Magnetic resonance diffusion–weighted imaging: extraneurological applications

Abstract

Diffusion-weighted (Dw) imaging has for a number of years been a diagnostic tool in the field of neuroradiology, yet only since the end of the 1990s, with the introduction of echoplanar imaging (EPI) and the use of sequences capable of performing diffusion studies during a single breath hold, has it found diagnostic applications at the level of the abdomen. The inherent sensitivity to motion and the magnetic susceptibility of Dw sequences nonetheless still create problems in the study of the abdomen due to artefacts caused by the heartbeat and intestinal peristalsis, as well as the presence of various parenchymal-gas interfaces. With regard to focal liver lesions, a review of the literature reveals that Dw imaging is able to differentiate lesions with high water content (cysts and angiomas) from solid lesions. With regard to the latter, although there are differences between benign forms [focal nodular hyperplasia (FNH), adenoma] and malignant forms [metastasis, hepatocellular carcinoma (HCC)] in their apparent diffusion coefficient (ADC) in the average values for histological type, there is a significant overlap in values when lesions are assessed individually, with the consequent problem of their correct identification. One promising aspect is the possibility of quantifying the degree of fibrosis in patients with chronic liver disease and cirrhosis given that the deposit of collagen fibres "restricts" the motion of water molecules and therefore reduces ADC values. However, even in this field, studies can only be considered preliminary and far from real clinical applications. The retroperitoneum is less affected by motion artefacts and similarly deserves the attention of Dw imaging. Here it is possible to differentiate mucin-producing tumours of the pancreas from pseudocystic forms on the basis of ADC values even though the limited spatial resolution of Dw imaging does not enable the identification of small lesions. Dw imaging may be applied to the study of the kidney to differentiate hydronephrosis from pyonephrosis and with regard to tumours, solid from pseudocystic forms. In addition, given that renal parenchyma has significantly variable ADC values on the basis of the anatomic section and physiological conditions, the possibility of assessing functional alterations is currently being studied. Indeed, a good correlation has been found between ADC values and glomerular filtration rate. With regard to musculoskeletal applications, the absence of motion artefacts in the regions studied has enabled the development of sequences less sensitive to magnetic susceptibility and with greater spatial resolution than EPI. Attempts have therefore been made to use Dw imaging in the characterization of soft-tissue tumours although the findings so far have been disputed. Greater agreement has been found regarding sensitivity of the technique in assessing response of these tumours to chemotherapy: tumour necrosis is thought to increase ADC whereas the persistence of vital neoplastic tissue tends to lower it. One of the most pr...

Diffusion‐weighted images differentiate benign from malignant thyroid nodules

“…The aforementioned microscopic motion in biological tissue includes not only molecular diffusion of water but also microcirculation of blood (perfusion). Therefore, ADC includes both the effects of ''real'' diffusion and perfusion (pseudodiffusion) (17). DWI can be sensitive to both of these parameters, but it can be modified to be more or less sensitive to one or the other.…”

“…The diffusion in the extracellular-extravascular space, cellular diffusion, and intravascular perfusion all contribute to the calculated ADC value (17). Thus, DW-MRI provides information on perfusion and diffusion simultaneously in any organ.…”

Diffusion-weighted MRI in a liver protocol: Its role in focal lesion detection

“…The acquisition of "black blood" images with DW SS-EP sequences makes it possible to easily differentiate vessels from focal small-sized lesions in the liver [8,9] . Parallel techniques reduce artifacts, such as "blurring" or magnetic susceptibility artifacts, increasing the signal-to-noise ratio (SNR) [10,11] ; moreover, respiratory-triggered sequences offer a better possibility of SNR compared to breath-hold techniques [12] . Many studies have recently started to compare DWI to T1-weighted acquisition in the detection and characterization of liver lesions [13][14][15] .…”

The Role of Diffusion Weighted MR for Differentiation Between Benign and Malign Pancreatic Masses

“…Böylece dokudaki normal ve anormal yapıları birbirinden ayırt edebilir ve karakterizasyonunu yapabilir. [23] Herhangi bir voksel içindeki kapiller yapıların rastlantısal dizilimi nedeniyle, kanın mikro sirkülasyonu da ADC üzerine etkilidir. Bu nedenle ADC hem gerçek difüzyonu hem de perfüzyonu (psödodifüzyon) gösterir.…”

“…Dolayısıyla yüksek b değerleri bu etkiyi minimalize eder. [21][22][23][24] Bihan ve ark.nın çalışmasında belirttikleri gibi, b değe-ri düşük tutulduğunda sekansın difüzyon ağırlığı az olmakta ve difüzyona bağlı sinyal kaybı azalmakta, ADC değeri daha yüksek ölçülmektedir. [25,26] Ichikawa ve ark.nın çalışmasında b değerleri 1,6, 16 ve 55 gibi oldukça düşük değerlerde olup, abdominal organlara ait ADC ölçümleri yüksek olarak ölçülmüştür.…”