Spectral CT in head and neck disorders

Jan Borggrefe, MD, Department of Diagnostic and Interventional Radiology, University Hospital of Cologne, Cologne, Germany

Nils Große Hokamp, MD, Department of Diagnostic and Interventional Radiology, University Hospital of Cologne, Cologne, Germany

Unenhanced brain imaging using virtual monoenergetic images from the IQon Spectral CT, a spectral detector computed tomography (SDCT)

Unenhanced CT examinations of the brain are a frequently performed tomographic study in the emergency department (ED) of hospitals. CT scans are commonly ordered in the ED to scan patients involved in accidents or suffering from an acute neurological condition such as a hemorrhagic or ischemic stroke, severe headaches, or seizures. CT scans are a fast and reliable way to identify or rule out acute life-threatening conditions within the neurocranium. On the other hand, unenhanced cranial CTs require the use of rather high radiation doses compared to other CT examinations, resulting in a significant radiation exposure to radiosensitive organs such as the eyes and the thyroid gland.1 Thus, optimization of radiation exposure is of great importance when performing head CTs. However, dose reduction often increases image noise, so the applied dose should be balanced with the acquisition for optimal image quality.2

Quality criteria of an optimal head CT examination are 1) the differentiation between gray and white matter (which exhibits an absolute difference of approximately 10 HU values), 2) low image noise so as not to impair differentiation (in studies with an image noise of approximately 10 HU, differentiation of gray and white matter becomes virtually impossible), and 3) little or no artifacts in the subcallosal space and within the posterior fossa of the neurocranium. Low keV virtual monoenergetic images (VMI) from the IQon Spectral CT address this challenging situation by improving soft tissue contrast and by reducing image noise compared to conventional CT. We recently demonstrated this in a study that included 40 patients, showing that low keV VMI can improve the differentiation between gray and white matter by more than 3-fold as determined by contrast-to-noise ratio (CNR).3 Furthermore, the study demonstrated that noise within these virtual monoenergetic images is approximately 30% lower compared to conventional CT images. These findings illustrate the superiority in image quality of the 65 keV images from the SDCT IQon scanner compared to conventional CT images, showing that SDCT imaging would work well as a recommended standard for brain imaging.

There is a need for dedicated studies to investigate just how much SDCT of the head allows for a reduction of applied dose in comparison to conventional CT. In several studies, 40 keV images showed even greater image contrast than 65 keV images; however, over-enhancement of the skull did frequently impair diagnostic assessment of the subcalvarial space. In an updated algorithm for neuroimaging on SDCT, this issue was successfully addressed; now it appears that 40 keV images can be used for diagnostic assessment in many cases and actually exhibit an outstanding image contrast (Figure 1). For other body regions or studies, it has been shown that virtual monoenergetic images require an adjustment of window settings as the absolute attenuation values (in HU) differ substantially from conventional image reconstructions.4,5 Such values are yet to be determined for cranial CT imaging.

In studies investigating assessment of focal parenchymal brain lesions, low keV VMI derived from SDCT improved lesion delineation independent of lesion type.6 In comparison to conventional CT, SDCT improved sensitivity for the detection of new, formerly unknown brain lesions, especially in regard to focal hemorrhages. This indicates that low keV images may be beneficial for the early detection of acute ischemic stroke, the detection of diffuse brain injuries in the acute phase, the depiction of calcifications, and the identification of cystic lesions.

At the other end of the keV scale, higher keV virtual monoenergetic images have successfully reduced artifacts from orthopedic implants. We recently investigated a benefit regarding metal artifact reduction in the presence of lead for deep brain stimulation.7 These patients routinely undergo unenhanced cranial CT (CCT) after the procedure in order to determine correct positioning of electrodes and to rule out post-procedural hemorrhage. We found that high keV images were especially helpful if combined with dedicated algorithms for metal artifact reduction. In these types of images, sensitivity, and specificity for blood detection, was significantly improved. However, regarding the severe artifacts caused by coiling material in cerebral aneurysms, the benefit from VMI is limited; here, further advances are highly desirable.

Unenhanced cranial CT of a 55-year-old patient in conventional image and virtual monoenergetic image reconstruction at 40 keV. Note that gray-white matter differentiation is clearly superior in 40 keV images.

Virtual non-contrast imaging and material quantification in the brain

Dual-energy CT allows an improved depiction, quantification, and separation of various tissue types such as iodine and calcium (see Chapter 1, Technical Aspects). In brain imaging, the virtual non-contrast (VNC) mode can be used for several indications. For dual-energy CT, Djurdjevic et al. proved that VNC is a powerful tool in order to predict the infarction development after endovascular stroke therapy by helping to detect the stasis of contrast medium within the brain tissue.8 This may be of particular interest in order to guide the surveillance and treatment of the patient after thrombectomy in regard to decision-making on the necessity of prolonged extubation, ICU surveillance, and early mobilization. Furthermore, as iodine removal may allow for the early detection of stroke hemorrhages after thrombectomy by separating blood from iodine, SDCT was shown to be a powerful tool for the detection of such complications.9,10 The VNC is further of value for the planning of brain biopsies, as calcifications of solid brain tumors or tumor cysts may impair biopsies and can be clearly differentiated from contrast enhancement using VNC images from SDCT (Figure 2).

In the planning of stroke treatments, an optimized imaging of the obstructing blood clot is of clinical interest as the relatively small clot structures may be partially obscured in CT by the stasis of blood, and as review of clot imaging may lead the clinician to draw conclusion with regard to clot compositions.

For example, numerous studies have shown that the hyperdensity of clots is strongly associated with the fraction of red blood cells (RBC) and improved functional outcome of stroke patients after 90 days. An ex vivo study showed that SDCT is a helpful tool for the differentiation of the blood clot components RBC and fibrin, which in contrast to RBC is

Figure 2

For stereotactic biopsies of cerebral cystic lesions, we routinely perform an intraoperative contrast enhanced study; however, the presence of contrast enhancement makes it difficult to differentiate calcified walls of such lesions from the surrounding cerebral vasculature (arrow). Here, virtual non-contrast images often enable identification of these calcifications, resulting in easier and shorter biopsy times.

strongly associated with contrast enhancement of clots due to protein binding.11 The VNC allowed the clinician to accurately detect clot densities, such as in unenhanced imaging, even in enhanced studies, enabling differentiation of the thrombus types with respect to their red blood cell fraction. Furthermore, iodine uptake within thrombi can be quantified, allowing for an estimation of the fibrin fraction within the clots. A patient study by Grams et al. documented that DECT is helpful for the detection of residual thrombi in patients after stroke treatment.12

In body imaging, VNC images have successfully demonstrated image qualities comparable to true non-contrast acquisitions;13 while in brain imaging, the aforementioned strong necessity for low contrast resolution and low image noise results in obscured separation of gray and white matter in VNC images. In addition, the current algorithm may not fully suppress iodinated contrast-media in small subcalvarial vessels. Thus, there is need for a further improvement of VNC imaging with dedicated fine-tuning with regard to the depiction of low contrasts of the brain.

Quantitative imaging with respect to contrast media uptake is of interest for all body regions including head and neck imaging. In contrast to clinical MRI, which has some limitations in this regard, CT in general and SDCT in particular, allow for quantitative imaging of tissue components.14,15 This is of interest, as iodine content may be used for the differentiation of tumor types for the guidance of targeted biopsies, or even to serve as a biomarker in virtual biopsies. In gliomas, the tumor grade is associated with the prevalence and intensity of tumor-tissue enhancement. Further, lymphomas are thought to differ from gliomas with a tumor enhancement of particular high intensity; here, iodine maps may allow for accurate separation of these two entities.

Knowing the iodine content of lesions in baseline studies further allows for the evaluation of tumor response (e.g., for the differentiation of remaining metastases after radiation in the differential to radiation necrosis). Hence, for this purpose, SDCT may overcome the need for additional and expensive (yet frequently performed) PET/CT examinations. In addition, iodine quantification may prove beneficial for the early detection of therapeutic response. Feasibility studies proved that dual-energy CT also allows for the calculation of edema maps within the brain.16 Although such maps are not yet available for DECT scanners, it shows the potential for SDCT to assist with multiparametric image analysis by detecting specific tissue features with correction for edema (i.e., analogous to MRI FLAIR sequences).

In preliminary studies, these edema maps allowed for an improved early depiction of brain infarction and thus for a more precise stroke scoring (i.e., when using the ASPECTS for pretherapeutic evaluation). Furthermore, edema maps could be used in tumor imaging and as an additional biomarker.

Contrast enhanced SDCT of the head and neck

Contrast enhanced CT of the head is a standard examination for the detection of vascular lesions in arterial and venous CT angiography and ischemic stroke. For the latter, CT perfusion is commonly used for the differentiation of penumbra and the core of infarction. Indisputably, in brain tumor imaging, MRI yields a better CNR in comparison to CT, and provides additional information such as diffusion imaging, fluid suppression, and susceptibility-weighted imaging. However, contrast enhanced CT is still used for various indications in cases of cerebral tumor diseases, including imaging of patients with contraindication for MRI, the use of CT perfusion for the detection of hyperperfused tumor foci, and treatment planning for stereotactic methods and bone-invading tumors. Thus, there is significant potential for SDCT to improve diagnostic tumor and stroke imaging in comparison to conventional CT.

The boost of CNR in contrast enhanced SDCT angiography was shown for intra- and extracranial vessels, enabling improved visual assessment of different vessels, including challenging segments in proximity to the skull base, extracranial vessels, or the cerebellar vessels.17 Here, the vessel depiction in areas susceptible to moderate artifacts was significantly enhanced, as the increase in image contrast at 40 keV outweighed the minimal increase in image noise. The low image noise is an inherent feature of the dual-layer detector technology used in the IQon Spectral CT scanner. In contrast to unenhanced CT, image noise in contrast enhanced CT does not appear to be clinically relevant. In comparison of 40 keV images with conventional images, we found only minimal blooming of carotid artery plaques, providing the ability to better assess the vessel lumen in the presence of strong calcifications and considerable improvements of vessel contrast, especially in investigations with suboptimal vessel contrast.18 Therefore, 40 keV images from SDCT angiography provide clear visualization advantages compared to conventional images and may provide helpful supplemental imaging information (Figure 3).

Coronal maximum intensity projection of a SDCT angiography of a 48-year-old patient. Note that vessel contrast is clearly enhanced in 40 keV image reconstruction compared to conventional images.

The contrast boost of SDCT can now be used in different ways:

• By yielding optimal contrast in low contrast structures or in images with suboptimal contrast (i.e., due to delayed contrast enhancement or vessels with stents and calcifications).

• By reducing the application of contrast media, which is helpful for patients with impaired kidney function.4,18,19

Furthermore, the high contrast of low keV images can be used to reduce radiation exposure for the patient, as CNR may be comparable to conventional imaging with considerable dose reductions. To date, however, there are no structured studies addressing optimal radiation doses or contrast protocols for low keV SDCT angiography.

Enhanced contrast may be able to help the clinician detect subtle contrast enhancement in blood brain barrier disruptions, thereby allowing optimized SDCT post-processing to aid in the detection of cerebral metastases, especially in patients with contraindications for MRI. Furthermore, contrast enhanced SDCT could be used by the clinician to assist with stereotactic therapy planning, as well as enhance the detection of early stroke demarcation in CT angiography and CT perfusion.

SDCT imaging of the neck and spine

One of the most frequent instances of metal artifacts in neck imaging are dental implants. Such artifacts can disrupt CT image quality, and thus decrease the clinician’s ability to provide a confident diagnostic assessment of pathology, such as a tumor or abscess. In this situation, high keV images were again found to be helpful to reduce both hypo- and hyperdense artifacts. However, the extent of artifact reduction shows a strong dependence on the metal alloy. A particular advantage of IQon Spectral CT in this respect is the ability to seamlessly adjust keV levels in real time, allowing the clinician to find an optimal balance between artifact reduction (toward higher keV) and maintained tissue contrast (toward lower keV).20

The diagnostic assessment of the spinal canal with respect to tumor masses is of particular importance in oncologic staging examinations, yet frequently presents challenges for clinicians (e.g., beam hardening caused by the surrounding bone, often with only slight differences in image contrast). We recently demonstrated that virtual monoenergetic images have been beneficial in this type of situation, as they can help to enhance visualization of intraspinal tumors.21 As learned from angiographic studies in abdominal CT angiography, the improved vessel delineation and visualization may be promising for assessment of spinal vessel lesions such as acute vessel occlusions or dural fistulas.

In spine imaging, we found that high keV virtual monoenergetic images are helpful in reducing artifacts caused by spinal fusions.22 These artifact-reduced images allowed for an improved assessment of the spinal canal in oncologic staging examinations, as well as improvements in image quality for myelography exams.

In conclusion, for all tumor indications within the soft tissue, SDCT provides enhanced benefits for assessing iodine uptake of tumoral diseases of the head and neck, including lymph nodes, in a more quantitative fashion, as well as improvements in the visualization of head and neck tumors.

References

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History Benefits or pitfalls of dual-energy CT