Spectral CT in emergency department

Begüm Demirler Șimșir, MD, visiting research fellow, Department of Radiology, Cliniques Universitaires St-Luc, Brussels, Belgium

Emmanuel Coche, MD, PhD, Head of Department of Radiology, Cliniques Universitaires St-Luc, Brussels, Belgium

Spectral CT provides additional information to conventional CT in emergency settings and offers the opportunity to help clinicians achieve a definitive diagnosis, as well as reducing the need for follow-up scans. With clinical results such as material decomposition, virtual monoenergetic imaging, and virtual non-contrast imaging, the IQon Spectral CT provides enhanced diagnostic benefits without increased radiation dose to the patient.

The advantage of this dual-layer spectral CT is when images are obtained at 120 or 140 kVp, spectral data is available for all patients without the need for prior selection of a specific protocol. Spectral CT has shown to add value in many emergency conditions, such as those discussed in other chapters. For example, in abdomen exams, we can highlight the interesting and unprecedented applications of spectral CT as assisting with the detection of gallstones (Figure 1), removal of iodine for detection of urinary tract stones (see clinical case Emergency: Abdomen discussed in this section), gallbladder wall (Figure 1), solid organ perfusion assessment (Figure 2), boosting of contrast media in low dose CT angiography for patients with poor renal functions (see clinical case Emergency: Abdomen and Vessels in this section), and detection of contrast extravasation.

In this chapter, we will focus on the role of spectral CT in emergency brain, head and neck, and musculoskeletal conditions.

Patient with recurrent severe cholecystitis: (a) Conventional CT with contrast, axial image: no stone detected in gallbladder. (b) Iodine density image: gallbladder wall defect related to severity of infection (blue arrow). (c) Z effective image: a large stone is visible and color coded in red corresponding to low atomic number (white arrow), and gallbladder defect is also demonstrated (blue arrow). (d) MRI and (e) ultrasound images were available on PACS and demonstrated the gallbladder stone (white arrows).

Upper row: Hepatic infarct in a patient with acute hypovolemic shock: (a) Portal phase conventional CT axial image. (b) Iodine density axial image: demonstrating lack of iodine within the liver parenchyma (0.07 mg/ml, arrow). (c) Liver parenchyma is color coded in yellow (arrow) on Z effective map indicating low atomic number and lack of iodine content.

Lower row: Previous CT examination of the patient before hepatic infarct: (d) Portal phase conventional CT axial image. (e) Iodine density axial image demonstrating iodine content of the liver parenchyma (1.59 mg/ml, arrow). (f) Liver parenchyma is color coded in light blue (arrow) on Z effective map indicating normal iodine content.

Emergency brain imaging

In the setting of acute neurologic conditions, non-contrast CT is the initial imaging modality to investigate acute intracranial hemorrhage (ICH) and stroke due to its availability and acquisition speed, and in the presence of MRI contraindications.1 As well as providing conventional images, spectral CT can contribute to diagnosis of acute conditions (e.g., detection of intracranial hemorrhage, differentiation of iodine from hemorrhage, and detection of infarction after intra-arterial revascularization therapy for acute stroke, reduction of metallic artifacts from aneurysm clips or coils, beam hardening artifacts in posterior fossa, and gray-white matter differentiation) with additional data sets generating virtual non-contrast images, virtual monoenergetic images at low and high keV, iodine, and Z effective maps.

Detection of intracranial hemorrhage

Non-traumatic intracranial hemorrhage primarily originates from hypertension, cerebral amyloid angiopathy, or anticoagulation. ICH can also arise secondarily from underlying conditions such as brain neoplasms and vascular malformations.2 An underlying tumor accounts for 10% of all spontaneous intracranial hemorrhage cases.3 However, when contrast enhanced conventional CT is performed with the suspicion of an underlying mass, a hyperattenuating hematoma could mask an enhancing solid lesion.4 Detection of an underlying cause is critical in patient management as well as patient prognosis.5 Spectral CT with virtual non-contrast images, iodine density, and iodine fusion images allows identification of iodine within the hyperattenuating lesion and aids in diagnosis of an underlying mass (see Head and Neck clinical case 3, Brain).

In the setting of intracranial hemorrhage, conventional true non-contrast images were compared with virtual non-contrast images obtained from spectral CT after contrast administration.6 Although contrast-to-noise ratio (CNR) was relatively lower, virtual non-contrast (VNC) images were found to be sufficient for the detection of the bleeding. Thus, spectral CT could be an ideal imaging modality for patients with acute ICH with the advantages of reducing both scanning time and radiation dose.5,6

Differentiating iodine from hemorrhage after intra-arterial revascularization in acute ischemic stroke

After intra-arterial stroke therapy, follow-up conventional non-contrast CT scans are obtained to rule out hemorrhage as a major complication of the procedure.7 However, it could be challenging to differentiate a hyperattenuation resulting from hemorrhage versus contrast extravasation or staining. Hemorrhage persists for several days to weeks, whereas early washout is seen in cases of contrast extravasation.7,8 Therefore, currently, a follow-up non-contrast CT scan is performed in 24 - 48 hours, and persistence of the hyperattenuation indicates hemorrhage, whereas early washout is indicative of contrast extravasation.7,9 Earlier accurate diagnosis of hemorrhage is crucial to allow an earlier decision-making regarding whether to continue or reverse anticoagulant therapy.10 Spectral CT was found useful in immediate differentiation of hemorrhage from contrast extravasation or staining on initial images in single acquisition with the help of virtual non-contrast images, iodine maps and overlay images.9,11,12 Hyperattenuating areas persisting as hyperdensities on virtual non-contrast CT were rated as hemorrhage, whereas hyperattenuating areas that were demonstrated as hyperdensities exclusively on iodine-only images and were not seen on virtual non-contrast images were rated as contrast staining or extravasation13 (See Head and Neck clinical case 4, Brain).

Detection of infarct after intra-arterial revascularization

Apart from detection of hemorrhage and blood brain disruption after intra-arterial revascularization, it has been shown that the infarct area could be depicted more accurately by spectral CT with iodine map and virtual non-contrast images compared to conventional CT.14-16

Artifact reduction

Metallic artifacts from clips and coils usually degrade the image, and evaluation of the patency of the adjacent vessels becomes challenging. The use of higher monoenergetic images could reduce these metallic artifacts as well as beam hardening artifacts in posterior fossa resulting in improved image quality.17,18

Assessment of gray-white matter differentiation

Evaluation of gray-white matter differentiation is important in suspected acute stroke patients. Loss of gray-white matter differentiation indicates cytotoxic edema, an early sign of ischemia and infarct.19 Virtual monoenergetic images at low keV were found to be superior to conventional CT in differentiating gray and white matter.20 Optimal image quality with higher contrast-to-noise and signal-to-noise ratios were observed at 65 keV reconstructed images which could enable earlier depiction of ischemic changes20-21 (Figure 3).

Patient with basilar artery occlusion presented with sudden onset of left sided hemiparesis and dysarthria. (a) Conventional CT without contrast, axial image at 120 kVp: basilar artery (arrow). (b) Virtual monoenergetic axial image at 40 keV: better demonstration of hyperdense basilar artery indicating a thrombus (arrow). (c) CT angiography, sagittal image: demonstration of the occluded basilar artery segment of 1.1 cm (arrow). (d) CT angiography, axial image at 120 kVp: subtle hypodensity on the right side of pons (arrow). (e) Virtual monoenergetic axial image at 40 keV: better demonstration of hypodensity of right side of pons (arrow). (f) Iodine density map: demonstrating lower iodine content in this area (arrow). (g) Z effective map: area with lower iodine content on the right side of pons is color coded in red (arrow). (h) Iodine overlay image: demonstrating corresponding area with lower iodine content (arrow). (i) Conventional CT without contrast, axial image at 120 kVp: 48 hours later, prominent hypodensity in the corresponding right side of pons (arrow) confirming ischemia.

Emergency head and neck imaging (infection and inflammation)

Infection and inflammation

Early and effective treatment is crucial in patients with suspected head and neck abscesses. CT is the modality of choice for evaluation of neck abscesses, and the neck spaces involved and diagnosis could be challenging in some cases.22 Spectral CT could contribute to better delineation of head and neck abscesses and could be helpful in early detection as well as better demonstrating its extent with the use of virtual monoenergetic images at low keV,

Z effective, and iodine density maps (Figure 4). Furthermore, monoenergetic images at lower keV and iodine maps could improve the attenuation of adjacent vasculature and help in prevention of bleeding during an incision or drainage.23

Artifact reduction

Higher keV monoenergetic images were found to be useful to reduce artifacts from dental implants and cervical spinal metallic implants which results in improved image quality and diagnostic confidence.24-26

Patient with dental abscess presented with pain and swelling in right submandibular region. Magic Glass coronal view. (a) Conventional CT coronal image: shows heterogeneous lesion in right submandibular area. (b) Virtual monoenergetic image at 40 keV: better delineates the borders of the abscess showing submental extension (arrows). (c) Iodine density coronal image: demonstrates increased iodine content of the abscess wall (2.30 mg/ml, blue arrow) compared to normal left submandibular area (1.34 mg/ml, open arrow), and central part of the abscess without iodine content (0.02 mg/ml, white arrow). (d) Z effective coronal image: wall of right submandibular abscess with increased iodine content is color coded in darker blue and has higher effective atomic number (8.53, blue arrow) compared to unaffected left submandibular area (8.08, open arrow) that is color coded in lighter blue-green. Central portion of the abscess without iodine content is color coded in yellow-orange (white arrow).

Left is not well demonstrated. (b) Virtual monoenergetic axial image at 40 keV: better demonstrates hypodense central area of the left psoas muscle (arrow). (c) Iodine density axial image: shows lack of iodine within the central area (0.00 mg/ml, white arrow), compared to peripheral areas of the muscle (1.18 mg/ml, blue arrow). (d) The central area of the muscle without iodine content has lower atomic number and color coded in yellow (white arrow) compared to periphery of the muscle color coded in light blue (blue arrow) on Z effective map.

Emergency musculoskeletal imaging

Currently, spectral CT is used in clinical practice to detect monosodium urate (MSU) crystals in and around joints in gout arthropathy as a non-invasive method which has also been shown to be valuable in acute settings.27 There are other several musculoskeletal acute conditions in which spectral CT can aid in diagnosis through different applications such as energy-specific imaging for metallic artifact reduction of implants and prosthesis, materialspecific imaging for detection of bone marrow edema in trauma patients with virtual calcium suppressed images, and assessment of tendons and ligaments.28 Additionally, assessment of iodine content could aid in diagnosis of intramuscular hematoma (Figure 5). Spectral CT with dual-layer detector has the advantage of performing dual-energy analysis on every data set acquired, eliminating the need for prospective selection of a dual-energy protocol.29

Acute gout arthritis

Acute gout arthritis presents as an acute periarticular inflammatory response to presence of monosodium urate crystals in soft tissues and joints. The classical presentation is pain of the first metacarpophalangeal joint (podagra), and it is the most common crystal arthropathy.30 Differential diagnosis includes septic arthritis, trauma, and pseudogout. Although it mainly involves the knee joint, exclusion of septic arthritis is difficult in some cases and requires arthrocentesis. Trauma is usually excluded by patient history. Pseudogout is diagnosed by calcium pyrophosphate presence in the synovial fluid. Although diagnosis of gout could be typically made invasively through detection of monosodium urate crystals in the involved synovial joint, in 25% of cases, it may not be identifiable.31 Early diagnosis is important, as it can lead to joint destruction if left untreated.32 Imaging methods such as radiography, ultrasound, conventional CT, and MRI have not been shown to be sensitive or specific enough to demonstrate urate crystals.33-35 Spectral CT allows material differentiation based on the attenuation difference of materials with high atomic number (calcium) and low atomic number (uric acid) at different energy levels; thus, characterization of urate and differentiation from calcium is possible.36,37 It has been shown that spectral CT with material decomposition is an accurate and specific non-invasive method to diagnose acute gout.32,38,39

Metal artifact reduction

Conventional CT evaluation of metallic prosthesis, implants and their loosening, and the assessment of fractures and infection around them could be challenging due to metal artifacts. Metallic implants also cause distortion in MRI images. Spectral CT provides energyspecific images and with higher monoenergetic images, metal artifacts could be reduced, and image quality could be improved without increased radiation dose to the patient.40,41

Bone marrow edema in acute trauma

Bone marrow edema related to trauma is presumed as trabecular microfractures, and MRI is the modality of choice for its evaluation.42-44 Conventional CT fails to detect bone marrow edema due to overlying trabecular bone, thus it is not possible to depict the age of the fracture or to detect a bone bruise in the context of trauma.45 It has been demonstrated that spectral CT with material decomposition and calcium suppressed images is successful in depicting traumatic bone marrow edema of the knee and ankle46-48 (see chapter on MSK diseases) as well as vertebral compression fractures.49-51 Although MRI maintains its position in demonstrating bone marrow edema, spectral CT is a promising method especially in emergency settings to detect occult fractures and differentiate acute fractures from older ones where MRI is contraindicated or not available.52

Evaluation of tendons and ligaments

Conventional CT has limitations in evaluation of soft tissues such as tendons and ligaments compared to ultrasound and MRI. Spectral CT, with enhanced characterization of collagenous structures, has been shown to be promising in the evaluation of tendons and ligaments which are composed of collagen, elastin, and glycosaminoglycan.53 Assessment of avulsion, thickening, and compression could be possible with material differentiation applications allowing differentiation of collagen.54,55 Although tendons and ligaments could be demonstrated, the data regarding spectral CT evaluation of tendons and ligaments is controversial and has some limitations.56-59 Future studies and technological advancements are required to show its true utility in this area.

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History Benefits or pitfalls of dual-energy CT

Key images Findings