Experimental Multi-Parametric MRI Characterization of Iliocaval Venous Thrombosis Pathological Changes

OBJECTIVE

Iliocaval thrombotic obstruction is a challenging condition, especially since thrombus age and corresponding pathological remodeling at presentation are unknown, which directly impacts management. Our aim was to assess the ability of Magnetic Resonance Imaging (MRI) in determining age thresholds of experimentally created inferior vena cava (IVC) thrombosis in pigs.

METHODS

We used a previously described swine model of IVC thrombosis. Animals underwent MRI at baseline, immediately after thrombosis creation and after a follow-up period extending from 2 to 28 days. Thirteen animals were divided into 3 groups according to disease's chronicity: acute (AG, N1 = 5), subacute (SAG, N2 = 4) and chronic group (CG, N3 = 4) with a mean thrombosis age of respectively 6.4 ± 2.5, 15.7 ± 2.8 and 28 ± 5.7 days. A T1 weighted volumetric interpolated breath-hold examination sequence was used to anatomically delineate IVC thrombus as a region of interest. Three other MRI sequences were used to assess thrombus signal.

RESULTS

Kruskal-Wallis showed a statistically significant difference in T1 relaxation times after contrast injection (P = .026) between the 3 groups of chronicity: AG (360.2 ± 102.5) was significantly different from CG (336.7 ± 55.2 ms; P = .003) and SAG (354.1 ± 89.7 ms) was significantly different from AG (P = .027). There was a statistically significant difference in native T2 relaxation times (P = .038) between the 3 groups: AG (160 ± 86.7 ms) was significantly different from SAG (142.3 ± 55.4 ms; P = .027) and SAG was significantly different from CG (178.4 ± 11.7 ms; P = .004).

CONCLUSIONS

This study highlighted MRI characteristics in a swine model that may have the potential to significantly differentiate a subacute and a chronic stage from an acute stage of deep venous thrombosis in humans. Further clinical studies in humans are warranted.

Endovascular Porcine Model of Iliocaval Venous Thrombosis

OBJECTIVE

To develop a large animal model of iliocaval deep venous thrombosis (DVT), which enables development and evaluation of interventional management and existing imaging modalities.

METHODS

The experimental protocol consisted of a total endovascular approach. Pigs were percutaneously accessed through the right internal jugular and bilateral femoral veins. Three balloon catheters were inflated to induce venous stasis in the infrarenal inferior vena cava (IVC) and bilateral common iliac veins (CIVs). Hypercoagulability was induced by injecting 10 000 IU of thrombin. After 2.5 hours, the balloon catheters were removed before animal recovery. After seven, 14, 21, 28, or 35 days, animals were euthanised; the IVC and CIV were harvested en bloc, cross sectioned and prepared for histological examination. Multimodal imaging was performed before and after thrombus creation, and before animal euthanasia.

RESULTS

Thirteen female domestic pigs with a mean weight of 59.3 kilograms were used. The mean maximum IVC diameter and area were 16.4 mm and 1.2 cm², respectively. The procedure was successful in 12 animals with occlusive venous thrombosis in the region of interest on immediate post-operative magnetic resonance venography and a mean thrombus volume of 19.8 cm³. Clinical pathology results showed platelet consumption, D dimer increase, and inflammatory response. Histological evaluation demonstrated a red cell, fibrin, and platelet rich thrombus on day 1, with progressive inflammatory cell infiltration from day 7. Collagen deposition appeared in week 2 and neovascularisation in week 3.

CONCLUSION

Endovascular occlusion combined with thrombin infusion is a reliable minimally invasive approach to produce acute and subacute DVT in a large animal model.

Quality assurance of quantitative cardiac T1-mapping in multicenter clinical trials - A T1 phantom program from the hypertrophic cardiomyopathy registry (HCMR) study

BACKGROUND

Quantitative cardiovascular magnetic resonance T1-mapping is increasingly used for myocardial tissue characterization. However, the lack of standardization limits direct comparability between centers and wider roll-out for clinical use or trials.

PURPOSE

To develop a quality assurance (QA) program assuring standardized T1 measurements for clinical use.

METHODS

MR phantoms manufactured in 2013 were distributed, including ShMOLLI T1-mapping and reference T1 and T2 protocols. We first studied the T1 and T2 dependency on temperature and phantom aging using phantom datasets from a single site over 4 years. Based on this, we developed a multiparametric QA model, which was then applied to 78 scans from 28 other multi-national sites.

RESULTS

T1 temperature sensitivity followed a second-order polynomial to baseline T1 values (R² > 0.996). Some phantoms showed aging effects, where T1 drifted up to 49% over 40 months. The correlation model based on reference T1 and T2, developed on 1004 dedicated phantom scans, predicted ShMOLLI-T1 with high consistency (coefficient of variation 1.54%), and was robust to temperature variations and phantom aging. Using the 95% confidence interval of the correlation model residuals as the tolerance range, we analyzed 390 ShMOLLI T1-maps and confirmed accurate sequence deployment in 90%(70/78) of QA scans across 28 multiple centers, and categorized the rest with specific remedial actions.

CONCLUSIONS

The proposed phantom QA for T1-mapping can assure correct method implementation and protocol adherence, and is robust to temperature variation and phantom aging. This QA program circumvents the need of frequent phantom replacements, and can be readily deployed in multicenter trials.

Functional assessment of bioprosthetic mitral valves by cardiovascular magnetic resonance: An in vitro validation and comparison to Doppler echocardiography

BACKGROUND

A comprehensive non-invasive evaluation of bioprosthetic mitral valve (BMV) function can be challenging. We describe a novel method to assess BMV effective orifice area (EOA) based on phase contrast (PC) cardiovascular magnetic resonance (CMR) data. We compare the performance of this new method to Doppler and in vitro reference standards.

METHODS

Four sizes of normal BMVs (27, 29, 31, 33 mm) and 4 stenotic BMVs (27 mm and 29 mm, with mild or severe leaflet obstruction) were evaluated using a CMR- compatible flow loop. BMVs were evaluated with PC-CMR and Doppler methods under flow conditions of; 70 mL, 90 mL and 110 mL/beat (n = 24). PC-EOA was calculated as PC-CMR flow volume divided by the PC- time velocity integral (TVI).

RESULTS

PC-CMR measurements of the diastolic peak velocity and TVI correlated strongly with Doppler values (r = 0.99, P < 0.001 and r = 0.99, P < 0.001, respectively). Across all conditions tested, the Doppler and PC-CMR measurement of EOA (1.4 ± 0.5 vs 1.5 ± 0.7 cm2, respectively) correlated highly (r = 0.99, P < 0.001), with a minimum bias of 0.13 cm2, and narrow limits of agreement (- 0.2 to 0.5 cm2).

CONCLUSION

We describe a novel method to assess BMV function based on PC measures of transvalvular flow volume and velocity integration. PC-CMR methods can be used to accurately measure EOA for both normal and stenotic BMV's and may provide an important new parameter of BMV function when Doppler methods are unobtainable or unreliable.