Common and uncommon abdominal applications of spectral CT

Routine use of spectral CT in abdominal examinations has helped give rise to a greater understanding of many conditions, in acute settings, in oncology, and in the management of chronic disorders such as kidney and biliary stones. Additionally, studies have shown that it can reduce the need for follow-up exams as clinicians benefit from access to a greater number of enhanced tools and imaging results available for image analysis on spectral imaging systems.17

There are multiple papers available concluding that spectral CT gives reliable image quality, without dose penalty for the patient.18-20

Kidneys: stone composition

More and more, spectral CT is being used for the characterization of kidney stones. These types of images require analysis using dedicated software that can assist in the detection of uric acid components within stones and distinguish it from calcium. Although this ability has been possible in CT since 2010, the struvite component remains difficult to assess.21,22

Kidneys: inflammation, ischemia, and tumors

Iodine maps, Z effective maps, and monoenergetic reconstructions at lower energy levels can be useful for better detection of renal infarcts and distinguishing infarction from acute pyelonephritis (Figure 3).

The availability of VNC contributes to a better diagnostic approach to initially nonsuspected cystic renal lesions, when the acquired series is only at the portal phase and the HU attenuation of the lesion is higher than 20. In these inconclusive CT examinations, the option is to call back the patient for an additional unenhanced acquisition, or for an abdominal sonography. The VNC images available with spectral CT are therefore helpful to attest the HU attenuation of the cystic lesion in the same session. Iodine maps help to confirm the absence of perfusion of the lesion.

Biliary stones

Ultrasound and MRI are the imaging methods of choice when gallbladder and biliary tracts stones are considered. Nevertheless, in many acute conditions, an abdominal CT is performed as it can help the clinician in diagnosing initially unsuspected cholecystitis and biliary tract obstruction. Stone detection is achievable with conventional CT images in a range from 25 to 88%, depending on the series;23 this detection is based on the kilovoltage of the CT system and the composition of the stone (presence of enough calcium and/or fat).

The role of CT is improved by the availability of virtual monoenergetic images, which have been recently noted as being valid in the better detection of gallbladder stones at different levels of energies, particularly the non-calcified forms.9,10,15

Bowel

All the tools available with the IQon Spectral CT have a valid application in the evaluation of bowel disease, including oncology, vascular disorders, infection, inflammation, and trauma.24

The availability of virtual non-contrast images in the data set of an abdominal CT focused on the bowel has several advantages:

• The true non-contrast acquisition can be deleted from the protocol when portal acquisition is included. This means a reduced radiation dose to the patient.

• Retrospective non-contrast images are useful when the chosen acquisition protocol does not include a non-contrast phase. In the acute setting, it helps to detect wall perfusion.25

• When oral contrast has been given before the CT examination, virtual extraction of the bowel content can help the clinician to detect or exclude a bowel suture leak.

• Colorectal cancer can be differentiated from stool.26

The use of low-energy reconstructed images contributes to a better assessment of the wall enhancement in patients suspected of intestinal ischemia, mainly when complicated bowel obstruction is questionable27 (Figure 4). The combined use of the low-energy images with iodine and Z effective maps is useful for better visualization of a gut wall perfusion and the detection of an acute bleeding source, as well as hypervascular lesions.28,29 Acute inflammation of the bowel could also be better detected when the injection quality is poor.

Colon cancer differentiation can be predicted by using the iodine concentration measured within the tumor, if an early arterial acquisition is performed.30 In poorly differentiated carcinomas, the vascularity of the tumor is increased, causing a higher iodine amount (1.59 mg/ml in poorly differentiated carcinomas vs 1.01 mg/ml in well differentiated carcinomas).

In rectal cancer cases, the malignant nodes have different spectral data compared to benign nodes, including iodine concentration measurement, Z effective values, and additional parameters, as well as the HU ratio between 80 and 140 keV values.31

Liver

In routine practice, liver CT protocols include non-contrast, arterial, portovenous, and late phases after contrast injection. VNC images can replace adequately true non-contrast images generated from arterial or portal phase.3 Lower energy reconstructed series are helpful for a better lesion detection (Figure 5).

Liver steatosis is a common clinical condition. Imaging methods are required to detect and follow steatotic patients and assess fibrosis. MRI and controlled attenuation parameters provided by Fibroscan (CAP) are considered as the reference for non-invasive methods. In this condition, ultrasound and CT have limited abilities. Currently, the use of spectral CT cannot replace MRI or CAP for liver steatosis assessment, but ongoing research shows promise.32, 33

Iron deposition can be evaluated with spectral CT in clinically relevant iron accumulation without interference of combined liver steatosis. However, it requires specific software to be implemented in the routine practice.34

Hepatocarcinoma (HCC) detection remains difficult to assess with CT. MRI is the non-invasive diagnostic and follow-up imaging method of choice; however, access to MRI is limited, and this technique also has several contraindications. Spectral CT could be an alternative method for the detection of HCC (including size < 10 mm) by using lower energy reconstructed images. This boost of contrast-to-noise ratio has already been demonstrated with iterative reconstructions in conventional imaging.35,36 Promising results have been reported and have demonstrated that the treated HCC could be optimally evaluated with spectral CT. Furthermore, spectral CT could be an additional method to assess response to therapy by measuring iodine concentration within the tumor.37

Various oncologic uses

Spectral CT can contribute to a better detection of metastatic melanoma within the abdominal cavity and in the bowel wall using arterial series reconstructed with low keV and with iodine map reconstructions.38 The response to therapy could be better assessed with a combination of RECIST criteria and spectral CT data.39

Local staging of uterine malignancies are generally evaluated with MRI. CT is used for the local and distant staging, as well as follow-up. It has been reported that the local myometrial invasion can be assessed with spectral CT. This imaging method can be an interesting alternative when MRI is not available40 (Figure 6).

Coronal conventional portal phase images (a and b) and VNC image (c) show a hypoattenuating bowel segment at the transition zone of obstruction (arrows) with preserved but reduced enhancement when HU point zones are compared between the VNC and portal phases. Reduced perfusion of one bowel segment (arrow) on 40 keV image (d), confirmed on iodine map (0.20 mg/ml, arrow (e). Surgical resection of the ischemic bowel segment (f).

Patient endometrial carcinoma. Note the close correlation between myometrial invasion detected by spectral CT series (a) iodine density image, (b) Z effective map, and (c) MRI (arrows). (d)

Conclusion

The information presented in this chapter helps to demonstrate how spectral CT has contributed to better management of abdominal conditions in adult patients. Spectral CT can replace true non-contrast acquisitions, provide an assessment of organ perfusion, assist in detection and following of hypervascular lesions at the arterial phase (HCC and melanoma), demonstrate the source of acute abdominal bleeding, characterize kidney stones, and provide an additional tool in colorectal and uterine cancer staging. Assessment of chronic liver disease (steatosis and iron accumulation) still remains under evaluation, but could soon be available in daily practice.

References

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History

Benefits or pitfalls of dual-energy CT

Key images

Findings