Imaging is often used to monitor patients on medical therapy and for assessment of response to therapy following PEA and BPA. Successful PTE is associated with immediate improvement in hemodynamics, with reduction in PVR and PASP.117 Several noninvasive tools can be used in conjunction with clinical tools such as 6-minute walk test to assess functional recovery. Echocardiography is the easiest and readily available test and can be used to assess posttherapy LV and RV remodeling. Newer techniques such as 2D speckle tracking may allow better assessment of cardiac function after therapy.118 One of the disadvantages of echocardiography, however, is that direct visualization of the disease burden is not possible. CTA and MRI are well suited for depiction of improvement of thrombus burden, although they are not routinely used. MRI, although slightly inferior to CTA in the assessment of pulmonary vascular thrombotic disease, can be extremely helpful in the evaluation of functional recovery. Several MRI-based noninvasive biomarkers such as PA maximum flow velocity, acceleration time/ejection time, and distensibility can be evaluated using MRI.119 RV remodeling and adaptation is also better evaluated with MRI than with any other imaging technique.120
A major role of imaging is in the assessment of complications. After PTE, some of the early complications include reperfusion pulmonary edema and PA steal syndrome. PA steal syndrome, seen in approximately 70% of patients after PTE, is characterized by new areas of V/Q mismatching and reflects the redistribution of blood flow from normally perfused lung to the newly endarterectomized segments.121 One of the intermediate to long-term complications of PTE is residual PH, seen in approximately one-third of patients.122 Causes include distal inoperable subsegmental disease and/or coexisting small vessel arteriopathy.122 Recurrent PH is less common and is due to a new thromboembolic event after successful PTE.116 CTA and MRI are promising tools for assessing residual/recurrent PH, with both having their respective strengths. DECT with its ability to provide perfusion information along with excellent depiction of thrombotic disease might evolve as a frontline test for evaluation of complications.
The role of imaging in the diagnosis and management of CTEPH continues to expand rapidly. With the success of newer minimally invasive therapies such as BPA, a variety of therapy options are available. One of the holy grails in the imaging of CTEPH remains the identification of microvascular disease. So far, there is no gold standard for direct identification of microvascular disease. Angiography-based partitioning of PVR by a PA occlusion technique may allow the identification of patients with small vessel disease but is invasive and technically demanding.111 A handful of studies have shown the potential role of echocardiography and MRI in identifying these patients. Using Pulsed Doppler and PC-MRI, the PA systolic profile is mapped and assessed for the presence of systolic notching. The timing of the systolic notch is used as a predictor of the site of obstruction, with a late systolic notch indicating the presence of microvascular disease.123 Although such advances are promising, larger studies are needed to fully assess the benefit of these noninvasive markers of microvascular disease.
Imaging plays an important role in the diagnosis of CTEPH, its preoperative evaluation, and in the assessment of a patient’s response to therapy. Although V/Q scanning continues to be favored as the initial screening test of choice, CT has emerged as the definitive imaging test of choice in depicting the structural and vascular abnormalities in CTEPH. MRI plays a complementary role, providing crucial functional and physiological information that carries prognostic value. The emergence of new methods such as DECT, SPECT V/Q, SPECT-CT V/Q, and newer MRI techniques heralds an exciting and promising shift in imaging paradigms that may improve clinical decision making and ultimately lead to more favorable patient outcomes.
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