Objectives There is increasing evidence that thrombotic events occur in patients with coronavirus disease (COVID-19). We evaluated lung and kidney perfusion abnormalities in patients with COVID-19 by dual-energy computed tomography (DECT) and investigated the role of perfusion abnormalities on disease severity as a sign of microvascular obstruction. Methods Thirty-one patients with COVID-19 who underwent pulmonary DECT angiography and were suspected of having pulmonary thromboembolism were included. Pulmonary and kidney images were reviewed. Patient characteristics and laboratory findings were compared between those with and without lung perfusion deficits (PDs). Results DECT images showed PDs in eight patients (25.8%), which were not overlapping with areas of ground-glass opacity or consolidation. Among these patients, two had pulmonary thromboembolism confirmed by CT angiography. Patients with PDs had a longer hospital stay (p = 0.14), higher intensive care unit admission rates (p = 0.02), and more severe disease (p = 0.01). In the PD group, serum ferritin, aspartate aminotransferase, fibrinogen, D-dimer, C-reactive protein, and troponin levels were significantly higher, whereas albumin level was lower (p < 0.05). D-dimer levels ≥ 0.485 μg/L predicted PD with 100% specificity and 87% sensitivity. Renal iodine maps showed heterogeneous enhancement consistent with perfusion abnormalities in 13 patients (50%) with lower sodium levels (p = 0.03). Conclusions We found that a large proportion of patients with mild-to-moderate COVID-19 had PDs in their lungs and kidneys, which may be suggestive of the presence of systemic microangiopathy with micro-thrombosis. These findings help in understanding the physiology of hypoxemia and may have implications in the management of patients with COVID-19, such as early indications of thromboprophylaxis or anticoagulants and optimizing oxygenation strategies. Key Points • Pulmonary perfusion abnormalities in COVID-19 patients, associated with disease severity, can be detected by pulmonary DECT. • A cutoff value of 0.485 μg/L for D-dimer plasma levels predicted lung perfusion deficits with 100% specificity and 87% sensitivity (AUROC, 0.957). • Perfusion abnormalities in the kidney are suggestive of a subclinical systemic microvascular obstruction in these patients.

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Imaging is used to detect, characterize, and monitor cancer. Recently, diffusion-weighted magnetic resonance imaging (DWI) has become a useful adjunct for assessing tumors with magnetic resonance imaging (MRI). DWI involves the acquisition of a magnetic resonance signal related to random thermal motion (Brownian motion) or the "diffusion" of water protons in tissue. The signal obtained with DWI is a measure of the net displacement of water molecules, e.g., the water path length in the extracellular, intracellular and intravascular spaces. This motion is largely random and can be reduced by structural barriers, such as cell membranes and collagen. Restricted diffusion in biological tissues is inversely proportional to tissue cellularity and the integrity of cell membranes, and it can be quantified with apparent diffusion coefficient (ADC) measurements (1). Depending on the microarchitecture, either restricted or increased diffusion can occur in malignant tumors. For instance, diffusion is restricted in cellular portions of the tumor, but it may be increased in necrotic portions (2, 3). Other causes of enhanced diffusion include increased water content arising from intratumoral edema and cystic tumor components (4-6).Although initially used to evaluate neurological diseases, the applications of DWI have been extended to oncologic imaging throughout the body made possible by improvements in MRI hardware and new sequences. The use of DWI as a complement to conventional MRI methods has led to improvements in the detection and characterization of tumors, treatment response monitoring, and detection of recurrence in oncology patients. Basic concepts of DWIIn 1965, Stejskal and Tanner (7) proposed the application of a symmetric pair of additional gradients on either side of the 180˚ refocusing radiofrequency pulse. The Stejskal-Tanner sequence is still the basis of modern DWI. For static water molecules, the phasing effect caused by the first gradient will be reversed by the second gradient, leading to no signal loss; however, this phasing effect will not be completely reversed if the water molecules are not stationary, which results in the observed well-known diffusion signal decay. The amount of diffusion weighting is determined by the b value, which describes the amplitude, duration of the applied gradient, and time interval between the two diffusion gradients (4). The degree of signal attenuation from water molecules is cor- ABSTRACT Diffusion-weighted magnetic resonance imaging (MRI), which involves the acquisition of a magnetic resonance signal related to the Brownian motion of water protons in tissue, has become a useful technique for assessing tumors. In this article, we review the basic concepts, imaging strategies, and body applications of diffusion-weighted MRI in detecting and monitoring cancer.

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