Quantitative MR Evaluation of Chronic Pancreatitis: Extracellular Volume Fraction and MR Relaxometry

Abstract

Objective: The purpose of this study was to determine if extracellular volume fraction and T1 mapping can be used to diagnose chronic pancreatitis (CP).

Materials and methods: This HIPAA-compliant study analyzed 143 consecutive patients with and without CP who underwent MR imaging between May 2016 and February 2017. Patients were selected for the study according to inclusion and exclusion criteria that considered history and clinical and laboratory findings. Eligible patients (n = 119) were grouped as normal (n = 60) or with mild (n = 22), moderate (n = 27), or severe (n = 10) CP on the basis of MRCP findings using the Cambridge classification as the reference standard. T1 maps were acquired in unenhanced and late contrast-enhanced phases using a 3D dual flip-angle gradient-echo sequence. All patients were imaged on the same 3-T scanner using the same imaging parameters, contrast agent, and dosage.

Results: Mean extracellular volume fractions and T1 relaxation times were significantly different within the study groups (one-way ANOVA, p < 0.001). Using the AUC curve analysis, extracellular volume fraction of > 0.27 showed 92% sensitivity (54/59) and 77% specificity (46/60) for the diagnosis of CP (AUC = 0.90). A T1 relaxation time of > 950 ms revealed 64% sensitivity (38/59) and 88% specificity (53/60) (AUC = 0.80). Combining extracellular volume fraction and T1 mapping yielded sensitivity of 85% (50/59) and specificity of 92% (55/60) (AUC = 0.94).

Conclusion: Extracellular volume fraction and T1 mapping may provide quantitative metrics for determining the presence and severity of acinar cell loss and aid in the diagnosis of CP.

Keywords: MR relaxometry; T1 mapping; extracellular volume fraction; pancreatitis.

Fig. 1

Histopathology of chronic pancreatitis (CP). Histologic hallmarks of CP are fibrosis, chronic inflammation, and loss of acinar cells. A, Photomicrograph (H and E, ×400) of mild CP shows perilobular (thick arrow) and intralobular (thin arrow) fibrosis. Most of pancreatic acinar cells (G) are still intact. B, Photomicrograph (H and E, ×200) shows amount of peri- and intralobular fibrosis (F) becomes more widespread as disease progresses, replacing normal acinar tissue (G). Nonstaining oval areas (arrow) indicate autodigestive fatty tissue necrosis. C, Illustration shows normal and increased extracellular space from fibrosis before (top) and after (bottom) enhancement with gadolinium. Normal pancreas is shown on left; chronic pancreatitis is shown on right. As amount of pancreatic fibrosis increases, lower T1 relaxation time is expected because of higher concentration of gadolinium (GD) in extracellular space. Extracellular volume imaging exploits this property of gadolinium-based contrast agents to calculate extracellular volume fraction of tissues. PD = pancreatic duct. (© Trustees of Indiana University)

Fig. 2

Patient selection algorithm.

Fig. 3

Bar graphs of mean extracellular volume (ECV) fraction and T1 relaxation time showing differences among study groups. Vertical lines and whiskers indicate 95% CIs. Normal = Cambridge grade 0; mild chronic pancreatitis (CP) = Cambridge grade 2; moderate CP = Cambridge grade 3; severe CP = Cambridge grade 4. A, ECV fractions of normal and three CP groups. Intergroup comparison (ANOVA) showed statistically significant difference between ECV fraction of each group (p < 0.001). Pairwise comparison showed that mean ECVs of all groups were statistically different (p < 0.05). B, Mean T1 relaxation times of normal and three CP groups. Mean T1 relaxation time of normal and severe CP groups were statistically different from all other groups. However, mild and moderate CP groups are similar.

Fig. 4

Comparison of ROC curves of different imaging variables used in this study for diagnosis of chronic pancreatitis. Best diagnostic performance was achieved by combining extracellular volume (ECV) fraction and T1 relaxation time (red curve) using logistic regression (0.94). AUC of individual variables were ECV, 0.90; T1 relaxation time, 0.80; anterior-posterior diameter in head (APD head), 0.70; arteriovenous enhancement ratio (AVR), 0.63; and signal-intensity ratio of pancreas to spleen (SIR P/S), 0.65.

Fig. 5

Quantitative imaging of pancreas. A, Axial quantitative extracellular volume (ECV) map in 67-year-old woman who was enrolled in pancreatic cancer screening program because of genetic susceptibility to breast cancer. Each pixel represents ECV fraction calculated by formula on scale of 0.0–1.0. MRCP was performed as annual screening test and showed no evidence of chronic pancreatitis (CP). Mean ECV fraction was 0.23. Border of pancreas is shown by white line. B, Axial quantitative ECV color map of pancreas in 47-year-old woman with history of pancreas divisum and repeated episodes of CP. Pancreas shows diffusely higher ECV fractions (mean, 0.49) compared with A. C, Unenhanced axial T1 mapping of pancreas in 55-year-old woman with family history of pancreatic cancer. MRCP was performed as annual screening and did not show evidence of CP or pancreatic cancer. T1 relaxation times were low throughout parenchyma, as indicated by blue color tones. Mean relaxation time of pancreas was 850 ms. D, Unenhanced axial T1 mapping of pancreas in 41-year-old woman with history of CP. Pancreas shows higher T1 relaxation times, as represented by turquoise and green tones. Mean T1 relaxation time was 1378 ms.

Fig. 5

Quantitative imaging of pancreas. A, Axial quantitative extracellular volume (ECV) map in 67-year-old woman who was enrolled in pancreatic cancer screening program because of genetic susceptibility to breast cancer. Each pixel represents ECV fraction calculated by formula on scale of 0.0–1.0. MRCP was performed as annual screening test and showed no evidence of chronic pancreatitis (CP). Mean ECV fraction was 0.23. Border of pancreas is shown by white line. B, Axial quantitative ECV color map of pancreas in 47-year-old woman with history of pancreas divisum and repeated episodes of CP. Pancreas shows diffusely higher ECV fractions (mean, 0.49) compared with A. C, Unenhanced axial T1 mapping of pancreas in 55-year-old woman with family history of pancreatic cancer. MRCP was performed as annual screening and did not show evidence of CP or pancreatic cancer. T1 relaxation times were low throughout parenchyma, as indicated by blue color tones. Mean relaxation time of pancreas was 850 ms. D, Unenhanced axial T1 mapping of pancreas in 41-year-old woman with history of CP. Pancreas shows higher T1 relaxation times, as represented by turquoise and green tones. Mean T1 relaxation time was 1378 ms.