The role of aortic wall CT attenuation measurements for the diagnosis of acute aortic syndromes

ACR Appropriateness Criteria® Suspected Acute Aortic Syndrome

Acute aortic syndrome (AAS) includes the entities of acute aortic dissection, intramural hematoma, and penetrating atherosclerotic ulcer. AAS typically presents with sudden onset of severe, tearing, anterior, or interscapular back pain. Symptoms may be dominated by malperfusion syndrome, due to obstruction of the lumen of the aorta and/or a side branch when the intimal and medial layers are separated. Timely diagnosis of AAS is crucial to permit prompt management; for example, early mortality rates are reported to be 1% to 2% per hour after the onset of symptoms for untreated ascending aortic dissection. The appropriateness assigned to each imaging procedure was based on the ability to obtain key information that is used to plan open surgical, endovascular, or medical therapy. This includes, but is not limited to, confirming the presence of AAS; classification; characterization of entry and reentry sites; false lumen patency; and branch vessel compromise. Using this approach, CT, CTA, and MRA are all considered usually appropriate in the initial evaluation of AAS if those procedures include intravenous contrast administration. Ultrasound is also considered usually appropriate if the acquisition is via a transesophageal approach. The American College of Radiology Appropriateness Criteria are evidence-based guidelines for specific clinical conditions that are reviewed annually by a multidisciplinary expert panel. The guideline development and revision include an extensive analysis of current medical literature from peer reviewed journals and the application of well-established methodologies (RAND/UCLA Appropriateness Method and Grading of Recommendations Assessment, Development, and Evaluation or GRADE) to rate the appropriateness of imaging and treatment procedures for specific clinical scenarios. In those instances where evidence is lacking or equivocal, expert opinion may supplement the available evidence to recommend imaging or treatment.

Investigation of Three-dimensional Printing Materials for Printing Aorta Model Replicating Type B Aortic Dissection

AIM

This study aims to determine a printing material that has both elastic property and radiology equivalence close to the real aorta for simulation of endovascular stent-graft repair of aortic dissection.

BACKGROUND

With the rapid development of Three-Dimensional (3D) printing technology, a patient- specific 3D printed model is able to help surgeons to make a better treatment plan for Type B aortic dissection patients. However, the radiological properties of most 3D printing materials have not been well characterized. This study aims to investigate the appropriate materials for printing human aorta with mechanical and radiological properties similar to the real aortic Computed Tomography (CT) attenuation.

OBJECTIVE

Quantitative assessment of CT attenuation of different materials used in 3D printed models of aortic dissection for developing patient-specific 3D printed aorta models to simulate type B aortic dissection.

METHODS

A 25-mm length of aorta model was segmented from a patient's image dataset with a diagnosis of type B aortic dissection. Four different elastic commercial 3D printing materials, namely Agilus A40 and A50, Visijet CE-NT A30 and A70 were selected and printed with different hardness. Totally four models were printed out and CT scanned twice on a 192-slice CT scanner using the standard aortic CT angiography protocol, with and without contrast inside the lumen. Five reference points with the Region Of Interest (ROI) of 1.77 mm2 were selected at the aortic wall, and intimal flap and their Hounsfield units (HU) were measured and compared with the CT attenuation of original CT images. The comparison between the patient's aorta and models was performed through a paired-sample t-test to determine if there is any significant difference.

RESULTS

The mean CT attenuation of the aortic wall of the original CT images was 80.7 HU. Analysis of images without using contrast medium showed that the material of Agilus A50 produced the mean CT attenuation of 82.6 HU, which is similar to that of original CT images. The CT attenuation measured at images acquired with the other three materials was significantly lower than that of the original images (p<0.05). After adding contrast medium, Visijet CE-NT A30 had an average CT attenuation of 90.6 HU, which is close to that of the original images without a statistically significant difference (p>0.05). In contrast, the CT attenuation measured at images acquired with other three materials (Agilus A40, A50 and Visiject CE-NT A70) was 129 HU, 135 HU and 129.6 HU, respectively, which is significantly higher than that of original CT images (p<0.05).

CONCLUSION

Both Visijet CE-NT and Agilus have tensile strength and elongation close to actual patient's tissue properties producing similar CT attenuation. Visijet CE-NT A30 is considered the appropriate material for printing aorta to simulate contrast-enhanced CT imaging of type B aortic dissection. Due to the lack of body phantoms in the experiments, further research with the simulation of realistic anatomical body environment should be conducted.

Virtual versus true non-contrast dual-energy CT imaging for the diagnosis of aortic intramural hematoma

PURPOSE

To assess whether virtual non-contrast (VNC) images derived from contrast dual-layer dual-energy computed tomography (DL-DECT) images could replace true non-contrast (TNC) images for aortic intramural hematoma (IMH) diagnosis in acute aortic syndrome (AAS) imaging protocols by performing quantitative as well as qualitative phantom and clinical studies.

MATERIALS AND METHODS

Patients with confirmed IMH were included retrospectively in two centers. For in vitro imaging, a custom-made phantom of IMH was placed in a semi-anthropomorphic thorax phantom (QRM GmbH) and imaged on a DL-DECT at 120 kVp under various conditions of patient size, radiation exposure, and reconstruction modes. For in vivo imaging, 21 patients (70 ± 13 years) who underwent AAS imaging protocols at 120 kVp were included. In both studies, contrast-to-noise ratio (CNR) between hematoma and lumen was compared using a paired t test. Diagnostic confidence (1 = non-diagnostic, 4 = exemplary) for VNC and TNC images was rated by two radiologists and compared. Effective radiation doses for each acquisition were calculated.

RESULTS

In both the phantom and clinical studies, we observed that the CNRs were similar between the VNC and TNC images. Moreover, both methods allowed differentiating the hyper-attenuation within the hematoma from the blood. Finally, we obtained equivalent high diagnostic confidence with both VNC and TNC images (VNC = 3.2 ± 0.7, TNC = 3.1 ± 0.7; p = 0.3). Finally, by suppressing TNC acquisition and using VNC, the mean effective dose reduction would be 40%.

CONCLUSION

DL-DECT offers similar performances with VNC and TNC images for IMH diagnosis without compromise in diagnostic image quality.

KEY POINTS

• Dual-layer dual-energy CT enables virtual non-contrast imaging from a contrast-enhanced acquisition. • Virtual non-contrast imaging with dual-layer dual-energy CT reduces the number of acquisitions and radiation exposure in acute aortic syndrome imaging protocol. • Dual-layer dual-energy CT has the potential to become a suitable imaging tool for acute aortic syndrome.

Experimental Insight into the Hemodynamics and Perfusion of Radiological Contrast in Patent and Non-patent Aortic Dissection Models

PURPOSE

In a curved vessel such as the aortic arch, the velocity profile closer to the aortic root is normally skewed towards the inner curvature wall, while further downstream along the curve, the velocity profile becomes skewed towards the outer wall. In an aortic dissection (AD) disease, blood velocities in the true lumen (TL) and false lumen (FL) are hypothesized to depend on the proximity of the entry tear to the root of aortic arch. Faster velocity in the FL can lead to higher hemodynamic loading, and pose tearing risk. Furthermore, the luminal velocities control the perfusion rate of radiological contrast media during diagnostic imaging. The objective in this study is to investigate the effect of AD disease morphology and configuration on the blood velocity field in the TL and FL, and on the relative perfusion of radiological enhancement agents through the dissection.

METHODS

Eight in vitro models were studied, including patent and non-patent FL configurations. Particle image velocimetry (PIV) was used to quantify the AD velocity field, while laser-induced fluorescence (LIF) was implemented to visualize dynamical flow phenomena and to quantify the perfusion of injected dye, in mimicry of contrast-enhanced computed tomography (CT).

RESULTS

The location of the proximal entry tear along the aortic arch in a patent FL had a dramatic impact on whether the blood velocity was higher in the TL or FL. The luminal velocities were dependent on the entry/reentry tear size combination, with the smaller tear (whether distal or proximal) setting the upper limit on the maximal flow velocity in the FL. Upon merging near the distal reentry tear, the TL/FL velocity differential gave rise to the roll up and shedding of shear layer vortices that convected downstream in close proximity to the wall of the non-dissected aorta. In a non-patent FL, the flow velocity was practically null with all the blood passing through the TL. LIF imaging showed much slower perfusion of contrast dye in the FL compared to the TL. In a patent FL, however, dye had a comparable perfusion rate appearing around the same time as in the TL.

CONCLUSIONS

Blood velocities in the TL and FL were highly sensitive to the exact dissection configuration. Geometric case A1R, which had its proximal entry tear located further downstream along the aortic arch, and had its entry and reentry tears sufficiently sized, exhibited the highest FL flow velocity among the tested models, and it was also higher than in the TL, which suggest that this configuration had elevated hemodynamic loading and risk for tearing. In contrast-enhanced diagnostic imaging, a time-delayed acquisition protocol is recommended to improve the detection of suspected cases with a non-patent FL.

[Pre- and postoperative imaging of type B aortic dissection]

Type B aortic dissections are serious diseases with a 60 to 80 % 5-year survival rate. Although typically managed with a medical treatment, surgery may be necessary in the acute/subacute or the chronic phase if significant complications are encountered. For these patients, CT angiography is the first-line imaging modality, used for indicating and preparing the surgical procedure as well as for follow-up. Physicians in charge of these patients should be familiar with the key reading points. Visceral malperfusion is the most common acute complication, while aneurysmal dilatation of the false lumen is the most common chronic complication, with surgical management generally indicated when the axial diameter of the aorta exceeds 55mm. Endovascular treatment tends to replace open surgery: it requires precise measurements and identification of the entry tear (contribution of 4D-MRA).

MDCT distinguishing features of focal aortic projections (FAP) in acute clinical settings

Focal aortic projections (FAP) are protrusion images of the contrast medium (focal contour irregularity, breaks in the intimal contour, outward lumen bulging or localized blood-filled outpouching) projecting beyond the aortic lumen in the aortic wall and are commonly seen on multidetector computed tomography (MDCT) scans of the chest and abdomen. FAP include several common and uncommon etiologies, which can be demonstrated both in the native aorta, mainly in acute aortic syndromes, and in the post-surgical aorta or after endovascular therapy. They are also found in some types of post-traumatic injuries and in impending rupture of the aneurysms. The expanding, routine use of millimetric or submillimetric collimation of current state-of-the-art MDCT scanners (16 rows and higher) all the time allows the identification and characterization of these small ulcer-like lesions or irregularities in the entire aorta, as either an incidental or expected finding, and provides detailed three-dimensional pictures of these pathologic findings. In this pictorial review, we illustrate the possible significance of FAP and the discriminating MDCT features that help to distinguish among different types of aortic protrusions and their possible evolution. Awareness of some related and distinctive radiologic features in FAP may improve our understanding of aortic diseases, provide further insight into the pathophysiology and natural history, and guide the appropriate management of these lesions.

The role of multidetector-row CT in the diagnosis, classification and management of acute aortic syndrome

The term "acute aortic syndrome" (AAS) encompasses several non-traumatic life-threatening pathologies of the thoracic aorta presenting in patients with a similar clinical profile. These include aortic dissection, intramural haematoma and penetrating atherosclerotic ulcers. These different pathological entities can be indistinguishable on clinical grounds alone and may be confused with other causes of chest pain, including myocardial infarction. Multidetector-row CT (MDCT) is the current modality of choice for imaging AAS with a sensitivity and specificity approaching 100%. Early diagnosis and accurate radiological classification is associated with improved clinical outcomes in AAS. We review the characteristic radiological features of the different pathologies that encompass AAS and highlight the vital role of MDCT in determining the management of these life-threatening conditions.