New Imaging Techniques for the Evaluation of Gastrointestinal Diseases
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MINI-REVIEW New imaging techniques for the evaluation of gastrointestinal diseases 1 1 3 2 Faranak Tafazoli MD , Jana Taylor MD , Elizabeth G McFarland MD , David Gianfelice MD , 2 1 Luigi Lepanto MD , Caroline Reinhold MD F Tafazoli, J Taylor, D Gianfelice, EG McFarland, L Lepanto, Nouvelles techniques d’imagerie pour C Reinhold. New imaging techniques for the evaluation of gas- trointestinal diseases. Can J Gastroenterol 2000;14(Suppl l’évaluation des maladies gastro-intestinales D):163D-180D. This article provides an overview of recently de- RÉSUMÉ : Le présent article donne un aperçu des nouvelles techniques veloped, noninvasive imaging modalities for the evaluation of gas- d’imagerie, non effractives, pour l’évaluation des maladies gastro-intesti- trointestinal disease processes. The advent of spiral computed nales. L’arrivée de la tomodensitométrie hélicoïdale, de la cholangio-pan- tomography, magnetic resonance cholangiopancreatography and créatographie par résonance magnétique et de l’imagerie usuelle par conventional magnetic resonance imaging has facilitated the résonance magnétique a facilité l’évaluation non effractive des maladies noninvasive assessment of pancreaticobiliary disease. Magnetic pancréato-biliaires. La cholangio-pancréatographie par résonance magné- resonance cholangiopancreatography provides projectional im- tique fournit des images de projection de l’arbre biliaire et du canal pan- ages of the biliary tree and pancreatic duct, similar to those créatique, semblables à celles obtenues par la cholangiographie directe achieved by direct cholangiography, without the need to adminis- sans l’administration d’une substance de contraste. La colographie par to- ter contrast medium. Spiral computed tomographic colonography modensitométrie hélicoïdale fournit des images virtuelles de la muqueuse provides virtual colonoscopic images of the colonic mucosa, al- du côlon, ce qui permet la détection de polypes mais sans les risques asso- lowing the detection of polyps without the risk associated with ciés à la coloscopie. colonoscopy. Key Words: Gastrointestinal disease; Magnetic resonance cholangio- pancreatography; Magnetic resonance imaging; Spiral computed tomo- graphy ith the advent of newer imaging techniques, the the biliary tree and pancreatic duct, similar to direct cholan- Wnumber of noninvasive diagnostic modalities avail- giopancreatography, without the need for contrast medium able to the clinician for evaluating patients with suspected injection. In addition, the role of spiral CT colonography gastrointestinal disorders has increased considerably. An (virtual colonoscopy), which provides a three-dimensional overview of a number of these new imaging modalities for simulated visualization of the colonic mucosa, is reviewed. the evaluation of gastrointestinal disease processes is provided. The availability of spiral computed tomography SPIRAL CT APPLICATIONS FOR (CT), magnetic resonance cholangiopancreatography HEPATOBILIARY IMAGING (MRCP) and conventional magnetic resonance imaging Introduction to spiral CT imaging: With recent techno- (MRI) has facilitated the noninvasive assessment of pancre- logical developments and the refinement of slip ring scan- aticobiliary disease. MRCP provides projectional images of ning, spiral CT imaging has become the preferred method of 1Department of Diagnostic Radiology, University of McGill and 2Department of Diagnostic Radiology, University of Montreal, Montreal , Quebec2; 3Mallinckrodt Institute of Radiology, University of Washington, St Louis, Missouri, USA Correspondence and reprints: Dr Caroline Reinhold, Department of Radiology, Montreal General Hospital, 1650 Cedar Avenue, Montreal, Quebec H3G 1A4. Telephone 514-937-6011 ext 2872, fax 514-934-8263, e-mail [email protected] Received for publication March 4, 1999. Accepted March 12, 1999 Can J Gastroenterol Vol 14 Suppl D November 2000 163D Tafazoli et al three-dimensional imaging for subsequent surgical and therapeutic planning (2,3). With the advent of increased speed and flexibility, image acquisition can be achieved in a region of interest during a single breath-hold (20 to 30 s). Optimal timing of image ac- quisition also allows studies to be obtained in the different phases of peak arterial and venous contrast enhancement pe- riods. This capacity allows increased lesion detection, and better visualization of arterial and venous structures in tu- mour encasement, with the added benefit of shorter imaging studies for the patient. Hepatobiliary imaging – Liver imaging: Lesion detection is the most frequent clinical indication for CT evaluation of liver parenchyma. Other indications, including lesion char- acterization and evaluation of perfusion disorders, are under clinical investigation and are beyond the scope of this arti- cle. Conventional CT imaging and spiral CT protocols capi- talize on the fact that most hepatic focal lesions are hypovas- cular compared with the normal liver parenchyma. Injection of iodinated contrast and scanning at peak enhancement of normal parenchyma accentuate this difference in vascularity with pathological lesions. Spiral imaging protocols offer sev- eral distinct advantages over classical CT techniques with the capacity to acquire a full examination of liver paren- chyma in both peak arterial (approximately 20 s after injec- tion of contrast) and venous enhancement phases (70 s after injection) in less time required for a single examination with the single slice method (4,5). Arterial phase images offer the distinct advantage of dem- onstrating early arterial enhancement in certain less com- mon lesions that receive their vascularity from the hepatic arteries directly (ie, carcinoid metastases, focal nodular hy- perplasia and some hepatomas) (Figure 1). Images acquired in the venous phase with spiral protocols closely resemble standard slice by slice CT images with two distinct differ- ences: all images in spiral CT protocols are obtained in the peak ‘enhancement window’ (hence optimal distinction be- Figure 1) Spiral computed tomographic arterial phase imaging in a tween normal parenchyma and focal lesion) and, secondar- patient with multicentric hepatoma involving the left and right lobes. Im- ily, volume spiral acquisition virtually eliminates the ages obtained late in the arterial phase (top) show the enhancing lesion possibility of ‘unscanned’ portions of the liver due to varia- (arrows) in the right lobe. The right lobe lesion (arrows) is less evident on tions in patient respiration. the portal phase images (bottom) and may be interpreted as a vascular de- Spiral CT arterial portography: CT images acquired during fect. Note the portal vein thrombosis (long arrow) arterial portography studies have proved to be superior in their ability to detect focal hepatic lesions (Figure 2). The main indication for these more invasive studies is the evalua- obtaining diagnostic CT images of the hepatobiliary system tion of liver parenchyma in patients with known localized (1,2). Slip ring scanning allows continuous rotation of the hepatic metastatic disease who may be candidates for cura- x-ray tube situated in the scanner gantry and, hence, allows a tive resection (6,7). Injection of iodinated contrast through a rapid acquisition of large volumes of information compared catheter placed in either the hepatic or splenic artery allows with that obtained with classic single slice CT imaging tech- more intense enhancement of normal liver parenchyma and, niques. Several important advantages result from this new hence, increased visibility of pathological nonvascular focal scanning method, namely increased speed of acquiring imag- lesions. Spiral CT rapid acquisition allows all of the liver pa- ing; virtual elimination of ‘unscanned’ areas due to varia- renchyma to be scanned in this short ‘peak enhancement tions in patient respiration; optimal use and timing of window’, as opposed to slice by slice protocols, where the last intravenous contrast administration; and acquisition of a images may be acquired well past the peak and into the equi- volume of data that can more efficiently be converted to librium phase of contrast enhancement. 164D Can J Gastroenterol Vol 14 Suppl D November 2000 Imaging techniques for gastrointestinal disease Figure 2) Top left, bottom left Spiral computed tomographic arterial portograph showing metastases (M) in segments 7 and 6, respectively. Top right, bottom right Images obtained later, during the equivalent of the portal venous phase of an intravenous infusion. Note how the metastases (M) are less well demarcated from normal liver parenchyma. IVC Inferior vena cava Volume data acquisition and three-dimensional applica- two-dimensional transverse plane views. Several features of tions: The objective of image processing and three- spiral CT are instrumental in allowing three-dimensional dimensional reconstruction is to obtain the greatest amount imaging. The continuous acquisition of image data is impor- of information possible from the data acquired by spiral CT. tant in yielding a volume data set, as is the speed of acquisi- The three-dimensional images augment the diagnostic con- tion. This allows the entire data set to be obtained during a tent of the data set compared with transverse plane images single breath-hold – a prerequisite for valid three-dimen- alone and can obviate the need for more invasive proce- sional reconstruction. In addition, the increased speed of ac- dures. quisition allows data to be obtained during different phases