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How to cite this article: Kocher MR, Kovacs MD, Stewart W, Flemming BP, Hinen S, Hardie AD. Added-value of iodine-specific imaging and virtual non-contrast imaging for the gastrointestinal assessment using dual-energy computed tomography. J Clin Imaging Sci 2021;11:68.
Dual-energy computed tomography (DECT) has become increasingly available and can be readily incorporated into clinical practice. Although DECT can provide a wide variety of spectral imaging reconstructions, most clinically valuable information is available from a limited number of standard image reconstructions including virtual non-contrast and iodine overlay. The combination of these standard reconstructions can be used for specific diagnostic tasks that provide added value over traditional CT protocols. In this pictorial essay, the added value of these standard reconstructed images will be demonstrated by case examples for diseases specifically related to the gastrointestinal system.
Dual-energy computed tomography (DECT) can be acquired through many different CT hardware setups, often specific to different manufacturers. Although a review of the different methods of DECT data acquisition is beyond the scope of this article, the cases presented here were all acquired using a dual-source technique. This method obtains DECT data from two different CT source emitters positioned at 90° to each other (one with a low kVp setting and the other a high kVp). The subsequent post-processing of this data allows for a greater degree of material characterization because of the differing relative X-ray absorption characteristics at high and low energies.[1,2] As such, post-processed DECT image sets can be of significant value when optimized for specific applications.
Although more advanced than traditional CT, DECT can be readily adapted to clinical workflow, particularly when standardized post-processed reconstructed images are incorporated into a routine CT protocol. Specifically, a prior published study has demonstrated that DECT increased diagnostic confidence when used in routine emergency department studies and leads to a reduction in unnecessary follow-up studies. In our experience, the standardized reconstructions that have the most value for routine CT protocols include a blended 120 kVp single-energy equivalent, a virtual non-contrast (VNC) series, and an iodine overlay (iodine map superimposed on a VNC) to identify the presence of iodine.
In this manuscript, we will highlight some of the features of DECT that aid in the evaluation of gastrointestinal structures and diseases. DECT specifically allows marked improvement in the ability to identify intravenous or orally administered iodinated contrast which improves diagnosis of bowel perfusion abnormalities, active gastrointestinal bleeds, and extraluminal contrast leaks. Applications for material characterization in the gastrointestinal system will also be reviewed.
Acute bowel ischemia
Acute bowel ischemia is an abdominal surgical emergency with prompt treatment highly reliant on timely diagnosis and imaging findings. One of the limiting imaging findings includes the increased density of the ischemic bowel wall which is likely secondary to hemorrhage in the setting of developing necrosis. This finding may cause ischemic bowel to appear as if it was enhancing on routine contrast-enhanced imaging, even though it is not [Figure 1]. Although intramural hemorrhage within the bowel wall with associated hyperattenuation on unenhanced CT is almost 100% specific for the diagnosis of bowel ischemia with necrosis, it would be difficult to make this diagnosis with a single-phase CT. The use of DECT may increase the conspicuity of subtle differences in attenuation between a hypoenhancing segment and adjacent normal bowel wall [Figures 2 and 3].
Active gastrointestinal bleed
Active gastrointestinal bleed can be difficult to diagnose on traditional CT because of confounding hyperattenuating bowel contents. When intraluminal material is present, the diagnosis of active gastrointestinal bleeding can be confirmed with the internal presence of iodine and will disappear on the virtual non-contrast reconstructions [Figures 4 and 5], whereas hyperattenuating ingested material such as bismuth will not have any internal iodine and will persist on virtual non-contrast reconstructions [Figure 6]. In addition, multiple phases are needed including non-enhanced and multiple contrast-enhanced images sets to confidently identify and diagnose a gastrointestinal bleed.
VNC imaging may act as a surrogate to conventional non-contrast imaging, which can provide additional information when unenhanced images are needed to establish a diagnosis, and could potentially decrease radiation dose from multiphase CT examinations. Prior studies have confirmed that virtual non-contrast reconstructions can allow for the omission of an initial unenhanced scan.[6,7]
Iodine-selective imaging can allow for greater characterization and detection of iodine-containing lesions and can confirm enhancement in otherwise indeterminate lesions in multiple solid organs. Like DECT application for assessing solid organs, a similar approach can be utilized to assess for enhancement within possible gastrointestinal tumors. Non-specific wall thickening on routine CT can be assessed for the presence of focal iodine uptake on DECT, thus increasing specificity [Figures 7 and 8].
Extraluminal oral iodinated contrast medium can be a difficult diagnosis to make. On traditional CT, it can be difficult to differentiate hyperdense extraluminal fluid or surgical material with extraluminal oral contrast material [Figure 9]. When virtual non-contrast images and iodine overlays can be utilized, the absence of iodine can be confirmed and extraluminal contrast can be confidently excluded from the study.
When iodine can be confirmed extraluminally with iodine maps either with active extravasation from the bowel or within extraluminal fluid collections, the diagnosis of bowel perforation or leak can be made [Figure 10]. Alternatively, if there is no iodine content within the fluid collection, bowel leak will be less likely [Figure 11].
It is important to remember that it is not possible to accurately distinguish barium-based oral contrast materials from iodinated intravenous contrast mediums because of their similar attenuation curves. Therefore, interpretation of bowel leak or extraluminal contrast should be done with caution if both IV and oral contrast are concurrently administered.
Using similar techniques as described above regarding the detection of extravasated iodinated oral contrast material to detect a leak, the same principles can be applied to the detection of a fistula. The presence of iodinated contrast medium communicating with other loops of the bowel [Figure 12] or outside of the bowel can help definitively diagnose the presence of fistulas [Figure 13].
GALLBLADDER DECT APPLICATIONS
Multiple prior studies have demonstrated the utility of DECT in the evaluation of gallstones. A study by Lee et al. (2016) demonstrated that virtual non-contrast images at DECT allow better visualization of cholesterol stones and another prior study showed the benefit of low keV virtual monochromatic imaging to increase the conspicuity of noncalcified stones.[8,9] [Figure 14] demonstrates the different appearance of cholesterol as well as calcified gallstones in different DECT reconstructions. Routine 120 kVp imaging can often mask the presence of cholesterol gallstones, whereas low keV virtual monochromatic imaging, VNC, and ultrasound imaging can confirm their presence [Figures 15 and 16].
MISCELLANEOUS PROBLEM-SOLVING FOR GASTROINTESTINAL APPLICATIONS WITH DECT
While there are many well-defined clinical uses for DECT reconstructions, there are additional problem-solving applications to assist search patterns and elucidate pathology. The integration of an iodine overlay search pattern can help quickly pinpoint areas of pathology and further interrogate the presence of abnormal enhancement [Figure 17]. In addition, intraluminal contents can be further characterized [Figure 18].
The application of DECT and specifically iodine overlay and virtual non-contrast reconstructions can help improve the ability to elucidate gastrointestinal pathology. By incorporating these reconstruction search patterns, further information can be gathered to better answer clinical questions. Not only can DECT reconstructions help to identify intravenous and orally administered iodinated contrast but they can also improve the detection of gallstones and intraluminal contents.
Declaration of patient consent
Patient consent is not required as patient’s identity is not disclosed or compromised.
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Conflicts of interest
There are no conflicts of interest.
FulwadhvaUP, WortmanJR, SodicksonAD. Use of dual-energy ct and iodine maps in the evaluation of bowel disease. Radiographics. 2016;36:393-406.
WongWD, MohammedMF, NicolaouS, SchmiedeskampH, KhosaF, MurrayN, et al. Impact of dual-energy CT in the emergency department: Increased radiologist confidence, reduced need for follow-up imaging and projected cost-benefit. AJR Am J Roentgenol. 2020;215:1528-38.
LeeHA, LeeYH, YoonKH, BangDH, ParkDE. Comparison of virtual unenhanced images derived from dual-energy CT with true unenhanced images in evaluation of gallstone disease. AJR Am J Roentgenol. 2016;206:74-80.
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