Coronary Iodine Concentration by Using Spectral CT and Success of Flow Restoration in Chronic Total Occlusion

See also the article by Lee et al in this issue.

Ronen Rubinshtein, MD, is the director of the Heart Institute at the Edith Wolfson Medical Center in Holon, Israel. He is an interventional cardiologist who also specializes in cardiac CT imaging. Dr Rubinshtein is a visiting associate professor at Tel Aviv University.

Ron Blankstein, MD, is director of cardiac computed tomography and associate director of the Cardiovascular Imaging Program at Brigham and Women’s Hospital in Boston, Massachusetts. He is also a professor of medicine at Harvard Medical School. Dr Blankstein is the immediate past president of the Society of Cardiovascular Computed Tomography and serves as an associate editor of JACC: Cardiovascular Imaging and Radiology: Cardiothoracic Imaging.

Chronic total occlusion (CTO) of a coronary artery is found in up to 15%–25% of patients undergoing invasive angiography and is associated with adverse prognosis. Although several historic nonrandomized studies failed to show a significant benefit of percutaneous coronary intervention (PCI) of a CTO, recent evidence suggests that it may be associated with a clinical benefit in selected patients, especially those who remain symptomatic despite optimal medical therapy or those with a large area of ischemia of the subtended myocardium (1). Historically, percutaneous recanalization of a CTO has been technically challenging, and such cases were referred to coronary artery bypass surgery or recommended for medical therapy alone. Recent advances in interventional equipment and innovative approaches to crossing CTO have significantly increased the success rate of percutaneous treatments.

While the success rate of CTO-PCI is lower than that for other coronary lesions, recent reports show that a success rate of up to 70%–75% may be expected in selected patients. However, the success rate of a CTO-PCI is highly variable and depends on many factors, including operator experience and the availability of newer PCI guidewires and techniques. Regardless of physician experience and competence, anatomic characteristics of a CTO are also important determinants of procedural success (1). Several anatomic features are known to be associated with improved procedural success, including blunt entry site (versus tapered), less severe calcifications, shorter CTO length, and reduced vessel tortuosity. The presence of a side branch at the CTO entry site is a common determinant of CTO-PCI failure. Several risk scores have been used to assess suitability and estimate the success of CTO-PCI, including the J-CTO score which incorporates anatomic characteristics at invasive coronary angiography (2).

Coronary CT angiography can provide important anatomic data to the operator, including vessel wall characteristics, amount of coronary calcifications, and length of CTO (3,4) (Table). However, poor opacification of an occluded, frequently calcified, coronary artery may limit the usefulness of coronary CT angiography in the assessment of CTO.

Coronary CT Angiographic Findings for Informing Management and Predicting Success of CTO PCI Lesions

In the current issue of Radiology: Cardiothoracic Imaging, Lee et al present an additional technique for predicting potential success of a CTO-PCI using spectral CT imaging (5).

Spectral imaging using a dual-layer detector (like the model used in this article) or a dual-energy CT scanner using other techniques (such as rapid kilovolt switching) allows for improved tissue characterization and differentiation between water, iodine, or fat (6). In theory, better identification of iodine concentration within a coronary artery using spectral CT imaging (especially while utilizing low monoenergetic images) can assess whether there is minimal flow in the proximal portion of a CTO. This finding would suggest the presence of intralesion microvessels and subsequently better likelihood for proximal cap penetration of a CTO using dedicated guidewires. Applying this concept, Lee et al evaluated the coronary iodine concentration at the proximal portion of a CTO (2 mm from entry of CTO, excluding the calcification) using spectral CT virtual monoenergetic image at 60 keV. Additionally, they evaluated other traditional factors known to be associated with CTO-PCI failure such as longer CTO length, severe calcifications, and vessel bending.

The authors retrospectively evaluated this concept among 50 eligible patients who underwent preprocedural coronary CT angiography using spectral CT followed by a CTO-PCI procedure. The authors evaluated whether spectral CT findings were associated with procedural success. Overall, 34 (68%) patients had a successful CTO-PCI with restoration of coronary blood flow in the affected vessel.

The authors found that coronary iodine concentration at the entry of a CTO lesion was significantly lower in failed PCI cases as compared with successful PCI cases. Moreover, the authors found that the optimal coronary iodine concentration cutoff point to predict failed PCI was 2.5 mg/mL, a threshold that resulted in a sensitivity of 87% (14 of 16), specificity of 79% (27 of 34), positive predictive value of 66% (14 of 21), and negative predictive value 93% (27 of 29). Other traditional markers of failed PCI (blunt stump, long CTO, severe calcification) were also more prevalent in the failed PCI groups. However, even when incorporating other findings into a multivariable model, low coronary iodine concentration (< 2.5 mg/mL) by using spectral CT at the CTO entry site remained significantly associated with failed CTO-PCI (3).

The authors should be commended for this interesting proof-of-concept work. Due to the small number of participants, and consequently the small number of failed PCI cases (16 patients), these findings deserve further evaluation. However, the idea that spectral CT imaging can allow better characterization of the proximal portion of a CTO lesion is highly appealing. The presence of microvessels (vessel diameter < 200 µm) within a CTO lesion is not infrequent and likely represents the presence of a “softer” proximal cap and a way to penetrate the CTO using dedicated tapered guidewires (7). In fact, “escalating wire technique” in the antegrade approach to CTO-PCI always starts with “microchannel probing” before upgrading to heavier and stiffer wires. Indeed, the association of even a small amount of contrast (and therefore higher iodine concentration) in the proximal portion of a CTO with smoother wire penetrance makes intuitive sense. While higher iodine concentration in the proximal CTO may be related to softer plaque and microvessel formation, one should remember that the spatial resolution of a 64-slice spectral CT used in the current study does not allow us to differentiate this favorable anatomy from bridging collateral vessels that are frequently present in CTOs. Whether newer technologies such as photon-counting CT will improve our ability to characterize CTO lesions with improved spectral imaging and higher spatial resolution is unknown, but in theory it may offer another technique to further evaluate CTO lesions.

Regardless of the coronary iodine concentration found to be useful in the current study, one should remember that multiple factors affect the success rate of CTO-PCI, and while operator experience is important, the most frequent anatomic cause for CTO-PCI failure is the presence of heavy calcifications (1).

In summary, the available data support the use of coronary CT angiography before CTO-PCI, particularly for procedure planning and for predicting procedure success. When coronary CT angiography is performed, it should evaluate all relevant anatomic features (Table). In centers where spectral imaging is available, coronary iodine concentration at the proximal part of the lesion may provide additional value to predict procedural success, thus further informing management decisions.