British Journal of Radiology (2005) 78, 894-898
© 2005 British Institute of Radiology
doi: 10.1259/bjr/28094700
A comparison of MR cholangiopancreatography at 1.5 and 3.0 Tesla
D P O'Regan, MRCP, FRCR,
J Fitzgerald, DCR(R),
J Allsop, DCR(R),
D Gibson, MRCS,
D J Larkman, PhD,
D Cokkinos, MD,
J V Hajnal, PhD and
S A Schmitz, MD, PhD
Imaging Sciences Department (Clinical Sciences Centre), Hammersmith Hospital, Faculty of Medicine, Imperial College, Du Cane Road, London W12 0HS, UK
Correspondence: Dr S A Schmitz
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Abstract
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Clinical MR systems operating at 3.0 Tesla have the potential to significantly improve spatial resolution due to the boost in intrinsic signal to noise ratio. However, body imaging at these field strengths presents a number of technical challenges. We performed a prospective pilot study in which 10 patients underwent an MR cholangiopancreatography (MRCP) examination consecutively on 1.5 and 3.0 Tesla systems (both Philips Intera). An axial half Fourier segmented turbo spin echo (HASTE) sequence and a coronal thick-slab 2D turbo-spin echo (TSE) sequence were compared on both systems. A reader measured the signal intensity (SI) ratios of common bile duct (CBD): liver, and CBD: fat on HASTE images and CBD: liver on the TSE images. A second reader performed a qualitative analysis of the intrahepatic and extrahepatic biliary anatomy. Quantitative data was compared using the paired t-test and qualitative data with the paired Wilcoxon signed rank test with p<0.05. The quantitative analysis of the HASTE sequences showed a slightly higher signal intensity ratio (CBD:liver) at 3.0 Tesla compared with 1.5 Tesla (8.1 vs 5.6, p=0.002). No significant difference was found between the SI ratios of (CBD:fat) on HASTE images or (CBD:liver) on TSE images. The qualitative analysis showed superior image quality of 3.0 Tesla over 1.5 Tesla images on both HASTE (31 vs 25, p=0.032), and TSE sequences (34 vs 28, p=0.043). This pilot study shows that MRCP is feasible at 3.0 Tesla with some improvement in image quality and signal characteristics. Further development may be achieved with sequence optimization and improved coil design.
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Introduction
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MR cholangiopancreatography (MRCP) has increasingly become established as an important modality for the clinical assessment of biliary and pancreatic disorders at 1.5 Tesla field strength [1, 2]. Its indications include the evaluation and diagnosis of choledocholithiasis, choledochal cysts, primary sclerosing cholangitis (PSC), pancreatitis and anatomical variations of the biliary tree. Its main advantage over conventional endoscopic retrograde cholangiopancreatography (ERCP) is that it is a non-invasive imaging modality. However, it is emerging as a more effective diagnostic method for a number of indications. MRCP has demonstrated a greater sensitivity for detecting intrahepatic calculi [3], depicts more peripheral strictures in PSC [4] and has advantages in imaging critical biliary strictures compared with ERCP [5]. MRI also allows a routine assessment to be made of the solid upper abdominal organs as well as the biliary tree. In some institutions the diagnostic advantages afforded by MRCP have led to ERCP being increasingly reserved for when therapeutic interventions are planned or histological samples are required [6, 7].
The new generation of clinical MR systems operating at very high field strengths has, in general, the potential to significantly improve spatial resolution due to greater signal to noise ratio (SNR). However, imaging of the biliary tree at 3.0 Tesla also presents a number of potential difficulties. At higher magnetic field strengths magnetic field inhomogeneities are increased, susceptibility effects are greater and the specific absorption rate of radiofrequency power deposition may become a limiting factor [8].
The purpose of this study was to evaluate the feasibility of performing MRCP at 3.0 Tesla, and to directly compare the image quality and signal characteristics of a sample of patients undergoing an examination on both 1.5 and 3.0 Tesla systems.
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Material and methods
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Patients
The study was approved by the local ethics committee and written informed consent was obtained from all study subjects. A prospective study was designed in which patients who had undergone an MRCP examination at 1.5 Tesla were invited to attend for a subsequent study at 3.0 Tesla. Patients were excluded if they had undergone a therapeutic procedure in the interim; had a diagnosis of a potentially evolving condition such as acute cholecystitis, cholangitis, pancreatitis or had ascites; or whose general clinical state precluded further imaging. Of 49 consecutive patients scanned at 1.5 Tesla between November 2003 and April 2004, 10 participated in the study (6 female, 4 male; age range 32–73 years; mean age 59 years). The interval between the two studies depended on patient availability and had a median of 31 days. The indications for the study were as follows: pain post cholecystectomy (n=3); cholelithiasis (n=2); unexplained right upper quadrant pain (n=2); suspected primary sclerosing cholangitis (PSC) (n=2); and biliary dilatation (n=1). Among the 10 patients 3 demonstrated biliary dilatation and 2 had normal anatomical variants.
Imaging technique
Patients were asked to fast before the examination for at least 2 h and were studied in the morning where possible. All patients underwent routine MRCP imaging at 1.5 Tesla using a body array wrap around coil, and then were subsequently re-imaged at 3.0 Tesla using the built-in body coil as no body array coil was available for this system at the start of the study. All patients were imaged on Philips Intera (Best, Netherlands) platforms at 1.5 and 3.0 Tesla with identical gradient performance at both field strengths. Patients were studied in the supine position and images acquired under breath hold. The following sequences were used: a two-dimensional (2D) half Fourier segmented turbo spin echo (HASTE) sequence with a repetition time (TR) of 658 ms, an echo time (TE) of 80 ms and a slice thickness of 4 mm; and secondly a thick-slab 2D turbo-spin echo (TSE) with a TR 8000 ms, TE 800 ms and 4 cm slab thickness. The thin slice technique was used in axial and parasagittal orientations to show the long axis of the common bile duct in-plane. The thick slab technique was performed in five separate breath hold phases with rotation around the longitudinal axis of the common bile duct. Identical sequence parameters were employed at both field strengths with only the field of view being adjusted to avoid aliasing artefact when necessary. Where specific absorption rate limits were encountered scan times were increased by up to 25%.
Qualitative image analysis
An independent reader (SAS) with 10 years of MR experience performed a blinded qualitative analysis of randomized images reviewed on a ViewForum workstation (Philips). Each pair of MR sequences were evaluated together for the quality of visualization of individual structures of the pancreaticobiliary system according to predefined criteria. Eight structures were analysed: the gall bladder, left hepatic duct, right hepatic duct, common bile duct, cystic duct, pancreatic duct, duct of Santorini and the segmental ducts. Each structure, apart from the segmental ducts, was rated using six image quality criteria scoring either zero or one point in each of the following categories: adequate visibility, continuity of structure, adequate contrast to surrounding background, border clarity, signal homogeneity within structure and presence of abnormality. The eight segmental ducts scored a point each if the visible length of the duct was at least one third of the common bile duct. The maximum score for image quality was thus 50 for each sequence. The presence of diagnostically important chemical shift or susceptibility artefact was also recorded.
Quantitative image analysis
Another independent reader (DPO'R) with 3 years of MR experience performed a quantitative image analysis. Operator defined circular regions-of-interest were placed in the centre of the common bile duct as it passed through the head of the pancreas, in apparently normal peripheral parenchyma in the right lobe of the liver and on the HASTE images in perinephric fat. In the thick slab TSE sequence, which generates a projectional image and does not allow background structures to be clearly distinguished, regions of interest were only placed within the common bile duct and over apparently normal liver. In the CBD the size of the region of interest was adjusted to the duct calibre to include the inner two thirds of the transverse diameter and typically was 5 pixels wide in a non-dilated system. Only duct segments which were perpendicular to the image plane were analysed. The regions of interest in the perinephric fat and liver parenchyma were 10 pixels in diameter. The mean pixel values in these regions of interest were used to calculate the following signal intensity ratios: DL=SI(common bile duct)/SI(liver), and DF=SI(common bile duct)/SI(fat).
The acquired images on both systems were subject to proprietary post-processing by the scanner software. This employs a noise-adaptive filtering algorithm to suppress background signal on each sequence which would have led to an underestimate of noise measured by conventional means [9]. In an MRCP examination, which generates high contrast between biliary fluid and background signal, signal intensity ratios should reliably reflect the contrast between these structures.
The signal intensity ratios and image quality scores were expressed as mean values with their standard deviation. A statistical comparison of mean values was performed with the Student's t-test for paired samples. The data of the qualitative image analysis was compared using the Wilcoxon signed rank test for paired samples. Statistical significance was assumed at p<0.05 in all cases.
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Results
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The study was completed by all patients and was well tolerated. All the images acquired were of diagnostic quality. None of the patients with intrahepatic or extrahepatic duct dilatation had undergone a therapeutic intervention or had spontaneously decompressed their biliary systems. No cause was found for the biliary dilatation in these three patients, however, the recent passage of a calculus appeared likely in one patient with gall stones. In no patients was chemical shift or susceptibility artefact noted to have a diagnostically significant effect on image interpretation at either field strength. Comparative images at each field strength are shown for TSE (Figure 1
) and HASTE sequences (Figure 2) in two patients.
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Figure 1. Examples are shown for two patients of the thick slab turbo spin echo sequence. The upper images were acquired at 3.0 Tesla and the lower images at 1.5 Tesla. (a) Improved contrast of the biliary tree relative to background noise at 3.0 Tesla. There is also clearer delineation of the pancreatic duct (arrowheads). (b) Improved continuity in the visualization of the pancreatic duct at 3.0 Tesla (arrowheads), and of the segmental ducts (arrows).
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Figure 2. Examples are shown for two patients of the axial half Fourier segmented turbo spin echo (HASTE) sequence. The upper images were acquired at 3.0 Tesla and the lower images at 1.5 Tesla. (a) Some improvement of contrast between fluid in the common bile duct (arrowhead) and surrounding liver at 3.0 Tesla. A slight reduction in contrast between liver and spleen at higher field strength is due to prolongation of T1 relaxation times. (b) Good visualization of the intrahepatic and pancreatic ducts on both systems, with similar appearances of the signal void created by the gall bladder calculus (arrowheads). The images at 3.0 Tesla are not appreciably affected by the presence of susceptibility or chemical shift artefacts.
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Qualitative image analysis
The qualitative analysis showed a superior image quality, as expressed by the mean of the previously defined summary measure, of 3.0 over 1.5 Tesla images on both the HASTE, 31±5 vs 25±9, p=0.032, and the TSE sequence, 34±7 vs 28±4, p=0.043. The summary measures for individual patients are listed in Table 1, and for each structure in Table 2.
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Table 2. Mean image quality scores for each structure analysed at 1.5 and 3.0 Tesla (n=10). All scores are out of 6, apart from the segmental ducts where the maximum score is 8
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Quantitative image analysis
The quantitative analysis of the HASTE sequences showed a significantly higher signal intensity ratio for DL (common bile duct/liver): 8.1±1.9 (3.0 Tesla) vs 5.6±0.7 (1.5 Tesla), p=0.002 on the two systems. The small increase in the signal intensity ratio for DF (common bile duct/fat): 2.1±0.5 (3.0 Tesla) vs 1.8±0.2 (1.5 Tesla) did not reach significance. In the TSE sequences no significant difference was found between the signal intensity ratios DL (common bile duct/liver): 15.0±5.6 (3.0 Tesla) vs 16.7±8.0 (1.5 Tesla).
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Discussion
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This study demonstrates that clinical MRCP at 3.0 Tesla is readily achievable and that it offers an improvement in image quality over conventional 1.5 Tesla systems. There is also an improvement in contrast between the common bile duct and liver on HASTE sequences, but this was not matched on TSE sequences. A similar trend in achieving improved contrast has been observed between 1.0 and 1.5 Tesla systems used for MRCP imaging [10]. In particular, the depiction of intrahepatic ducts was improved at 3.0 Tesla compared with conventional systems and this may have specific diagnostic applications – for instance in the evaluation of primary sclerosing cholangitis.
The development of MRI at 3.0 Tesla has brought the potential advantage of increased signal to noise ratios which can be traded for improvements in spatial or temporal resolution. Compared with brain and orthopaedic imaging, where a significant improvement has already been demonstrated over 1.5 Tesla, several theoretical physical characteristics make abdominal imaging at 3.0 Tesla challenging [8, 11]. Susceptibility effects increase at higher field strength and in abdominal imaging this may cause artefact at gas-tissue interfaces. Signal-intensity variation may originate from B1 field-inhomogeneities, which also increase with field strength, and this may explain the inconsistent improvement seen in image quality at 3.0 Tesla amongst different patients. In addition, greater phase shifts may produce more fat/water cancellation resulting in poor border definition. Interestingly, MRCP is a fortunate exception with regard to some of these limitations since it exploits the intrinsically high contrast of fluid within the biliary system against the liver parenchyma. Similarly, the increase in fat/water phase shift has relatively less effect on the MRCP sequences due to the limited amount of body fat next to bile and pancreatic ducts, the fat suppression of the TSE sequences and the low overall signal intensity of fat in the HASTE sequences.
During a pre-study phase it was observed that depiction of the common bile duct and cystic ducts might significantly suffer from susceptibility artefacts from neighbouring duodenal bowel gas in non-fasting volunteers. By allowing at least 2 h of fasting before the scan this artefact was not observed in the study. Similarly, intrapulmonary air caused no degradation of image quality in our study. MRCP studies also require fast T2 weighted breath-hold imaging, which typically employ turbo spin echo or HASTE techniques using a long series of refocusing pulses in a short time period. The radio-frequency power deposition resulting from such sequences frequently reaches specific absorption rate (SAR) limits of 4 W kg–1. To reduce SAR, wait periods may be built into the sequence resulting in an increase in scanning time. While this was not necessary for TSE sequences, wait periods were included in the HASTE sequence resulting in a 25% increase in acquisition time. New sequence developments for multiecho sequences with variable refocusing flip angles have shown promising results and may overcome this limitation [12]. The TSE sequences did not demonstrate an improvement in signal intensity ratios and this may reflect the disadvantage of using a built-in body coil at 3.0 Tesla compared with a wrap-around body array coil at 1.5 Tesla. Further improvements at 3.0 Tesla would be expected with the development of more efficient phased array surface coils, which have the signal to noise advantages of a small coil, but over a much larger field of view [13]. Array coils also offer the opportunity of using parallel imaging acquision techniques to shorten scan times.
A limitation of our study is the quantitative measure of signal intensity ratios instead of contrast-to-noise ratios which are more commonly used. However, the background noise in the image has been subjected to a proprietary image domain filter as part of the scanner manufacturer's image reconstruction process which would result in an underestimate of noise and was therefore avoided.
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Conclusion
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This preliminary study demonstrates that the characteristics of very high field strength MRI can be successfully addressed in abdominal imaging. MRCP at 3.0 Tesla can provide equal or superior image quality compared with 1.5 Tesla, and shows improved contrast characteristics on HASTE sequences. MRCP is readily achievable at 3.0 Tesla and has the potential to offer diagnostic advantages in selected cases.
Received for publication January 26, 2005.
Revision received March 24, 2005.
Accepted for publication May 6, 2005.
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Abstract
Introduction
