Imaging Techniques
Imaging Techniques for AbdomenCHAPTER 1
INTRODUCTION
The imaging techniques for evaluation of abdomen have evolved dramatically over the past few decades. Conventional radiographs and barium studies now have a limited role with the cross-sectional imaging modalities such as computed tomography (CT) and magnetic resonance imaging (MRI) being the mainstay of diagnosis. Ultrasound remains the first line investigation for suspected hepatobiliary and pancreatic pathologies. The recent advent of ultrasound contrast agents and elastography techniques has improved the diagnostic yield of ultrasound. The routine contrast-enhanced CT and MRI have been modified in the form of multiphase acquisition and enterography/colonography for an optimal delineation of hepatic/pancreatic and bowel pathologies respectively. There has been a significant advancement in the nuclear imaging studies with the advent of hybrid scanners like positron emission tomography (PET)-CT and PET-MRI that are capable of providing both anatomical and functional information.
CONVENTIONAL TECHNIQUES
Conventional radiographs of the abdomen are now limited to the emergency setting for the evaluation of suspected perforation and intestinal obstruction.
BARIUM STUDIES
Endoscopy is the investigation of choice for most esophageal pathologies with a complimentary role for barium studies. However, fluoroscopic barium studies (Fig. 1.1) are essential in the evaluation of motility disorders.
The stomach and duodenum are also best evaluated by endoscopy in cases of dyspepsia and abdominal pain. However, barium studies (Fig. 1.2) are required for assessing functional abnormalities like reflux and delayed emptying; submucosal masses and infiltrative processes. The barium/gastrografin oral contrast study is also the technique of choice for evaluation of early postoperative complications following gastric surgery.
For radiological evaluation of small bowel, both barium follow through (Fig. 1.3) and enteroclysis (Fig. 1.4), are superior to CT/MRI examination in terms of demonstration of fine mucosal detail. The conventional enteroclysis has the advantages of shorter examination time, better distension of small bowel and better visualization of pathology but requires more radiologist time, greater technical skill, gives more radiation dose to patient and operator and is relatively uncomfortable for the patient.
Fig. 1.1: The lower thoracic esophagus and gastroesophageal junction well seen in the distended state in a normal barium swallow
Fig. 1.2: Supine radiograph of double contrast barium swallow showing well-distended stomach and normal mucosal pattern
Fig. 1.3: Barium followthrough examination showing well-distended jejunal loops with normal villous pattern
Fig. 1.4: Conventional small bowel enteroclysis showing normal distended jejunal and ileal loops in double contrast
Fig. 1.5: Double contrast barium enema. Supine radiograph showing normal mucosal outline of the cecum, ascending colon, part of transverse colon and descending colon. The hepatic flexure and rectosigmoid colon are in a dependent position and hence seen in a barium-filled state
Small bowel follow through study is easy to perform, allows estimation of transit time and is more comfortable to the patient. However, complete and adequate distension of small bowel cannot be achieved in followthrough study.1,2
Colonoscopy is the investigation of choice for evaluation of colorectal diseases. However, barium enema (Fig. 1.5) is useful for delineating colonic caliber and configuration.3 Also barium contrast enema is preferred in patients with question of large bowel obstruction, need for localization of colonic disease preoperatively or to assess the status of colon anastomosis.
Prior to undertaking any barium procedure, appropriate medical history along with results of previous investigations (including imaging and endoscopic procedures) should be obtained. Table 1.1 highlights the indications, contraindications, brief technique along with special modifications of the commonly performed barium procedures.4–7
ULTRASONOGRAPHY EVALUATION OF THE LIVER
Sonography is used as the initial imaging modality for suspected liver pathology.8 It plays a vital role in the evaluation of focal liver lesions, screening for liver metastases in a patient with known malignancy, screening for hepatocellular carcinoma in the setting of hepatitis/cirrhosis, portal hypertension, surgical obstructive jaundice, hepatic veno-occlusive disease and preoperative work-up and postoperative follow-up of liver transplant patients.3
5Sonography effectively differentiates a solid from a cystic lesion and provides significant information regarding the internal architecture of indeterminate lesions.
The addition of color Doppler flow imaging further helps in characterizing mass lesions and assessing patency of vessels.9 Doppler is particularly helpful in liver transplants, Budd-Chiari syndrome and cirrhosis.
Technique
The liver is scanned in the supine or left decubitus position with a 3.5–5 MHz convex transducer in the transverse, sagittal and oblique planes, from a subcostal approach. The sub-costal approach may not suffice in all patients and intercostal scanning may have to be done with a small footprint transducer.9 This is especially true in shrunken cirrhotic livers. An attempt is made to delineate the venous landmarks so that all the liver segments can be identified and scanned sequentially.
Sonographic Anatomy
Liver parenchyma is imaged as fine homogeneous mid-level echoes interrupted only by fissures and vessels. The liver echogenicity is usually greater but may be equal to renal echogenicity. Hepatic veins (Fig. 1.6) are well-defined tubular structures without significant marginal echoes, whereas portal veins have echogenic walls due to fibrous and fatty tissue (Fig. 1.7). Bile ducts run parallel to portal vein branches, but their location in relation to veins is variable, and the axiom that ducts are always anterior to portal vein branches is not correct. The state of art electronically focused ultrasound equipment demonstrates normal intrahepatic biliary radicles quite well and a bile duct must be at least 40% of the size of its neighboring portal vein branch for it to be considered dilated. Therefore, the mere visualization of parallel channels in the liver parenchyma does not indicate biliary dilatation.9
Fig. 1.6: Transverse US scan showing the three hepatic veins (arrow) joining the interior vena cava (IVC)
The gallbladder is located in the interlobar fissure and is well evaluated by ultrasound. The wall of the gallbladder should not be thicker than 3 mm in the fasting state. The common duct lies anterior to the portal vein at the porta hepatis and should be measured at the place where the hepatic artery crosses perpendicularly between them. This level has been used because of the consistent acoustic window provided by the surrounding liver, which ensures reproducibility of the measurement. The common duct should not be wider than 6 mm at this point.
On Doppler examination, the normal hepatic vein trace reflects the transmitted right heart pressure changes leading to flow reversal during the right atrial contraction (Fig. 1.8). The portal vein trace is continuously antegrade with a mean peak velocity of 15–22 cm/s. The portal venous flow shows slight undulation related to the cardiac cycle and respiration (Fig. 1.9).
Intraoperative Ultrasonography
Intraoperative and laparoscopic ultrasonography (US) are very useful techniques that have had major impact on patient management. A 5–7 MHz T-shaped linear array probe is used for intraoperative scanning. The probe can be sterilized using ethylene oxide gas sterilization. The probe is applied directly to the liver surface and no gel or acoustic coupling agent is necessary.
Fig. 1.7: Ultrasonography scan showing the bifurcation of main portal vein (MPV) into the right (RPV) and left portal veins (LPV)
Fig. 1.8: Color Doppler flow imaging of hepatic vein showing normal triphasic flow pattern with a short phase of reversed flow
The liver is scanned from the dome to the caudal edge and from left to right in a sequential manner. Color Doppler can be coupled with the gray scale scan and is very useful to identify vascular landmarks and assess their patency.
Intraoperative ultrasonography (IOUS) provides useful real time information to the surgeon and can alter the surgical approach. Current applications for IOUS include tumor staging, metastases evaluation, tumor ablation processes, evaluation of patency of vessels, intrahepatic biliary disease, and in liver transplantation.9
Endosonography
The development of endoscopic US (EUS) has rendered the gastrointestinal tract amenable to high resolution US examination and histopathologic sampling when required in the same session. Higher frequency probes (6–10 MHz) are used during EUS and the echoendoscopes have biopsy channels for performing fine-needle aspiration (FNA).10 It is useful for assessing the depth of tumor invasion and sampling of smaller lesions and adjacent lymph nodes. It is being used to stage esophageal and stomach malignancies in many centers.
Similarly, transrectal ultrasonography can be used for staging of colorectal carcinomas. Anal endosonography is useful in patients with injury to the external anal sphincter and presenting with fecal incontinence.
Endoscopic US is also used for the characterization of cystic lesions of the pancreas by identifying the ultrasound morphology and performing FNA with cyst aspiration.10
USG Elastography
Elastography is a recent development, which gives a measure of tissue stiffness.11,12 Depending on the deforming force applied and the method used to study the resultant deformation, it can be classified as:
- Static elastography
- Impulse elastography
- Unidimensional transient elastography (Fibroscan)
- Acoustic radiation force impulse (ARFI) imaging mode elastography
- Shear wave elastography.
Fig. 1.9: Color Doppler flow imaging of portal vein showing continuous antegrade flow with minimal phasicity
Acoustic radiation force impulse (ARFI) elastography gives a quantitative measure of the shear wave speed (in m/s) induced by acoustic radiation through an external vibrator. The measurement area is a 1 × 0.5 cm rectangle placed on the B-mode ultrasound image. These measurements are taken in the right lobe of liver through the intercostal space. Its main use is in the assessment of liver fibrosis and hepatic masses. However, this technique is not real time, does not give standard deviation and the size of the measurement area cannot be changed.11
Shear wave elastography measures the speed of shear waves that are produced in the tissue perpendicular to the direction of displacement when a focused ultrasound beam provides an acoustic ‘push’. The acquisition is real time with simultaneous display of the B-mode image and the corresponding shear wave elastography image (two- dimensional box with elastography color map). The speed of shear waves is used to calculate the elasticity in kilopascals (kPa and real time measurements can be made by placing the Q box over the region of interest (Fig. 1.10). The system generates the maximum, minimum and mean values along with the standard deviation. The size and position of the Q-box can be altered and in addition, retrospective measurements can be made with this system.7
USG Contrast Agents
Contrast-enhanced ultrasound (CEUS) uses microbubble-based contrast agents to improve the echogenicity of blood. It has major clinical applications in echocardiography, hepatology and vessel evaluation. It can be easily peformed after baseline ultrasound in the same sitting and provides functional vascular information without the use of ionic radiation.
Fig. 1.10: Shear wave elastography of the liver seen as dual display. The color in the box corresponds to the velocity of the shear waves. The velocity is measured in kilopascals and is displayed in the vertical bar on the right
The microbubbles have an average diameter of 1–4 µm and they oscillate at ultrasound frequencies of 1–15 MHz. These nonlinear oscillations generate echoes which are detected by the imaging system. With the availability of phase modulation and amplitude modulation techniques, the sensitivity of ultrasound equipment for microbubble detection has consistenly improved in the last decade.13
Examples of commonly used contrast agents include SonoVue (sulfur hexafluoride microbubble) and Levovist (microcrystalline suspension of galactose and palmitic acid). The contrast agent is available in the lyophilized powder form, which is to be mixed with 5 mL of 0.9% normal saline and injected as a rapid IV bolus followed by 5–10 mL of saline flush. USG scanning is started immediately and takes approximately 4–5 minutes. For the evaluation of liver lesions, real time imaging is performed through the arterial, portal venous and parenchymal phases (Figs 1.11A and B). Contrast is seen to pass through the hepatic artery and its branches with liver echogenecity increasing from the arterial to the portal phase. Lesion echogenecity is compared to the parenchyma at the same depth and their enhancement pattern is compared with that of the blood pool.13,14
USG Image Fusion and Navigation
The higher end ultrasound machines provide integrated image fusion and instrument navigation capabilities. The image navigation system provides real time guidance for soft tissue biopsy and ablation procedures in ultrasound combining the cross-sectional data available.
Figs 1.11A and B: Small HCC in a 30-year-old man with chronic liver disease: (A) Arterial phase US image obtained 20 seconds after contrast material injection demonstrates diffuse homogeneous enhancement of the nodule (arrow); (B) Delayed phase US image obtained 192 seconds after injection shows washout (arrows)
8Image fusion techniques makes it possible to target those lesions which are not well visualized on USG but are seen on another modality like CT or MRI. The CT/MRI images of the patient are added onto the ultrasound fusion system. The two-dimensional cross-sectional images are then transformed into dynamic, fused imaging maps that combine CT or MR imaging with live ultrasound.
CT EXAMINATION OF THE ABDOMEN AND PELVIS
For complete CT evaluation of abdomen and pelvis, images are acquired in the axial plane from the dome of diaphragm to below the ischial tuberosities in suspended respiration. The slice thickness is 5 mm or less. CT examination is optimized for each patient according to the relevant history and clinical indication. This includes determining the need for non-contrast scan and planning of the study as a single phase or multiphase study.15
Enteric contrast agent is used for better delineation of bowel loops and can be administered orally, via nasogastric tube, per rectally or in varying combinations. Positive contrast agents include dilute barium (1–2%) and water-soluble iodinated contrast material (2–3%). These help to differentiate intraluminal from extraluminal pathology and detect suspected leaks. However, bowel wall enhancement characteristics are difficult to interpret when positive contrast agents have been used. Similarly, they also interfere with the 3D reformatted angiographic images and should not be used in cases of suspected GI bleed.15,16
Neutral contrast material is more useful when bowel wall characteristics need to be evaluated and in vascular studies. These include water and agents with water density like mannitol, lactulose and methylcellulose. However, the evaluation of abscesses and hypodense collections becomes more challenging with use of neutral contrast agents.
Negative contrast agents like air and carbon dioxide are used routinely in CT colonography for optimal colonic distension.
When single-phase acquisition is planned, intravenous nonionic iodinated contrast (100 mL of 350 mg I/mL) is administered and images are acquired after 50–70 seconds. When evaluating CT abdomen, care should be taken to use different widow width and window level settings for viewing the visceral organs, the intra-abdominal fat, bony anatomy and lung bases as well. Multiphase acquisition is planned according to the area of interest as described below.15
CT Evaluation of Hepatic Pathology
Multiphase CT scanning should be performed for the evaluation of liver pathology.
Multidetector CT Evaluation
Typically a triple phase CT is performed in case of suspected liver lesion.
Non-contrast scan has a limited role, and is of use predominantly in patients treated with TACE or other ablative therapies. It can help to assess post-contrast enhancement in patients with siderotic nodules of increased attenuation. However, in order to keep the radiation dose low, most studies now do not recommend acquisition of the pre-contrast scan.17,18
Intravenous contrast is administered in a dose of 1.5–2 mL per kg body weight. The total iodine dose should be 525 mg iodine per kg or more; assuming iodine concentration of contrast to be 350 mg iodine per milliliter. Contrast should be administered at a flow rate of 3–4 mL/s using dual head mechanical injector and followed by saline flush. Most scanners now provide bolus-tracking software and passage of contrast through the descending aorta should be monitored to start the image acquisition.19,20
The images are acquired in the arterial, portal venous and delayed phases. The early arterial phase (about 10 seconds after the injection) is characterized by enhancement of the hepatic artery but not the portal vein. This phase is reserved for patients in whom CT angiography is required, such as potential transplant recipients, candidates for hepatic resection or cryoablation, or candidates for chemo- embolization.21 CT angiography images can be created through three-dimensional reconstructions of the thin slice, isotropic images obtained in the early arterial phase. Water is preferred as the oral contrast agent for multiplanar CT angiography to allow optimal reconstructions from the 3D data sets. Maximum intensity projection, surface-shaded display, and volume-rendered techniques are used in the post-processing of CT angiography to depict vascular anatomy (Figs 1.12A and B).
The late arterial phase, which is acquired 20 seconds after contrast injection, is characterized by enhancement of hepatic artery, its branches and the portal vein.22 This phase is critical for picking up hypervascular tumors. A combined CT angiogram/CT portogram can be obtained from the late arterial imaging phase and is used for delineation of the extrahepatic portal venous system in transplant cases and patients with pancreaticobiliary malignancy.21 Venous compression, venous stenosis or thrombosis, and portal systemic venous collateral vessels can be displayed.
This is followed by the portal venous phase acquisition at 60–80 seconds after the start of injection of contrast. In this phase, there is peak parenchymal enhancement with optimal opacification of both the portal veins and the hepatic veins.19,23
The delayed phase is acquired to study the washout characteristics of liver lesions, capsular enhancement and to observe the delayed contrast enhancement of certain tumors like cholangiocarcinomas.9
Figs 1.12A and B: Coronal maximum intensity projection [MIP] (A) and volume rendered technique [VRT] (B) reconstructed from the axial MDCT images obtained during early arterial phase of contrast enhancement demonstrates abdominal aorta and its branches.Abbreviations: CA, celiac axis; SMA, superior mesenteric artery; SA, splenic artery; GDA, gastroduodenal artery; RHA, right hepatic artery; LHA, left hepatic artery; RRA, right renal artery; LRA, left renal artery
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This is acquired at 3–5 minutes post-contrast injection.19
Dual Energy CT Evaluation
Triple-phase CT on a dual energy CT scanner is performed similarly as described above for the multidetector CT. The main advantage is generation of virtual unenhanced images without adding to the radiation dose (Figs 1.13A to D). These help to detect the presence of calcification and fat within lesions. Few studies have reported that it is feasible to quantity iron deposition in the liver, with dual energy CT.24
VOLUME ESTIMATION
Volume estimation of the liver (Fig. 1.14) helps in accurate planning of hepatic resection by delineating the exact 3D anatomy of the liver.21 Estimation of liver volume using three- dimensional techniques, when combined with clinical and laboratory evaluation of liver function, can aid in predicting postoperative liver failure in patients undergoing resection, assists in embolization procedures, and planning of staged hepatic resection for bilobar disease.25
MULTIDETECTOR COMPUTED TOMOGRAPHY EVALUATION OF PANCREATIC PATHOLOGY
For the evaluation of the pancreas, multiphase acquisition (Figs 1.15A and B) is recommended.26
The precontrast images are followed by the arterial phase at 17–25 seconds after the start of contrast injection. This phase is recommended wherever optimal delineation of the arterial supply is required.
The peak parenchymal enhancement of the pancreas is seen at 35–50 seconds after the start of contrast injection. This pancreatic phase is best for peak tumor-parenchymal attenuation difference. Curved multiplanar reformat (MPR) can be obtained along the pancreatic duct for showing relationship of the duct with the tumor (Table 1.3).
The portal venous phase is acquired 55–70 seconds after the start of contrast injection and is ideal for depiction of venous involvement and liver metastases.26,27 Minimum intensity projection (MinIP) images are used to delineate the anatomy of the pancreatic and biliary tree, which are seen as low-density structures.10
Figs 1.13A to D: Multiphase CT acquisition for liver performed on a dual energy scanner. Virtual non-contrast (A), arterial (B), portal venous (C) and delayed (D) phase images reveal widening of the fissures in this known case of chronic liver disease. No focal lesion was detected
BOWEL EVALUATION
CT Enteroclysis
Computed tomography (CT) enteroclysis involves the insertion of a nasojejunal tube under fluoroscopic guidance. The catheter tip is placed in the proximal jejunum and 1.5 L saline infused at 100 mL/min followed by on table infusion of 1.5 L saline with 100–120 mL intravenous contrast administration and single portal venous phase acquisition.28 It has been replaced by CT enterography at most institutions.
CT Enterography
This is the preferred examination technique for the evaluation of small bowel pathology and is replacing traditional barium fluoroscopic studies, especially in cases of Crohn's disease.29
Neutral oral contrast agents are administered for optimal distension of small bowel. Bowel is prepared by prior fasting for 8 hours and intake of 50–100 mL of laxative diet solution (Polyethylene glycol lavage) one day prior to the study.16 The neutral oral contrast agents used include water, mannitol, lactulose, methylcellulose, polyethylene glycol and 0.1% barium suspension. The CT attenuation of these agents is 0–30 HU and provides optimal contrast between bowel wall and lumen.29
The oral contrast should be consumed over 30–60 minutes. The protocol varies with institutions and a suggested protocol is given in Table 1.4.16 Intravenous contrast is given (100–120 mL @ 3–4 mL/sec) and single-phase images are acquired between 50–70 seconds (either enteric or portal 11venous phase). However, in cases of obscure GI bleeding, both the arterial and portal venous phases must be acquired. Multiplanar reconstructions in the coronal plane (Fig. 1.16) better depict the anatomical relation of bowel loops and maximum intensity projection (MIP) images provide angiographic images for suspected GI bleed.29
CT Colonography
CT colonography (Table 1.5)16 is a minimally invasive procedure for evaluation of the large bowel. It can be used as a screening, surveillance or diagnostic modality depending on the patient profile. Colonic preparation is essential and consists of low fiber diet (for 1–3 days), hydration and 50–100 mL of laxative diet solution (Polyethylene glycol lavage), one day prior to the study. Fecal tagging is usually performed with oral administration of water-soluble iodinated contrast or dilute barium. This is done to incorporate positive contrast into residual fecal material and increase its CT density. The increased density helps to distinguish fecal material from polyps or malignancy.30
The patient should be asked to evacuate prior to the study. Soft and flexible rectal catheter is inserted and the colon is insufflated with carbon dioxide (mechanical) or room air (manual).
Fig. 1.14: CT volumetry in a patient planned for partial liver resection. Segments II and III are manually mapped out on contiguous axial slices and the volume is calculated using volumetric software
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Figs 1.15A and B: Multiphase CT acquisition for pancreas performed for suspicious lesion on USG. Pancreatic phase (A) is acquired at 35 seconds and portal venous phase (B) at 65 seconds after the injection of contrast medium. No focal lesion was detected
Fig. 1.16: CT enterography (coronal reformatted image) showing normal distension of the small bowel loops with no abnormal enhancement
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CT scout film should be taken to check the adequacy of colonic distension. Low dose pre-contrast scan is acquired in the prone position, followed by intravenous contrast administration. Images are then acquired at 50–70 sec in the portal venous phase in the supine position. Optimal colonic distension is indicated by pencil thin colonic wall with thin haustral folds. Axial and coronal reconstructed images can be analyzed using CAD software, wherever available.16,30
MAGNETIC RESONANCE IMAGING
Magnetic resonance (MR) imaging is an excellent imaging modality because of its high contrast resolution, multiplanar capability, sensitivity to blood flow and lack of ionizing radiation. Technical advances in MR hardware and software have allowed the introduction of faster pulse sequences without the motion artifacts that previously posed limitations to abdominal MR imaging. The use of parallel imaging techniques with multi-channel phased array coils has significantly reduced the scan time. MRI of the abdomen (Table 1.6) is now an important modality for imaging the abdomen encompassing a variety of sequences, which are continuously being improvised.31
Higher field strengths are preferred (1.5 T or 3T) for abdomen evaluation. The advantage of 3T imaging systems is increased signal-to-noise ratio, better delineation of post- contrast enhancement and reduced time for chemical shift sequences (echo times of in and opposed phase sequences is less compared with 1.5 T). At the same time, in comparison to 1.5T, 3T scanners have increased specific absorption rate (SAR) and suffer from greater magnetic field inhomogeneity making images more prone to susceptibility and chemical shift artifacts.32,33
Phased array surface coil is used for abdomen evaluation. T1 and T2 weighted imaging is performed using breath-hold or respiratory triggered sequences. The acquisition is usually multiplanar with slice thickness of 5–8 mm and interslice gap not more than 3 mm. Either spin-echo (TSE or FSE) or gradient echo sequences can be used for T1-weighted imaging, with at least one in and out-of-phase gradient T1 sequence. T2-weighted images include either spin-echo sequences (TSE or FSE) or hybrid gradient and spin-echo (GRASE) sequences. Fat suppression is required during T2 weighted imaging, and can be achieved by chemical selective fat saturation, short tau inversion recovery (STIR), or spectral presaturation inversion recovery (SPIR). 3D sequences (both T1 and T2) are also available and provide higher SNR with better fat suppression. Intravenous contrast (gadolinium chelates) is used depending on the study protocol. Post-contrast administration, 2D or 3D T1-weighted sequences with fat suppression are acquired.31
Heavily T2-weighted magnetic resonance cholangio-pancreatography (MRCP) sequences are used for evaluation of the biliary and pancreatic ductal system. The sequences used are breath-hold rapid acquisition relaxation enhancement (RARE) or half-Fourier single-shot echo train spin-echo sequence as a thick slab acquisition (Fig. 1.17) in multiple projections or as multiple thin (less than 5 mm) slices in coronal or axial plane.13
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Fig. 1.17: Choledocholithiasis. Coronal thick slab projectional MRCP reveals multiple calculi in the common bile duct and right hepatic duct
3D respiratory-triggered T2- weighted FSE techniques with maximum intensity projection (MIP) reconstruction (Fig. 1.18) can also be used.31
Diffusion-weighted imaging for abdomen uses single-shot echo-planar sequences, breath-held or respiratory triggered. At least two b-values are used (b = 0 to 50 s/mm2 and b = 500 to 1,000 s/mm2) and ADC maps are generated for proper interpretation. DWI has short acquisition time and is now a part of the routine protocol. In patients with contraindications to MR contrast this sequence is particularly advantageous.16,31
Fig. 1.18: MRCP (MIP) image showing normal intrahepatic biliary radicles, common bile duct and pancreatic duct in a case of carcinoma gallbladder
MRI of the Liver
MRI has evolved from a problem solving technique to first line investigative modality for liver lesions.14
Pelvic phased array surface coil is used with field of view including the entire liver. Acquisition is predominantly in the axial plane with additional coronal plane imaging to better depict the vascular anatomy and the hepatic dome.34
T1-weighted imaging is performed as in-phase and out-of-phase chemical shift gradient-recalled echo sequences prior to contrast administration. This is useful in evaluation of focal fat containing lesions and diffuse pathology like hepatic steatosis and iron overload.35 In-phase (IP) and out-of-phase (OP) imaging is performed with a TE of 2.1 msec and 4.2 msec, respectively on 3T magnets. Both the images can be obtained in one TR by use of a double echo technique.35
T2-weighted images are acquired in the axial and coronal planes using a breath-hold or non-breath-hold technique. These include the accelerated fast spin-echo or single-shot accelerated fast spin-echo (half-Fourier single-shot turbo spin-echo (HASTE) or single shot fast spin-echo (SSFSE)) sequences. Respiratory compensation or respiratory triggering is employed whenever non-breath-hold sequences are used. An echo train length (ETL) of 16–20 provides optimal T2-weighted images in short scan time.35 Echo-train spin echo sequences acquired as contiguous thin 2D sections or as a thick 3D volume slab, also form the basis for MR cholangiography.
The TE of T2-weighted images is usually around 80-90 msec and longer TE (150–250 msec) T2W imaging can be used to differentiate between cysts and solid lesions.
The liver parenchyma appears homogeneous on both T1 and T2-weighted images. The liver shows moderate signal intensity on T1W images, similar to the pancreas but brighter than spleen and kidneys (Figs 1.19A and B). On T2W images (Fig. 1.20), the liver appears dark and has signal intensity less than that of spleen.
Post-contrast administration, dynamic fat-suppressed MR imaging (Figs 1.21A to D) is performed using a 2-D or 3-D technique. Contrast is administered at a dose of 0.1 mmol/kg at a rate of 2 mL/s and followed by saline flush.36 Pre-contrast scan is followed by the acquisition of scans in the late arterial (20 seconds after administration of contrast), portal venous (60–80 seconds) and delayed (2–5 minutes) phases. The bolus timing technique (includes automated bolus detection system or fluoroscopic triggering) or acquisition of multiple repeated arterial-phase datasets is used for planning the arterial phase. Additional delayed images (more than 5 minutes) may be required in cases of hemangiomas, vascular malformations, or cholangiocarcinomas.37,38 Subtracted images (subtraction of the pre-contrast scan from the post-contrast scan) should be generated in cases of T1 hyperintense lesions.
The use of hepatobiliary specific MR contrast agents is recommended for liver imaging. When gadobenate dimeglumine (Multihance) is used, one hour delayed scan is performed. However, with the use of gadoxetate (Eovist), hepatobiliary phase is obtained at 20 minutes post-contrast injection. This phase is used to document retention of contrast in lesions like focal nodular hyperplasia and to better delineate the biliary anatomy. However, it is important to obtain MRCP images (T2-weighted imaging of biliary tree) before biliary excretion of contrast, as the enhanced bile is not visualized on the MRCP images. For optimal time utilization, T2-weighted and diffusion-weighted images can be obtained after the injection of gadoxetate disodium.34
Diffusion-weighted images are obtained using echo-planar imaging (EPI) with breath held, free breathing, or respiratory-gated techniques. Imaging time can be reduced with the use of parallel imaging. DWI is performed with at least two b-values (b=20–50 s/mm2 and b=400 to 1000 s/mm2).
Figs 1.19A and B: T1-weighted imaging for the liver. In-phase (A) and out-of-phase (B) chemical shift gradient-recalled echo sequences are performed prior to contrast administration. Liver shows homogeneous moderate signal intensity, greater than that of the spleen
Fig. 1.20: Axial T2 WI fat suppressed sequence reveals homogeneous signal intensity of the liver. Note the signal intensity of liver is less than that of spleen
The use of ADC maps and calculation of ADC values is recommended to identify T2 shine through effect, which can be a confounding factor in the interpretation of DW images. DWI is useful in characterization of focal liver lesions, liver fibrosis and screening of the entire abdomen for additional lesions. It is of particular use in patients with contraindications to MR contrast medium. However, it is important to view the DWI images in conjunction with the other sequences and amalgamate, its findings with the morphology and enhancement characteristics for increased accuracy of diagnosis.34
MRI of Pancreatic Pathology
For the evaluation of pancreatic pathology, T1-weighted images are acquired as dual-echo GRE images in the axial plane and T2-weighted images as turbo spin-echo sequences in the axial and coronal plane. MRCP sequences using both 2D and 3D fast SE techniques are used to delineate the pancreaticobiliary anatomy.
Figs 1.21A to D: Multiphase MRI liver. Post-contrast (Gadobenate dimeglumine/Multihance) administration, T1-weighted fat suppressed gradient echo images are obtained at 20 seconds [late arterial phase] (A), 65 seconds [portal venous phase] (B), 180 seconds [delayed phase] (C) and 1 hour [hepatobiliary phase] (D) in a patient of chronic liver disease. No focal lesion was detected
16Intravenous gadolinium based contrast medium is injected at a dose of 0.1 mmol/kg of body weight and dynamic scanning is performed. This includes the acquisition of arterial (20 sec after injection of contrast medium), portal venous (45–65 seconds) and equilibrium phase (3–5 min) images. Some authors recommend a coronal plane (parallel to the bifurcation of the portal vein) T1 GRE acquisition two minutes after the injection of contrast medium.26,39
MAGNETIC RESONANCE ENTEROGRAPHY
This is a noninvasive technique for the evaluation of small bowel pathology. Since there is no radiation exposure, it is the modality of choice for pediatric patients and monitoring of patients of inflammatory bowel disease.
Patient preparation includes fasting for 6 hours and intestinal cleansing as for CT enterography.
Biphasic contrast agents (hyperintense on T2-weighted and hypointense on T1-weighted sequences) are preferred as they provide maximal contrast with bowel wall on the post-contrast sequences. These include agents like water, mannitol, locust bean gum, low-dose barium, manganese compounds, and polyethylene glycol. The protocol varies with institutions and a suggested protocol is given in Table 1.7. Some centers advocate prone position as it decreases respiratory excursion and enables better separation of bowel loops.16,40
Images are acquired in both the axial and coronal planes (Figs 1.22A and B). T2W imaging can be performed as spin-echo sequence (TSE or FSE), gradient-echo (GRE) sequence, or a hybrid gradient and spin-echo (GRASE) sequence.
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Figs 1.22A and B: MR enterography. Coronal T2-weighted fat suppressed fast spin echo (A) and post-gadolinium coronal T1 fat suppressed gradient echo (B) acquisition shows good bowel distension and no abnormal enhancement
17Single shot technique is more useful as it is relatively insensitive to motion. Fat suppressed sequences are preferred for the evaluation of active inflammation. T1 imaging can be performed as spin echo or gradient echo sequences with T1W 3-D gradient-echo sequence having the advantage of short scan time allowing image acquisition within a single breath-hold.
“Cine imaging” of the bowel is real time, dynamic imaging of a predefined region of interest by employing a heavily T2W coronal slab or a single-shot steady-state free precession sequence. This is useful for the identification of fibrotic strictures.40
NUCLEAR IMAGING
Neuroendocrine tumors (NET) like carcinoids and pancreatic NET express somatostatin receptors (SSTR) at the cell membrane.41 Functional imaging uses the affinity of somatostatin receptor analogs for tissues, which express SSTR. There are six main subtypes of SSTR (1–5 including 2A and 2B) with SSTR-2 and SSTR-5 being the most commonly expressed in NET.
Octreotide scintigraphy (using octreotide labeled with indium 111) is the standard functional imaging modality for NET's. It is combined with single photon emission computed tomography for improved spatial resolution. It is also used to evaluate the response to treatment and for metastatic work-up of NET.42 This technique has greater sensitivity for carcinoids and gastrinomas than insulinomas.42
Iodine 123 (123I) MIBG scintigraphy: This technique uses Metaiodobenzyl guanidine, which is a norepinephrine analog and binds to the norepinephrine transporter in NET.
PET/CT: Newer analogs labeled with gallium-68 are now used in combination with PET-CT imaging. However, they have more affinity to the SSTR-2 receptors than to the SSTR-3 and SSTR-5 receptors. Compared with octreotide scintigraphy, PET/CT requires less time and has a higher spatial resolution. In addition, the production of 68Ga-DOTA peptides is relatively less costly as it does not require a cyclotron.
Three types of 68Ga-DOTA-peptide analogs are available:
Out of these, DOTA-NOC has good affinity for SSTR-3 in addition to SSTR-2 and SSTR-5 receptors and is preferred as a broad-spectrum agent.43
68Ga-DOTANOC PET enterography is useful for delineation of small bowel carcinoids.16
18F FDG is also used as a glucose analog in FDG PET (can be combined with CTE/CTC for evaluation of bowel neoplasms and inflammatory bowel disease) and 18F DOPA is used as a catecholamine analog for DOPA PET in the evaluation of NET.16,44
ANGIOGRAPHY
Conventional hepatic/mesenteric angiography is now seldom used for diagnostic purpose as CT and MR imaging can generate high quality angiographic images in a non-invasive manner. The use of catheter angiography is now restricted to vascular interventions.
Technique
Coagulation profile and serum creatinine should be available prior to the procedure. The femoral artery is the preferred approach and Seldinger technique is used for the puncture. Angiography of the liver is performed by selective injections of the celiac axis and superior mesenteric artery (SMA) or one or more of their branches. A 5-French right angle or reverse curve catheter such as Cobra is commonly used. The volume of contrast used is about 20–30 cc injected at a rate of 5–6 cc per second. The portal venous system is visualized by injecting the splenic artery or SMA coupled with prolonged filming.
Anatomy
The celiac axis (Fig. 1.23) is the first major branch of the abdominal aorta and it gives rise to three major branches (the common hepatic, left gastric, and splenic arteries). The common hepatic artery gives rise to the gastroduodenal artery after which it becomes the proper hepatic artery. The proper hepatic artery divides into the right and left hepatic arteries, which further divide to supply the corresponding segments. The main branches of the gastroduodenal artery are the retroduodenal, superior pancreaticoduodenal and right gastroepiploic arteries. The splenic artery gives rise to short gastric branches and the left gastroepiploic artery. The superior mesenteric artery (SMA) arises approximately 1 cm caudad to the celiac trunk and supplies the small bowel through the inferior pancreaticoduodenal, jejunal, and ileal arteries. The right colon is supplied by the ileocolic, right colic and middle colic artery branches of the superior mesenteric artery forming an arcade (marginal artery of Drummond), which anastomoses with the branches of the inferior mesenteric artery (IMA). The branches of the IMA include left colic artery, sigmoidal artery, rectosigmoid artery and superior rectal artery.45
CONCLUSION
The imaging techniques for abdomen have evolved considerably in the past few years. Conventional radiographs and barium studies now have limited role in the evaluation of GI pathology. Recent advances in ultrasound like endosonography, elastography and contrast studies have significantly improved the diagnostic yield of a noninvasive and radiation free modality like ultrasound.18
Fig. 1.23: Intra-arterial DSA showing selective celiac axis injection, splenic artery (SA), left gastric artery (LGA), common hepatic artery (CHA), gastroduodenal artery (GDA), right hepatic artery (RHA) and left hepatic artery (LHA)
Cross-sectional modalities like CT and MRI remain the mainstay in the diagnosis and follow-up of most abdominal pathologies. A judicious use of the currently available imaging modalities is required for appropriate diagnosis and treatment of various abdominal diseases.
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