A. M. Wallnoefer, MD; C. J. Zech, MD; K. A. Herrmann, MD Department of Clinical Radiology, University Hospitals – Gross hadern, Ludwig Maximilian University of Munich, Marchioni nistr. 15, 81377 Munich, Germany
C O N T E N T S
22.1 Introduction 233
22.2 Clinical Applications of MRCP 233 22.2.1 Biliary Stones 234
22.2.2 Biliary Stenosis 234
22.2.3 Benign and Malignant Stenosing Disease 234 22.2.4 Post-Operative Assessment of the
Biliary System 234
22.2.5 Normal Variants of the Biliary System 235 22.2.6 Pancreatic Duct System 235
22.3 Standard MRCP Sequences 235 22.4 Technical Developments and Value of
Parallel Imaging for MRCP 235 22.4.1 Thick-Slab Single-Shot RARE
Sequences 236
22.4.2 Two-Dimensional Multi-Slice Acquisitions – HASTE Sequence 236
22.4.3 Three-Dimensional Turbo-Spin-Echo Sequences – 3D-TSE 237
22.4.4 Diffusion Weighted Imaging of the Liver with Black-Blood Echo-Planar Imaging 242 22.5 Future Developments for MRCP 242 22.6 Pancreatic Imaging 243
References 244
High-Resolution Imaging of 22
the Biliary Tree and the Pancreas
Astrid M. Wallnoefer, Christoph J. Zech and Karin A. Herrmann
22.1
Introduction
Endoscopic retrograde cholangio-pancreaticogra- phy (ERCP) is still referred to as the gold stand- ard in the diagnosis of pancreatico-biliary pathol- ogy. It was introduced as a diagnostic modality in 1968. As an invasive procedure it is associated with a morbidity of 7% and a mortality of 0.2%
to 1.0%. With the introduction of pulse sequences for the non-invasive magnetic resonance cholangio- pancreaticography (MRCP) in 1986 by Hennig et al. (1986), a competitive diagnostic modality has become available that meanwhile is well accepted as a less intricate alternative to the invasive ret- rograde endoscopic cholangio-pancreaticography.
Since MRCP has experienced tremendous technical improvement over the years, the method has evolved into a quick, simple, low-risk non-invasive imag- ing modality with the potential to replace ERCP in many respects. In certain aspects, it can also be considered superior to percutaneous ultrasound, computed tomography and percutaneous transhe- patic cholangiography (PTC).
The following outline is designed to fi rst give an overview on the current technical standards of MRCP imaging and the traditional and conventional state- of-the-art MRCP sequences and secondly to focus on the distinct technical advances and improvements with parallel imaging techniques with respect to the image acquisition and image quality of MRCP in diagnosing pancreatico-biliary pathologies.
22.2
Clinical Applications of MRCP
Numerous indications can be cited for MRCP as a non-invasive technique to assess the entire pancrea- tico-biliary system. The most frequent indication is to
rule out or confi rm intra- or extrahepatic cholestasis, cholelithiasis and cholecystolithiasis.
22.2.1 Biliary Stones
Compared to ERCP, cholelithiasis can be diagnosed on standard MRCP with a sensitivity and specifi city of 92% (80%-97%) and 97% (90%-99%), respectively (Guibaud et al. 1995; Holzknecht et al. 1998; Kim et al. 2002; Soto et al. 2000). The sensitivity in detect- ing biliary stones strongly correlates to the size of the stone and varies from 64% to 100% with a cut- off criterion of either greater or lesser than 3 mm (Mendler et al. 1998).
22.2.2
Biliary Stenosis
MRCP is also very sensitive in the detection and pre- cise localization of stenoses within the pancreatico- biliary system. Sensitivities ranging from 91%–99%
(mean 97%), and specifi cities around 84% have been reported with typical sequences currently proposed for this purpose (Holzknecht et al. 1998; Magnuson et al. 1999; Rosch et al. 2002). The underlying cause of the stenosis can be identifi ed as benign or malignant with a sensitivity of 88% (70%-99%) (Holzknecht et al. 1998; Rosch et al. 2002; Urban et al. 2002). The sensitivity in the detection of stenosis depends on the degree of the luminal narrowing and the degree of the resulting cholestasis. High-grade and intermedium- grade stenoses are generally easily detected. Diffi cul- ties may be encountered in detecting low-grade sten- oses in cases of minor or absent cholestasis as it is often the case in primary sclerosing cholangitis (PSC).
With conventional MRCP imaging, pathologic changes of the intra- and extrahepatic bile ducts in primary sclerosing cholangitis (PSC) can be iden- tifi ed with a sensitivity of 88% and a specifi city of 97% (Fulcher et al. 2000). However, MRCP is still limited in the depiction of early pathologic changes in PSC presenting as discrete lumen irregularities in the second and higher-order branches. Owing to the availability of simultaneous cholangioscopy and endosonography and the option of direct injection and active dilatation of the biliary system with con- trast medium, ERCP is still superior to MRCP for the assessment of low-grade stenoses without preste- notic dilatation. With ERCP, the volume charge and
increased intraluminal pressure created by the injec- tion of the contrast medium will induce active dila- tation of the duct system and enlarge the lumen to its maximum diameter. Subtle parietal irregularities and low-grade stenoses are more easily displayed.
However, forced injection of contrast medium may result in iatrogenically induced pancreatitis or cholangitis (Loperfi do et al. 1998). In a large trial including 2,769 patients with diagnostic and thera- peutic ERCPs, the rate of major complications was 1.38% in the diagnostic investigations (n=942) with a mortality rate of 0.21% (Loperfi do et al. 1998).
Major complications were moderate and severe pan- creatitis, cholangitis, hemorrhage and duodenal per- foration.
22.2.3
Benign and Malignant Stenosing Disease
Differentiating between benign and malignant underlying pathology is successful with MRCP with a reasonably high sensitivity of 88% (70–99%) (Rosch et al. 2002; Urban et al. 2002). In contrast to ERCP, MRCP can be combined with further morphologic imaging sequences such as spin-echo and gradient- echo sequences in the same examination so as to complete the diagnostic work-up and include infor- mation on the hepatic parenchyma. By depicting also the tissue adjacent to the biliary system, potential mass effects, tumors or infl ammatory reactions can be identifi ed with an extended MRCP protocol, which signifi cantly supports the differentiation between benign and malignant processes.
22.2.4
Post-Operative Assessment of the Biliary System Further strengths of MRCP as a non-invasive procedure lie in the imaging of postoperative conditions when the access to the biliary system is limited for ERCP or PTC. MRCP is the modality of choice in the primary workup of postoperative complications to evaluate postoperative biliary leakage, stenoses or strictures of the bilio-digestive anastomosis, ischemic strictures as an early complication after liver transplantation and post-infl ammatory or post-interventional fi brosteno- sis after cholecystectomy. These pathologies are well identifi ed and distinguished with MRCP due to its high soft tissue contrast and the capability for imaging of the iuxta-luminal structures.
22.2.5
Normal Variants of the Biliary System
A non-invasive assessment of the biliary system is also warranted to rule out normal variants prior to biliary surgery or liver transplantation in order to prevent potential anatomy-related intra-operative complications. MRCP is ideal in this context, and normal anatomy, anatomic variants and congenital abnormalities of the biliary system and the pancre- atic duct system such as a pancreas divisum have been shown to be easily identifi ed with MRCP.
22.2.6
Pancreatic Duct System
In addition to the imaging of the biliary system, MRCP plays a major role in the non-invasive assessment of recurrent and chronic pancreatitis in ruling out or confi rming potential underlying causes such as stric- tures of the biliary duct or pancreatic duct system, choledocholithiasis and tumors. The non-invasive- ness of MRCP is again particularly advantageous in these cases, since with ERCP, imaging of the pancre- atic duct can be diffi cult and prone to complications in the presence of chronic pancreatitis and strictures.
ERCP implies a certain risk to induce pancreatitis as an iatrogenic complication in these cases.
22.3
Standard MRCP Sequences
Wallner et al. (1991) were the fi rst to describe the value of T2-weighted sequences in the depiction of the biliary system with MRI. Thanks to the long T2 relaxation times of static or low-fl ow fl uid, bile can be easily depicted on T2-weighted images, while fl uid with shorter T2 relaxation times is less well appre- ciated due to lower signal on T2-weighted images.
With ongoing improvements of the gradient systems, increased spatial resolution and shorter acquisition times have been achieved. State-of-the-art MRCP imaging currently uses ultra-fast turbo-spin-echo sequences. Two typical MRCP sequences are pivotal representatives of those:
the (single-slice) thick-slab RARE-sequence (rapid acquisition with relaxation enhancement) (Nitz et al. 1999) and
●
the (multi-slice) two-dimensional (2D) fast-spin- echo/turbo-spin-echo sequence with half-Fourier acquired k-space (single-shot FSE/TSE).
The RARE-sequence represents the so-called
“projection technique.” This type of MRCP is based on a 2D single-shot technique using one single slice with a high slice thickness of 30 mm to 100 mm (Laubenberger et al. 1995; Obenauer et al. 1999).
The resulting T2-weighted image is similar to projec- tion radiography. 2D-TSE sequences are used for the multi-slice approach. The most frequently employed sequence type of the multi-slice technique is the T2-weighted 2D half-Fourier-acquired single-shot turbo-spin-echo (HASTE) sequences. This acronym is subsequently used as a synonym for all thin-slice 2D-TSE sequences.
22.4
Technical Developments and Value of Parallel Imaging for MRCP
The major issues addressed with parallel-imaging tech- niques are the reduction of acquisition time, reduc- tion of image blurring and simultaneous increase in spatial resolution. For MRCP, the use of 12-channel phased-array coils will allow acceleration factors (R) of, e.g., 6; however, signal-to-noise constraints favor the use of a maximum acceleration factor of 3. With 32- channel phased-array coils and higher fi eld strengths, an acceleration factor of 6 is feasible without major limitations in image quality. In addition, high-reso- lution three-dimensional (3D) imaging of the biliary system with an isotropic voxel size of 1×1×1 mm³ or less has become feasible with parallel imaging using a T2-weighted 3D-TSE sequence with ultra-long echo trains, variable fl ip angles and restore pulses. This sequence opens new horizons for 3D post-processing and 3D image display and can be expected to revolu- tionize the diagnostic capabilities of MRCP in imaging the pancreatico-biliary system.
The benefi ts of parallel imaging for the quality and performance of MRCP are manifold. In the fol- lowing sections, the distinct effects of parallel imag- ing on standard MRCP sequences will be described in detail. Furthermore, newly developed parallel- imaging sequences for MRCP will be presented and discussed as to their additional diagnostic value in current MR pancreatico-biliary imaging.
●
22.4.1
Thick-Slab Single-Shot RARE Sequences
The RARE sequence is a single-shot TSE technique.
It was fi rst established by Hennig et al. (1986) and introduced into clinical practice by Laubenberger et al. (1995) as an MRCP sequence in 1995. The RARE sequence is characterized by one 90° RF pulse fol- lowed by a single, particularly long echo train com- prising, e.g., 256 or more spin echoes. All data are acquired after one single RF excitation pulse within one single echo train; every single spin echo will be separately phase-encoded. The echo time usually exceeds 635 ms and, thus, practically all signal from tissue apart from the bile is suppressed. The acquisi- tion time of one single measurement is in the order of 1 s. In order to include the pancreatico-biliary system in its entire extent within one single planar projection, the measurement slab has a thickness of 30 mm to 100 mm (Laubenberger et al. 1995;
Obenauer et al. 1999). Three different projections are acquired, which are angulated along the relevant anatomic structures of the biliary system such as the intra-hepatic bile ducts, the bile duct bifurcation, the choledochal duct and the pancreatic duct. These projections can be planned on standard T1- or T2- weighted sequences of the liver and the pancreas in transverse orientation.
The large slice thickness of RARE-sequences usually results in a certain impairment of the over- all image quality. Subtle pathologic changes of the pancreatico-biliary system and small intra-ductal concrements may be missed on these projections,
especially when surrounded with fl uid. Furthermore, the detailed diagnostic analysis of the of the biliary system may suffer from projection effects of overlay- ing structures with high T2-weighted signal intensi- ties such as ascites or other pathologic fl uid collec- tions that occur in exudative pancreatitis, pancreatic rupture or postoperative exudates. The diagnostic value of this projection technique is signifi cantly lim- ited in those cases.
The immanent advantage of RARE sequences is the ultra-fast image acquisition in the order of 1 s.
Hence, for severely ill patients with limited respira- tory capacities, the RARE sequence may be the only imaging technique to obtain suffi cient image quality with the least artefacts. For RARE imaging, parallel- imaging techniques shorten the echo train length and thus reduce the blurring of the image (Fig. 22.1).
22.4.2
Two-Dimensional Multi-Slice Acquisitions – HASTE Sequence
The HASTE sequence is a single-shot TSE sequence allowing for sequential acquisition of high-resolution T2-weighted images. If used in the context of MRCP, this sequence is applied in transverse and coronal ori- entations and typically in conjunction with spectral fat saturation. Due to its high echo train length, this sequence type is particularly appropriate to imag- ing fl uid. The entire 2D image data are acquired in a single echo train employing half-Fourier acquisi- tion, which relies on the symmetry of the k-space.
Fig. 22.1a–c. Infl uence of parallel imaging techniques on blurring artefacts and visualization of peripheral biliary ducts in a T2- weighted RARE sequence as used for MRC with a standard phased-array coil on a 32-channel 1.5-T system (Magnetom Avanto, Siemens Medical Solutions). Without parallel imaging a only the common bile duct and the biliary tree up to the fi rst order are visualized in this 47-year-old female patient without a pathological fi nding. The signal extinction in the common bile duct (arrow) is caused by a fl ow artefact arising from the hepatic artery. Note the blurry depiction of the biliary structures; the pancreatic duct cannot be visualized. With increasing acceleration factors of b R=3 and c R=4 these blurring artefacts disap- pear because of the reduced echo train length. Note the depiction of the biliary tree up to the second to third order and faint visualization of the pancreatic duct (small arrows) in the images with parallel imaging
c a b
The acquired data fi ll half of the k-space while the remaining k-space data are reconstructed exploiting k-space symmetry. Usually, slightly more than half of the k-space is fi lled with primary data to perform a phase correction of the mirrored data. Since only half of the k-space is actively measured, this technique is very time effective. As a single-shot technique, HASTE is less susceptive to motion artefacts.
If used in the context of MRCP, HASTE images can be acquired using respiratory gating. To implement respiratory gating, a navigator is positioned at the dome of the diaphragm. The diaphragmatic move- ments are monitored with 2D gradient echo sequences using low spatial resolution and high temporal reso- lution with an image acquired every 150 ms (Zech et al. 2004). Navigator-based respiratory motion correc- tion allows for both a free-breathing mode (respira- tory gating) or a breath-hold mode for multi-stack breath-hold examinations, the latter being less fre- quently employed for MRCP imaging. Application of respiratory gating in multi-breath-hold examinations guarantees the correct display of the positions of the sequentially acquired slices within one slice stack with- out slice gaps or overlay. Without respiratory gating in this context so-called “serious misregistration arte- facts” may occur. These artefacts represent gaps in the image information between subsequent image slices due to a different position of the diaphragm in differ- ent breath-holds. Respiratory gating for multi-breath- hold examinations helps to reduce these artefacts to a certain degree (Zech et al. 2004) since the patient’s respiratory cycles may vary in depth and differ in the positioning of the diaphragm at each single breath hold, which may impair the diagnostic analysis.
HASTE sequences are distinctly robust against susceptibility artefacts, which is particularly advanta- geous in postoperative imaging of the biliary system if metallic clips have been used after cholecystec- tomy, liver resection or liver transplantation. As com- pared to the RARE and the subsequently mentioned 3D-TSE sequences, HASTE is characterized by a rela- tively short echo time of, e.g., 100 to 150 ms and a good depiction of soft tissue structures. The detailed evaluation of anatomic background allows for the differentiation of benign and malignant stenosis.
HASTE is superior to other above-mentioned MRCP sequences in distinguishing structures adjacent to the biliary system. Periductal edema as an early indicator of initial cholangitis may be particularly appreciated in HASTE imaging with fat saturation (Fig. 22.2).
The major drawbacks of HASTE imaging are fl ow artefacts, which are likely to occur in the trans-
verse slice orientation. To date, their origin remains unclear. One primary hypothesis is that they could be the consequence of arterial pulsation of adjacent vascular structure such as the hepatic artery or the portal vein (Holzknecht et al. 1998). Furthermore, the typically applied slice thickness of 3 mm in the z- axis may be considered a limitation in the detection of very small intra-ductal concrements.
Apart from this, HASTE imaging suffers from the presence of image blurring. With increasing duration of the echo train, the relevant signal in the tissue con- tinuously decreases because of T2 relaxation, while image blurring increases. This effect is increased if tissue with relatively short T2 relaxation times (com- pared to the long T2 of fl uids) is imaged, which is the case in HASTE sequences, while the thick-slab RARE techniques suppress virtually all signal from non-fl uids. The essential benefi t of parallel imag- ing in HASTE imaging lies in the reduction of these blurring artefacts, cf. Chap. 10. With parallel imaging, the echo train length can be signifi cantly shortened, which results in a reduction of the image blurring.
With higher acceleration factors, however, the gain in image quality is counterbalanced by the dispropor- tionately increasing inherent noise created by parallel imaging at higher acceleration factors, since the dis- tinct reconstruction algorithms additionally contrib- ute to the lowering of the signal-to-noise as compared to non-accelerated imaging techniques (Heidemann et al. 2003). Currently, acceleration factors up to 3 are recommended and considered appropriate with a 32-channel acquisition system at 1.5 T. These param- eters seem to represent an acceptable compromise between reducing image acquisition time and blur- ring artefacts on the one hand and decreasing signal- to-noise ratios on the other hand. With acceleration factors of 4 and more, the image noise increases to an unacceptable degree, mainly in the central por- tions of the resulting MR image. Higher acceleration factors of R=6 are possible when increasing the fi eld strength from 1.5 to 3 T. An example of typical blur- ring artefacts with HASTE and the positive effect of parallel imaging in reducing the blurring are shown in Fig. 22.3 for a 3-T system.
22.4.3
Three-Dimensional Turbo-Spin-Echo Sequences – 3D-TSE
With the availability of parallel imaging, 3D imag- ing of the pancreatico-biliary system became both
Fig. 22.2a–d. A 50-year-old female patient presenting with laboratory signs of infl ammation after cholecystectomy is shown.
HASTE sequence (slice thickness 3 mm; parallel-imaging acceleration factor R=2) discloses diffusely increased signal intensity in the central portion of the porta hepatis paralleling the vascular structures corresponding to a periductal edema and thus suggesting cholangitis a. The identical fi nding is shown in the coronal plane b of the HASTE-sequence (slice thickness 3 mm;
acceleration factor R = 3). The corresponding slice in 3D-TSE imaging (slice thickness 1.5 mm; acceleration factor R=2) does not depict the periductal edema in single slice c or MIP display d. The intra-hepatic bile ducts show no irregularities
reachable and a future challenge. A T2-weighted 3D- TSE sequence has been proposed for the purpose of MRCP imaging (Barish et al. 1995; Soto et al. 1995).
In 3D imaging, one single volume is acquired with contiguous slices and no gaps. 3D imaging requires an additional phase-encoding gradient along the slice-selection z-axis in addition to the frequency- encoding and phase-encoding gradients in the x-y plane. The spatial resolution of 3D-TSE sequences is signifi cantly higher in the z-axis as compared to 2D imaging. On modern multi-channel MR-scanners with the option for parallel imaging, high-resolution imaging with isotropic voxel size and a spatial resolu- tion of 1×1×1 mm³ are feasible.
By convention, the 3D slab for the MRCP imaging is oriented in the coronal plane, since the most distal portions of the pancreatic duct and the biliary ducts have orthogonal positions. Due to a longer time of acquisition, the application of navigator-based respira- tory triggering is mandatory. The 3D properties of this
T2-weighted TSE sequence and the isotropic voxel size allow for the 3D-post-processing of all data sets with multi-planar reformats (MPR), maximum intensity projections (MIP) and volume-rendering techniques and other 3D functions (Fig. 22.4). These functions provide new facilities in displaying the pancreatico- biliary system that have not been accessible up to now.
Volume-rendering techniques seem distinctly helpful to improve the display of variations in the caliber of the intra-hepatic biliary ducts more conspicuously (Fig. 22.5). This improvement is particularly obvious in the second and higher order branches, which are not adequately displayed on 2D sequences.
High-resolution imaging requires longer acquisi- tion times. The overall acquisition time of 3D-TSE sequences is generally substantially higher than with all previously mentioned sequence types and reduces to only 3-5 min with parallel imaging. This compara- bly long acquisition time of 3D-TSE sequences will result in severe motion artefacts, which is why respi-
c
b a
d
ratory triggering is mandatory. For an effi cient image acquisition and good image quality, a deep and regu- lar respiration cycle is favorable to accelerate the data acquisition, since irregular breathing may increase the acquisition time to over 5 min. Without parallel imaging, the acquisition time with this sequence type usually exceeds 8 min.
In conventional imaging, motion artefacts are reduced by increasing the number of averages. This approach can also be employed with parallel-imaging techniques. Doubling the averaging and using parallel imaging at an acceleration factor of 2 will still allow keeping the acquisition time constant, e.g., at 5 min.
The resulting diagnostic image will be less disturbed with motion artefacts. With an acceleration factor of 4 and an 8-channel coil system at 3 T, the overall acqui- sition time for a 3D-TSE can be further diminished to values of 2 to 3 min. Nonetheless, motion artefacts cannot be entirely eliminated with these techniques.
According to initial preliminary results, 3D-TSE sequences with parallel imaging and respiratory trig- gering yield signifi cantly better image quality compared to RARE and 2D HASTE imaging in breath-hold tech- nique (Miller et al. 2004). Our own experience sup- ports these results according to which the 3D-TSE with parallel imaging yields better results in image quality and the detection of small details such as the smallest intra-ductal concrements (Wallnoefer et al. 2005).
Isotropic spatial resolution allows for multi-planar reconstruction and reformats of projection images in any virtual plane. In this same preliminary study, the conspicuity of all structures except the duodenum and the gallbladder was signifi cantly better with respira- tory-triggered 3D-MRCP (Miller et al. 2004).
3D-TSE with the free-breathing mode was superior to the breath-hold 3D-TSE and thick-slab 2D-RARE breath-hold technique regarding signal-to-noise and contrast-to-noise ratios (Zang et al. 2004). However,
Fig. 22.3a–d. Infl uence of parallel imaging techniques on blurring artefacts in a T2-weighted HASTE sequence as used for MRC with a standard phased-array coil on a 32-channel 3-T system (Magnetom Tim Trio, Siemens Medical Solutions). Without par- allel imaging severe blurring artefacts obscure the margins of the liver a, with increasing acceleration factors of b R=4, c R=6 and d R=8 these blurring artefacts disappear, because the echo train length can be reduced effectively from 1,756 ms without parallel imaging to 220 ms with an acceleration factor of R=8; however, due to increased image noise in the center of the fi eld of view – which is a typical artefact from parallel imaging – the optimal image quality is reached at a factor of R=6. The high image quality despite such a high acceleration factor is owing to the fi eld strength of 3 T in this example
c
b a
d
Fig. 22.4. 3D post-processing options for 3D-TSE sequences on a standard 3D Workstation (Leonardo, Siemens Medical Erlan- gen): MPR, MIP and volume-rendering techniques
Fig. 22.5. A 42-year-old male patient with primary sclerosing cholangitis (PSC). Volume rendering of a submillimeter 3D dataset with 0.9×0.9×0.9 mm3 voxel size acquired on a 3-T MR system (Magnetom Trio, Siemens Medical Solutions, Erlangen). On the left side a view from the anterior and on the right side a view from the oblique anterior-lateral is shown. The anterior view shows the severe intra-hepatic biliary duct stenoses (arrows) with pre-stenotic dilatation in both liver lobes. The anterior-lateral view shows the stenosis in the common bile duct (larger arrow), suspicious of a cholangiocarcinoma, which was confi rmed by surgery
in case of an unstable respiration, drop-outs in image quality can occur with the free-breathing technique (Masui et al. 2004).
Preliminary results have shown 3D-TSE sequences to provide a higher sensitivity in the detection of small gall stones when compared to the HASTE and RARE sequences. Especially prepapillary concrements are more reliably detected on 0.6-1.2-mm slices and repre- sent a signifi cant increase in diagnostic performance.
Multi-planar reformations are particularly helpful in doubtable cases of pathologic fi ndings identifi ed in one plane. In these cases multi-planar reconstructions help to specify the diagnosis without additional data acqui- sition. When compared to the HASTE sequence, 3D- TSE images are less susceptible to intra-luminal fl ow artefacts according to our own experience, which sup- ports the high performance in diagnosing small stones.
Figure 22.6 shows an example of a 68-year-old patient presenting with a small prepapillary concrement.
Owing to higher spatial resolution and options of 3D post-processing, the assessment of low-grade peripheral stenoses that do not reveal cholestasis is facilitated especially in patients with primary sclero- sing cholangitis (PSC). These patients profi t from 3D- TSE and parallel imaging. The combination of paral- lel imaging techniques and higher fi eld strength will most probably enhance this diagnostic benefi t and increase the diagnostic quality as has been shown in Fig. 22.5 (a 42-year-old patient with PSC and cholan- gio-cellular carcinoma). Further studies have to clar- ify if MRI at its latest state-of-the-art imaging with the newest technological advances can challenge the standard of ERCP.
One major disadvantage of the 3D-TSE sequence is its relatively long acquisition time and therefore its potential susceptibility to motion artefacts. Strong T2-weighting and long echo time result in excel- lent depiction of fl uid-fi lled structures, but suppress
Fig. 22.6a–d. 3D-TSE MRCP of a 68-year-old female patient with acute onset of jaundice is shown. All images (thick MIP, thin MIPs and MPRs) are derived from one single dataset, which was acquired in 4 min 15 s. The thick MIP clearly shows the intra- and extrahe- patic cholestasis b; however, only the morphology of the common bile duct disruption raises suspicion of a pre-papillary concrement, which cannot be appreciated in this image. An MPR shows multiple concrements in the gallbladder a. The thin MIP reconstructions d, c can clearly visualize this pre-papillary concrement (arrow) and a further concrement in the common bile duct along with multiple biliary concrements in the gall-bladder; note also the slight dilatation of the proximal pancreatic duct (small arrows)
c
b a
d
clear delineation of surrounding organic and soft tissue structures. Anatomic relationships, parenchy- mal structures and the characterization of the tissue adjacent to the biliary system are more conspicuous on HASTE sequences than on RARE or 3D-TSE imag- ing.
22.4.4
Diffusion Weighted Imaging of the Liver with Black-Blood Echo-Planar Imaging
The advent of parallel imaging techniques opens the access to new sequence types. Diffusion-weighted black-blood echo-planar imaging (BB-EPI) sequences were introduced for liver imaging to determine their potential diagnostic purposes in pancreatico-bil- iary pathologies. The use of parallel imaging allows implementing this type of sequences without major image distortion, cf. Chap. 10. Apparently, BB-EPI sequences present as favorable in the imaging of the biliary system and open new horizons of diagnostic information.
According to our own preliminary experience in ten healthy volunteers, BB-EPI sequences are capable of distinguishing dilated and cholestatic segments of the bile system from non-dilated bile ducts. Intra- hepatic cholestasis and limited and reduced bile fl ow result in an increase in signal intensity within the corresponding bile ducts. The underlying cause of cholestasis does not have any infl uence on this effect and does not affect the presence of this fi nding. In
our study design, ten healthy volunteers underwent MRCP with standard sequences and BB-EPI after fasting for >6 h before and after a high-caloric meal.
In all situations, the bile ducts were depicted with low signal intensity on BB-EPI. When compared to patients with cholestasis proven at ERCP, dilated and cholestatic bile ducts exhibited high signal intensi- ties within the corresponding depending segments (Fig. 22.7). Hence, BB-EPI seems to be capable of differentiating between cholestatic and non-choles- tatic segments, independently of the diameter of the respective bile duct.
22.5
Future Developments for MRCP
Breath-hold sequences are indispensable in abdom- inal MR imaging. With parallel imaging, the total acquisition time for those sequence types can be sig- nifi cantly reduced. Typical acquisition times are in the range of 20 to 25 s per breath-hold. In patients with respiratory impairment and in severely ill patients, this may exceed their limits of compliance.
Parallel imaging is very helpful to compensate for these inabilities.
A major motivation of parallel imaging is to increase spatial and temporal resolution simultane- ously. Signal-to-noise is limited with short acquisition times. High-fi eld scanners at 3 T will provide a higher
Fig. 22.7a,b. 3D-TSE sequence (acceleration factor R=2) with MIP projection a in a 40-year-old female patient with small intraductal concrements in the prepapillary section of the choledochal duct associated with intra- and extra-hepatic cholestasis. b Black-blood echo planar imaging (EPI; b-value: 50 s/mm²; acceleration factor R=2) in the same patient shows increased signal within the dilated biliary system indicating limited free motion and reduced bile fl ow, therefore supporting the diagnosis of cholestasis
b a
signal-to-noise ratio. Besides the advantageous effects on image quality and image acquisition time, high- fi eld imaging is linked to higher specifi c absorption rates (SAR), which may rise in critical regions.
With respect to MRCP imaging with HASTE sequences, this is particularly of concern since this type of sequence requires multiple refocusing 180°
pulses and thus causes high SAR. Lowering the fl ip angles of these refocusing pulses, e.g., with an opti- mized series of refocusing pulses <180° (hyperecho technique, Hennig and Scheffl er 2001) could con- tribute to solving this problem. The increase in the signal-to-noise ratio at 3 T in conjunction with the hyperecho technique and higher acceleration factors would allow HASTE imaging to be improved and more advanced in spatial and temporal resolution. 3D-TSE imaging will profi t from 3 T in that higher spatial res- olution will improve the depiction of fourth and fi fth level intrahepatic bile ducts so that a higher sensitiv- ity and diagnostic reliability may be achieved in the early stages of PSC and other benign or malignant pathologies in their early stages. However, one draw- back is the occurrence of signal loss from dielectric artefacts in the abdomen at 3 T (Table 22.1).
22.6
Pancreatic Imaging
Magnetic resonance imaging is an established diag- nostic procedure in the assessment of pancreatic dis- ease. With the help of a gadolinium-enhanced MRI, pathological conditions such as ductal adenocarci-
noma, chronic pancreatitis, neuroendocrine tumors, cystic neoplasms and intraductal papillary mucinous tumors can be detected and differentiated with high confi dence (Pamuklar et al. 2005; Pilleul et al.
2005). Due to the small extension of the pancreas and of pancreatic lesions, a high spatial resolution is mandatory for the exact evaluation of pancreatic pathologies. This fact makes modern multidetector- CT (MDCT) scanners a strong competitor for pan- creatic MRI, since excellent image quality and the highest spatial resolution can be provided by MDCT robustly (Prokesch et al. 2003). Therefore, all tech- niques that can increase the spatial resolution, that can overcome the problem of respiratory motion and that can decrease the time of acquisition are of high interest for pancreatic MRI (Fig. 22.8). Along with the innovations described in Chap. 21, parallel imaging helps to achieve that goal. If image quality and spa- tial resolution can be kept at a high level, MRI has several advantages over CT. On the one hand tissue characterization and differentiation are made easier by the intrinsic high signal intensity differences of pancreatic tissue, fl uid, fat or tumor tissue. This capa- bility can be increased by using extracellular contrast agents or even tissue-specifi c contrast agents such as, for example, mangafodipir trisodium (Teslascan, GE Healthcare, Norway) (Schima et al. 2002) (Fig. 22.9).
On the other hand, imaging of the pancreatic duct structures is non-invasively possible without differ- ences in image quality in comparison to the invasive procedure of ERCP (Calvo et al. 2002).
As pointed out for the liver in Chap. 21, parallel imaging can be recommended as a standard with an acceleration factor of R=2 for the transversal as well as the coronal orientation. Aliasing artefacts have to
Table 22.1 Sequence parameters for MRCP with parallel acquisition techniques
Sequence parameters RARE sequence T2-HASTE 3D TSE
TR 4,500 ms 1,040 ms 2,000 ms*
TE 635 ms 114 ms 889 ms
FOV 320 mm 380 mm 400 mm
Slice thickness 50 mm 3 mm 1.5 mm
Number of slices 1 36 40
Resolution 1.1×0.9×50 mm³ 1.4×1.4×3.0 mm³ 1.2×1.1×1.5 mm³
Matrix 384 320 384
Time of acquisition 0.5 s 2-3×18-15 s 3-5 min
Respiratory acquisition mode Breath-hold Breath-hold or respiratory gating Respiratory gating
*Effective time of repetition with respiratory gating depends on the length of the individual respiratory cycle of the patient and ranges between 4 and 5 s
be avoided by applying GRAPPA or choosing a suf- fi ciently large fi eld of view, because any artefacts in the center of the fi eld of view will obscure the region of the pancreatic head and might result in an insuffi - cient study. The background of these phenomena has already been displayed in Chap. 21.4.1. In contrast to MRI of the liver, the coronal orientation is very helpful in pancreatic imaging; therefore, the applica- tion of higher acceleration factors, as is possible for coronal acquired sequences or the combination of parallel imaging in two orientations in 3D sequences, is of relevance in the daily practice. This holds true both for T1-weighted 3D gradient-echo sequences, which are required for the detection of pancreatic adenocarcinoma and which allow an excellent depic- tion of the pancreatic parenchyma, as well as for T2- weighted 3D TSE sequences for the depiction of the pancreatic duct.
Fig. 22.8. Depiction of the pancreas with a T1-weighted 2D FLASH sequence with fat saturation on a 3-T system (Magnetom Trio, Siemens Medical Solutions) in a healthy volunteer. The sequence with 5-mm slice thickness and a 320×256 matrix (left) shows already a very good visualization of the anatomical area of the pancreatic head and the pancreatic tail (arrows). However, on the 3-T system a substantial increase in spatial resolution with 3-mm slice thickness and a 384×320 matrix is still possible without a visible loss of signal (right). Note the superior visualization of the pancreas on the right side. Both sequences are acquired with parallel imaging (GRAPPA; acceleration factor of R=2)
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