Shear Wave Elastography in Diagnosis of Inflammatory and Malignant Pancreatic Diseases

Only a few studies that have assessed the normal range value for pancreatic stiffness, inflammatory and malignant pancreatic lesions, shear wave velocities of healthy parenchyma, acute and chronic pancreatitis, malignant lesions of the pancreas will be evaluated and compared with other conventional imaging modalities, and evaluate its role in assessment of severity and prediction of clinical course/ outcome in patients with inflammatory pancreatic diseases.

The pancreas is a retroperitoneal organ that has several serious inflammatory and malignant diseases can occur, inflammatory diseases include acute and chronic pancreatitis (Janssen et al., 2007). Acute pancreatitis (AP) is one of the most frequent gastrointestinal causes for hospital admission in the US. Chronic pancreatitis, although lower in incidence, significantly reduces patients' quality of life (yadav et al., 2013). Pancreatic cancer has high mortality and is 1 of the top 5 causes of death from cancer. The burden of pancreatic disorders is expected to increase over time (yadav et al., 2013). AP is an inflammatory condition of the pancreas presenting as abdominal pain and increased levels of pancreatic enzymes in the blood, most commonly caused by bile stones or excessive use of alcohol (Leppäniemi et al., 2019). It has been associated with significant morbidity, mortality, and hospitalization costs (Janisch et al., 2016). AP is a common disorder that leads to large number of admissions in Egypt and elsewhere in the world (Ahmed et al., 2016). The overall prevalence rate is 45.1 per 100,000 population (satoh et al., 2011). The severe form comprising about 20-30% of the patients with hospital mortality rates of about 15% (van santvoort et al., 2011). Chronic pancreatitis (CP) is an inflammatory pancreatic disease characterized by pancreatic infiltration with inflammatory cells, progressive fibrosis, and loss of pancreatic parenchymal tissue (Ito et al., 2015). The global pooled incidence of CP is 40-50 per 100,000 general population per year (Xiao et al., 2016). The diagnosis of CP is mainly based on the demonstration of the morphological and/or functional changes that develop during the course of the disease. CP is sometimes diagnosed at the progressed stage because no standard diagnostic criteria or method of diagnosing CP at an early stage has been established (Stevens et al., 2010). As regard neoplastic lesions of pancreas, the global annual incidence rate for pancreas cancer is about 8/100,000 persons. Adenocarcinoma is the most frequent type of pancreatic cancer (Raimondi et al., 2009). The overall age-adjusted pancreatic cancer mortality rate in Egypt was 1.47/100,000 population (Soliman et al., 2006). The diagnosis of pancreatic pathology based on a combination of clinical signs and symptoms, imaging techniques and laboratory investigations (Lippi et al., 2012). Imaging provides a significant contribution to the diagnosis as well as to the assessment of disease severity in patients with inflammatory pancreatic lesions, also can assess the neoplastic pancreatic lesions with accurate detection of extension and follow up of chemotherapeutic effects and survival prognosis (D'Onofrio et al., 2010; Lippi et al., 2012). Imaging techniques for pancreatic diseases include ultrasound (US) B-mode, contrast enhanced computed tomography (CECT), magnetic resonance imaging (MRI), secretin-magnetic resonance cholangiopancreatography (S-MRCP), endoscopic ultrasonography (EUS), and endoscopic retrograde cholangiopancreatography (ERCP). All the techniques have variable sensitivity and specificity, with certain disadvantages (Mateen et al., 2012). Ultrasound B-mode (US) is effective in detection of etiological causes including presence of gallstones, common bile duct calculi, pseudocysts, pseudoaneurysm, and guiding percutaneous aspiration during the follow-up US also depicts the secondary signs for diagnosis of chronic pancreatitis like calcification and dilatation of pancreatic ducts (Munsell et al., 2010). CECT is more accurate following the onset of acute pancreatitis in assessment and quantification of necrosis. Although MRI is superior to CECT in this respect, however CECT is less expensive and readily available (Mateen et al., 2012). Typical CECT findings in acute pancreatitis include focal or diffuse enlargement of the pancreas, heterogeneous enhancement with irregular contour of the margins, increased density of peripancreatic fat planes, as well as the presence of intraperitoneal or retroperitoneal fluid collections (Lippi et al., 2012). There's increasing use of ultrasound elastography for measuring tissue elasticity (hardness). Shear wave elastography (SW-EG), a form of ultrasound elastography used in transabdominal ultrasonography, can measure tissue elasticity by generating shear waves inside the organ using the acoustic radiation force impulse (ARFI) (Hirooka et al., 2015; Kuwahara et al., 2016). The ultrasound machine monitors shear wave propagation and measures the velocity. The shear wave velocity, displayed in kilopascals (kPa) or meters per second (Hirooka et al., 2015; Kuwahara et al., 2016). Using ARFI, qualitative and quantitative determination of the tissue stiffness can be obtained (Bamber et al., 2013). Qualitative assessment through generation of elastogram (gray scale or red-green-blue) map in which hard tissue areas are shown in black or dark blue, soft tissue areas in white or red and intermediate stiffness tissues are characterized by a yellow/green color indicator. There is no absolute scale of tissue stiffness (Göya et al., 2014). Quantitative assessment generates numerical value related to the stiffness of tissue within the region of interest (Rees, 2008). SW-EG is effective for the non-invasive assessment of liver fibrosis in patients with HCV infection especially in advanced stages (F3 and F4) (Moustafa et al., 2017) However, studies on the role of shear wave in a deep-seated organ like pancreas are limited (Hiroki et al., 2009; D'Onofrio et al., 2009). Galloti et al. (2010) recorded normal values of shear-wave speed for healthy pancreas to be 1.40 m/s. Another researcher group compared the diagnostic success of shear wave elastography with B-mode ultrasound and CT scan in patients with AP (Bollen et al., 2012). They found that when using a cut-off value of 1.63 m/s, the sensitivity and specificity of VTQ for the diagnosis of AP were 100% and 98%, respectively (Bollen et al., 2012). Also, D'Onofrio et al. (2009) diagnosed pancreatic cystadenoma, which mimicked a solid neoplasm at conventional imaging (US and CT), as cystic at shear wave imaging . Kuwahara et al. (2018) stated that chronic pancreatitis may be diagnosed noninvasively and objectively using SW-EG without performing EUS. Only a few studies that have assessed the normal range value for pancreatic stiffness, inflammatory and malignant pancreatic lesions; from this point of view, shear wave velocities of healthy parenchyma, acute and chronic pancreatitis, malignant lesions of the pancreas will be evaluated and compared.