracted and/or Divided IR) sequences, and thirdly incorporating division into the image processing needed for scMR to increase synergistic T 1 contrast. These updated concepts are then used to explain and improve contrast at tissue boundaries, as well as to develop imaging regimes to detect and monitor small changes to the brain over time and quantify T 1 . The CCT is in two parts: the first part states that contrast produced by each TP is the product of the change in TP multiplied by the TP sequence weighting which is the first partial derivative of the TP-filter. The second part states that the overall fractional contrast is the algebraic sum of the fractional contrasts produced by each of the TPs. Subtraction of two IR sequences alone about doubles contrast relative to a conventional single IR sequence. Division of this subtraction can amplify contrast 5-15 times compared with conventional IR sequences. Dividing sequences can be problematic in areas where the signal is zero but this is avoided by dividing the difference in signal of two magnitude reconstructed IR sequences by the sum of their signals. The basis for the production of high contrast, high spatial resolution boundaries at white-gray matter junctions, between cerebral cortex and cerebrospinal fluid (CSF) and at other sites with subtracted IR (SIR) and divided subtracted IR (dSIR) sequences is explained and examples are shown. A key concept is the tissue fraction f, which is the proportion of a tissue in a mixture of two tissues within a voxel. Contrast at boundaries is a function of the partial derivative of the TP-filter, the partial derivative of the relevant TP with respect to f, and the partial derivative of f with respect to distance, x.Location of tissue boundaries is important for segmentation and is helpful in determining if inversion times have been chosen correctly. In small change regimes, the high sensitivity to small changes in T 1 provided by dSIR images, together with the high definition boundaries, afford mechanisms for detecting small changes due to contrast agents, disease, perfusion and other causes. 3D isotropic rigid body registration provides a technique for following these changes over time in serial studies. Images showing high lesion contrast, high definition tissue and fluid boundaries, and the detection of small changes are included. T1 maps can be

Targeted magnetic resonance imaging (tMRI) of small changes in the T1 and spatial properties of normal or near normal appearing white and gray matter in disease of the brain using divided subtracted inversion recovery (dSIR) and divided reverse subtracted inversion recovery (drSIR) sequences

“…The dSIR and drSIR sequences are specifically T 1 -weighted rather than conventionally T 1 -weighted as with SE sequences of the brain. These conventional sequences also have ρ m -weighting and may actually be T 2 -weighted when imaging the Achilles tendon ( 23 );…”

“…The choice of TI may be based on a knowledge of tissue T 1 s and nulling TIs, experience with similar cases and recognition of the technical factors which change the nulling TI as well as conditions which change the T 1 of tissues to be nulled [see Tab . 8 in ( 23 )].…”

“…In four previous papers published in 2020 and 2022, a formalism was described to understand the signal, contrast and weighting of MRI pulse sequences using the concepts of tissue property filters (TP-filters) and the central contrast theorem (CCT) ( 20 - 23 ). This approach uses a quantitative definition of sequence weighting to understand the contrast produced by commonly used clinical sequences as well as more complex sequences.…”

“…Using narrow mD dSIR sequences in cases of MS, extensive abnormalities were demonstrated in areas of normal appearing white matter [e.g., Fig . 31 - 35 in ( 23 )].…”

Ultra-High Contrast MRI: Using Divided Subtracted Inversion Recovery (dSIR) and Divided Echo Subtraction (dES) Sequences to Study the Brain and Musculoskeletal System

“…In 2022 and 2023 we described the dSIR technique, which is even more sensitive to small changes in T 1 [19,20]. This sequence takes two images obtained using the IR technique and generates a third image by performing a voxel-by-voxel division of their difference by their sum.…”