Dual-Energy CT in Hemorrhagic Progression of Cerebral Contusion: Overestimation of Hematoma Volumes on Standard 120-kV Images and Rectification with Virtual High-Energy Monochromatic Images after Contrast-Enhanced Whole-Body Imaging

Discussion

The major findings from our study in patients after admission contrast-enhanced WBCT are the following: 1) follow-up 120-kV head CT images, which are akin to SECT images, significantly overestimate the hematoma volume and hence the rate of HPC, 2) mean and median percentages of hematoma volume overestimations were 29% and 22%, respectively, and 3) the best predictor of percentage of volume overestimation was the percentage of hemorrhagic progression on follow-up CT (ie, the magnitude of overestimation is proportional to the magnitude of HPC).

Our study demonstrated that the true occurrence of HPC in patients after contrast-enhanced WBCT, as measured from the 190-keV images, was 43%. This number is similar to the 45% that was reported in conservatively managed patients by Alahmadi et al,¹⁸ who had similar selection criteria and cutoff values for progression. However, the rate increased to 60% if measured on 120-kV images (images comparable with SECT images) because there were 7 patients (18%) who were wrongly diagnosed with HPC. This higher rate of HPC is caused by “pseudohematoma.” We describe pseudohematoma as the volume of nonhemorrhagic hyperattenuating brain parenchyma caused by iodinated contrast leak from the permeable capillary endothelium. The leaked contrast from the bolus administered during the admission WBCT is retained in the brain parenchyma and would be seen in the follow-up studies as an area of hyperattenuation on 120-kV images, simulating a hematoma. The exact duration of this parenchymal contrast retention has not yet been analyzed and reported.

The attenuation contribution of iodine is negligible at the higher end of the monochromatic energy spectrum.^13,16,17 Hence, 190-keV images demonstrate the actual hematoma size by displaying attenuation contributed predominantly by hematoma. It was recently demonstrated that virtual noncontrast images are comparable with 190-keV images in minimizing the attenuation contribution of iodine to negligible levels.¹³ Hence, virtual noncontrast images, if used in place of 190-keV images, may have yielded similar results in obtaining true hematoma volumes. Enhancement of cerebral contusion on contrast-enhanced SECT was described by Huang et al²⁰; however, the authors did not describe how they were able to differentiate the hyperattenuation caused by iodinated contrast from that of the hematoma.

Cerebral contusions can be divided into 3 distinct regions: 1) epicenter, 2) penumbra, and 3) parapenumbra.² The epicenter receives the peak energy from impact, and surrounding regions receive progressively less energy with distance. Energy deposited in the epicenter is sufficient to fracture capillaries, resulting in an immediate hemorrhagic lesion. In the penumbra and parapenumbra, the energy is not enough to fracture the capillaries but activates the mechanosensitive molecular processes that will lead to the delayed structural failure of capillaries.^21,22 Animal studies have shown that the mechanosensitive molecular process begins with transcriptional up-regulation, followed by opening of the sulfonylurea receptor 1 (SUR1) SUR1-regulated channel.^2,21 Opening the channel has been linked to gradual oncotic cell swelling and death of the endothelial cells, resulting in increased capillary permeability, and progression to capillary fragmentation. Increased permeability results in vasogenic edema, while capillary fragmentation results in extravasation of blood, contributing to HPC. HPC rates are directly related to the degree of endothelial damage and capillary fragmentation. The penumbra and parapenumbra usually manifest as vasogenic edema on CT because of increased permeability of the damaged endothelium.² However, after contrast administration, enhancement of the parenchyma in the penumbra and parapenumbra results from the leak of iodinated agents through the same damaged endothelium, resulting in pseudohematoma.²⁰ The phenomenon of HPC rates being proportional to the degree of capillary disruption caused by the mechanosensitive molecular process likely explains our other finding (ie, the magnitude of overestimation of hematoma is proportional to the magnitude of HPC because both are directly related to the degree of endothelial damage and capillary fragmentation).

Several authors have reported HPC as the reason for an operation, sometimes in up to 20% of patients.^{3⇓⇓⇓⇓–8} Studies have also reported that patients with progression of injury on a repeat CT underwent more frequent interventions related to intracranial pressure management changes, ventriculostomy placement or adjustment, addition of antiseizure medication, repeat CT, intensive care unit observation, delay in extubation, and initiation of thromboprophylaxis, even in patients with stable or improved Glasgow Coma Scale scores.^{12,18,23⇓–25} Similarly, Alahmadi et al¹⁸ reported a significant association between radiographic progression of contusions and the need for neurosurgical interventions, even though the mean Glasgow Coma Scale score and mean change in Glasgow Coma Scale score during the hospital course showed no difference.¹⁸ Hence, an accurate assessment of HPC has important clinical implications in preventing unnecessary interventions and radiation from follow-up imaging.

Using strict criteria in describing HPC, including indicating whether there was prior contrast administration, and detailing image sets used for measurement (SECT, 120 kV, or 190 keV) would minimize potential errors in identifying contusion progression. Health care providers should account for overestimation of hematoma volumes if the follow-up studies were performed on SECT. We also suggest that future research articles describe in detail the imaging techniques in their methods section, because contrast-enhanced WBCT has become a widely used technique for the work-up of the patient with blunt polytrauma.