Diffusion Imaging and Tensor Physics for the Clinician
Andrei I. Holodny in Functional Neuroimaging, 2019
The motion of the molecules in a glass of water is isotropic; on average it is the same in all directions. There is no direction that the molecules would prefer to move in. This can be demonstrated by carefully injecting a drop of ink into the water; over time, the ink will spread out as a sphere. In this situation the degree of diffusion can be represented by a single numerical value, the apparent diffusion coefficient (ADC). The ADC is a measure of how much a water molecule moves because of its environment and temperature. At higher temperatures the water molecules have more energy, move faster, and have a higher ADC. If the water molecules are restricted in their motion, for example, because of surrounding cell membranes, then the ADC will be reduced. It is called the apparent diffusion coefficient because strictly speaking the diffusion coefficient of water is a physical constant that depends only on temperature, and not on its environment. However, it is a good approximation to say that the diffusion coefficient appears to be reduced by restrictions.
Multiple myeloma
Anju Sahdev, Sarah J. Vinnicombe in Husband & Reznek's Imaging in Oncology, 2020
DWI analyses the freedom of movement of interstitial water molecules which depends on many factors such as cell density, the relative content of fat and marrow cells, water content, and bone marrow perfusion or the presence of organized structures (80,81) (Figure 26.8). The signal intensity relies on diffusion of water molecules at microscopic level within tissues (82). Initial assessment of bone marrow disease by DWI is made by visually assessing the signal intensity on high b value images (usually b 1,000), with high signal corresponding to marrow infiltration. Studies have shown that diffusion is impaired within tumours and that a decrease in diffusion may indicate disease progression. Effective therapy may cause a transient decrease in diffusion owing to toxic cell swelling but thereafter as the cellular load is reduced diffusion increases significantly. This method of assessment is practically useful and clinically appealing for both radiologists and referring physicians because lesions are more conspicuous (83). The quantified parameter derived from DWI is the apparent diffusion coefficient (ADC), which is a direct indicator of water motion within extracellular and intracellular space and is thus directly related to tissue cell density (84,85). Yellow marrow has low signal on high b value images with low ADC values, red marrow has higher signal intensity on high b value images and higher ADC values (63,81).
Imaging in Rhinology
John C Watkinson, Raymond W Clarke, Louise Jayne Clark, Adam J Donne, R James A England, Hisham M Mehanna, Gerald William McGarry, Sean Carrie in Basic Sciences Endocrine Surgery Rhinology, 2018
Essentially, this is an extension of MRI predicated on the degree to which free water molecules diffuse in tissues. In general, the more cellular the tissue the less extracellular space available for water to diffuse. The degree to which diffusion occurs in tissues can then be expressed in terms of the apparent diffusion coefficient (ADC). The more cellular the tissue the lower the ADC and therefore it might be hoped that measuring the ADC of a tumour might help distinguish benign tumours from, usually more cellular, malignancies. Unfortunately, the picture is complicated by factors such as tumour necrosis and the presence of keratin, collagen or debris. A considerable overlap in ADC values has been observed between benign sinonasal tumours and malignancies53 although other observers report improved differentiation between inflammation, benign lesions and malignancy through the use of ADC mapping (a visual map of ADC values assigned to individual pixels rather than an overall ADC)54 particularly if interpreted alongside the analysis of time-signal intensity curves generated from dynamic contrast-enhanced MRI (DCE MRI).55
Predicting neoadjuvant chemoradiotherapy response with functional imaging and liquid biomarkers in locally advanced rectal cancer
Published in Expert Review of Anticancer Therapy, 2022
Trang Thanh Pham, Stephanie Lim, Michael Lin
DWI is the most studied functional MRI modality for prediction of CRT response in rectal cancer. Apparent diffusion coefficient (ADC) measures water diffusion through tissue, and has an inverse relationship with tissue cellularity. Viable tumor cells have low ADC values, whereas necrotic tumor cells have high ADC values. Single-center small prospective DWI studies assessing mean ADC in general have shown that a lower ADC before CRT [46–48], higher ADC after CRT [49–52], and a greater percentage increase in ADC over time-points [46–49,52–57] was associated with good response to CRT in rectal cancer. Patients with a high ADC before CRT are hypothesized to have radio-resistant tumor necrosis, leading to poorer CRT response. Selected prospective DWI studies for response prediction in rectal CRT are shown in Table 1. A meta-analysis of 6 DWI studies, compared DWI with standard morphologic T2-weight MRI, and found that ADC outperformed T2-weighted imaging in assessment of response after CRT [28]. Our group assessed tumor heterogeneity by performing a histogram analysis of ADC quantiles (10th, 25th, 50th, 75th, 90th), skewness and kurtosis before, during, and after-CRT. We found that ADC 75th and 90th quantile values were significantly higher in responders than nonresponders to CRT [50].
A diffusion weighted imaging study of basal ganglia in schizophrenia
Published in International Journal of Psychiatry in Clinical Practice, 2018
Giuseppe Delvecchio, Alessandro Pigoni, Cinzia Perlini, Marco Barillari, Amelia Versace, Mirella Ruggeri, A. Carlo Altamura, Marcella Bellani, Paolo Brambilla
However, up to date, only one diffusion imaging study has been conducted investigating basal ganglia in schizophrenia, showing disrupted microstructure in globus pallidus (Hashimoto et al. 2009). Diffusion weighted imaging (DWI) is a valid non-invasive technique able to evaluate microarchitecture organization of brain tissue. Specifically, the rate of water molecule diffusion is measured allowing to quantify the apparent diffusion coefficient (ADC). High ADC measures correspond to relatively unrestrained water diffusion (e.g. ventricles or demyelinated plaques), while low ADC measures reflect high concentration of cellular structures, such as preserved myelinated axons (Assaf and Pasternak 2008). Other than diffusion imaging, resting-state and whole-brain connectivity studies also showed altered functional and white matter connectivity of basal ganglia with other cortical and subcortical areas in psychotic patients (Duan et al. 2015; O’Hanlon et al. 2015).
New challenges in integrated diagnosis by imaging and osteo-immunology in bone lesions
Published in Expert Review of Clinical Immunology, 2019
Concetta Schiano, Andrea Soricelli, Filomena De Nigris, Claudio Napoli
Diffusion-weighted MR image (DWI) represents in vivo images of biological tissues weighted with the local microstructural characteristics of water diffusion providing an additional promising dimension to the conventional anatomical MRI. DWI delivers an excellent tissue contrast through detection of differences in the Brownian motion of water molecules in tissues. The apparent diffusion coefficient (ADC) is a parameter calculated from DWI and altered by any architectural changes in the proportion of extracellular to intracellular water molecules because the diffusion of water molecules is disturbed by intracellular organelles and macromolecules [67]. Malignant tumours usually show decreased ADC values because they are characterized by increased cellularity, larger nuclear/cytoplasmic ratio, and less extracellular space relative to normal tissues, which restrict the diffusion of water molecules [68]. Generally, except for cellularity, ADC is correlated directly with tumour necrosis because of increased presence of free water in the necrotic area [69]. Quantitative DWI is useful for monitoring the therapeutic response of primary bone tumours [70,71]. The authors correlated conventional MRI imaging findings and ADC values pre- and post-treatment with histological findings observing that the change in the ADC value was statistically greater in the group that manifested tumour necrosis exceeding 90% than in the group with less than 90% necrosis. The ADC value on DWI therefore, is a tool for monitoring the therapeutic response of primary bone sarcomas [70,71].
Related Knowledge Centers
- Contrast
- Diffusion
- In Vivo
- Macromolecule
- Tissue
- Tractography
- Mri Sequence
- Biological Membrane
- White Matter
- Spin–Lattice Relaxation