Evaluation of the FOCUS Diffusion's Added Clinical Value Compared to Conventional MRI

Overview

The radiologist plays a key role in the management of pancreatic tumours, which are potentially serious. While the scanner, with its high spatial resolution, plays a major role in pancreatic pathology, and in particular in the assessment of operability, MRI, with its good contrast resolution, has proven its contribution to the detection and characterization of focal lesions. Each MRI examination consists of several series of images called sequences, each with its own particularity, to highlight different types of abnormalities such as edema, bleeding, tumor content or vascularization. All the sequences performed constitute a "protocol". The diffusion sequence is a technology that allows the microscopic random movements of water molecules to be translated into images. It thus makes it possible to differentiate between certain aggressive tumours which are characterised by a higher cell density than healthy tissue, in which water molecules do not circulate freely, benign lesions such as cysts in which the circulation of water molecules is not hindered. The calculation of the Apparent Diffusion Coefficient (ADC), an estimate of the diffusion rate of water molecules, is a quantitative diagnostic tool validated in many fields of application and in particular in oncology.

This diffusion sequence has shown its usefulness in pancreatic pathology in the detection and characterization of focal pancreatic lesions, particularly for neuroendocrine tumors, the evaluation of chronic and autoimmune pancreatitis or the early detection of malignant transformation of certain high-risk cystic lesions. The diffusion sequence usually performed is called "wide field" because it covers the entire abdomen. The radiologist can thus analyse the neighbouring organs and in particular the liver, which makes it essential when a pancreatic tumour is suspected because of the risk of liver metastases. This wide-field sequence suffers from several limitations: difficulty in differentiating adenocarcinomas from chronic pseudo-mass pancreatitis due to the overlap of CDA values and difficulty in defining the boundaries of cephalic or corporal adenocarcinomas due to the hypersignal of upstream chronic obstructive pancreatitis (9)(10). Technically, it is subject to movement artifacts related to respiration, adjacent organs and in particular the duodenum, and to an average spatial resolution of the pancreas. Over the last 10 years, a diffusion sequence called "FOCUS" has been developed allowing a reduction of the field of view in the direction of phase coding and therefore a "zoomed" image with higher resolution. This sequence has shown interest in neuroradiology and prostate cancer detection in reducing artifacts and achieving better spatial resolution than the usual "wide field" diffusion sequence. 3 minutes and 30 seconds more are required to complete this sequence. The pancreas is a good candidate for FOCUS diffusion imaging because of its small size, susceptibility to movement artifacts and spatial orientation, allowing antero-posterior reduction of the field of view. Initial work has established the feasibility of this FOCUS diffusion in pancreatic imaging and shown an improvement in image quality (more accurate and less artefacts) in FOCUS diffusion compared to the usual "wide field". Due to its potential in pancreatic imaging, the FOCUS diffusion sequence has been performed in current practice since 2014 in the imaging department of the Groupe Hospitalier Paris Saint-Joseph (GHPSJ) in addition to the "wide field" diffusion sequence. The protocol thus created is called "combined protocol" as opposed to "standard protocol" which does not contain the FOCUS broadcast sequence. Kim H et al showed an improvement in the subjective clinical utility of readers in the diagnosis of benign or malignant pancreatic lesions. However, it should be noted that in this study the authors compared the two sequences by opposing them rather than comparing the usual wide-field sequence with the combination of the two sequences.