7.7 Patency Control of Coronary Stents 239

7.7

Patency Control of Coronary Stents

K. Nieman, N. Mollet, F. Cadamartiri, P. de Feyter

ter resolution have resulted in further improvements (Nieman 2002, Ropers 2003, Leschka 2005).

Patients who previously underwent PCI often developed recurrent symptoms due either to re-ste- nosis at the location of the previous obstruction or to progression of atherosclerosis at other sites. Post-PCI patients are more likely to require repeated angio- graphic coronary evaluation, so that a non-invasive technique would therefore be desired. Possible clini- cal indications for multi-slice coronary CTA could include suspected early occlusion of stents after the procedure, late in-stent restenosis, or progression of coronary artery disease in non-stented vessel seg- ments.

7.7.2

CT Imaging Characteristics of Stents

Stents are small catheter-delivered expandable devices that maintain the lumen diameter after dil- atation or restore endothelial integrity after vessel dissection. The devices are expected to sustain considerable inward radial force, while maintain- ing longitudinal flexibility. Most stents are laser- cut stainless steel meshes, of which recent designs include coating with a drug to prevent restenosis.

Currently used stents have variable radiopacity, and consequently, variable visibility on conventional X- ray angiography.

Most stents are well-visible on CT images. The CT density of natural tissues in and around the coronary artery is much lower than the density of the metal stent material. Intravenous contrast enhancement used in multi-slice CT results in a lower concentra- tion of contrast medium and in less X-ray attenua- tion of the intra-coronary lumen compared to con- ventional angiography with selective intra-coronary contrast injection. As a consequence, stents appear as bright structures on multi-slice CT images.

In vitro sub-millimeter multi-slice CT imaging of static coronary stents (Nieman 2003) has shown that the stent struts appear significantly larger than their actual size (Fig. 7.39). A combination of partial voluming caused by the limited spatial resolution in relation to the diameter of the stent struts, beam hardening artifacts, and a slightly widened slice-sen- sitivity profile related to spiral scanning are likely

C o n t e n t s

7.7.1 Introduction 239

7.7.2 CT Imaging Characteristics of Stents 239 7.7.3 Post-processing and Analysis 240 7.7.4 In Vivo Coronary Stent Imaging 241 7.7.5 Conclusion 242

References 244

7.7.1 Introduction

The use of percutaneous intervention (PCI) for the treatment of ischemic coronary artery disease has expanded dramatically over the last two decades.

Initially, balloon angioplasty offered an alternative to bypass surgery in focal lesions, but it involved a considerable risk of acute dissection, thrombosis, or late coronary re-stenosis. Continuous technical innovation has expanded the indications for PCI and reduced both the procedural risk and the occurrence of post-procedural re-stenosis. Nowadays, most interventions involve intra-coronary expansion of stents. Still, until the introduction of coated stents, neointimal hyperplasia caused clinically significant re-stenosis in at least 20% of patients (Kiemeneij 2001). Although the occurrence of re-stenosis may be less in the future with increasing use of drug- eluting stents, the progression of atherosclerotic degeneration in the remaining coronary arteries is not affected (Morice 2002).

Multi-slice spiral CT allows minimally invasive

angiographic imaging of the coronary arteries. The

diagnostic accuracy to detect coronary stenoses is

good (Nieman 2001, Achenbach 2001), particularly

in the absence of extensive vascular calcification and

in patients with low heart rates: The introduction of

16-slice and 64-slice CT scanners with sub-millime-

Fig. 7.39. In vitro imaging (4-slice CT, 0.5-mm detector colli- mation) of stents in contrast-enhanced silicon tubes: Synthesis Star 4.0 and 3.0 mm (CardioVascular Dynamics, Irvine, USA).

While the magnitude of the blooming effect seems similar in both stents, only a very limited area of relatively artifact-free lumen remains in the 3.0-mm stent

responsible for this blooming effect, or increased CT density outside the actual location of the stent itself.

As a result the coronary lumen within the stent has a higher density that extends from the vessel wall and decreases towards the center. Depending on the size of the vessel and the metal density of the

stent, depiction of a variable central part of the vessel will remain unaffected by the stent-related artifacts (Fig. 7.39). Particularly small radiopaque stents (3.0-mm diameter) result in a density incre- ment throughout the lumen (Nieman 2003). In gen- eral, stent-related artifacts and visualization of the in-stent lumen strongly varies with different stent types, depending on stent diameter, caliber of the stent struts, and stent material (Maintz 2003a).

Use of the thinner slice-collimation available with the newer 16-slice and 64-slice CT scanners in com- bination with special edge-preserving reconstruc- tion kernels for higher in-plane resolution improves visualization of the in-stent lumen (Maintz 2003a, Maintz 2003b).

7.7.3

Post-processing and Analysis

Various post-processing techniques exist for evalu- ation of CT coronary angiograms, including MPR, thin-slab MIP, curved reformations, various 3D reconstructions such as shaded-surface display (SSD), VRT, and virtual endoscopy. Cross-sectional images, either the axial source images or (curved) MPRs, are best-suited for evaluation of the coronary lumen within the stent. The anatomical location of stents can be nicely visualized with VRT (Fig. 7.40).

When evaluating a stented coronary artery, the exaggerated stent appearance, and consequently the decreased lumen diameter, need to be taken into account and do not necessarily imply in-stent re- stenosis (Fig. 7.41). Complete or near-complete stent obstruction will become clear by an overall density that is lower than that of the other coronary seg- ments (Fig. 7.42). Visualization of tissue within the stent without significant obstruction requires the highest possible resolution, beyond the capabilities of 4- and 16-slice CT scanners. The latest 64-slice CT scanners, however, with spatial resolution below

LM LAD

CX

Fig. 7.40. Three-dimensionally reconstructed CT angiogram (4-slice CT with 1.0-mm detector collimation) of a patient with three stents in the LM, LAD, and CX

Fig. 7.41. A 3.0-mm diameter stent in the LAD, imaged with contrast-enhanced 16-slice CT. Visualization of the in-stent lumen is compromised due to blooming artifacts

Fig. 7.42a,b. Occluded stent in the CX coronary artery exam- ined with 16-slice CT using clipped 3D reconstruction (a) and curved MPR (b). The low-density material (arrow) represents the occlusion in the mid-section of the stent

a

b

0.4 mm, may provide sufficient resolution to also visualize mild and moderate in-stent stenosis.

More advanced post-processing techniques are less suitable for the evaluation of coronary stents.

External 3D display does generally not show the intra-coronary lumen. Adjustment of the display settings to correct for the presence of the stent and particularly the stent-related artifacts is not readily done and does not substantially improve interpreta-

tion. MIP of thin slabs is a useful method for ini- tial evaluation of the coronary arteries, but the high density value of the stent material leads to further over-projection and precludes assessment of the coronary lumen. Therefore, curved MPR and cross- sectional MPR perpendicular to the vessel lumen are the most useful techniques for visualizing the in-stent lumen.

7.7.4

In Vivo Coronary Stent Imaging

While the feasibility and diagnostic accuracy of detection of obstructive coronary disease by multi- slice CT imaging has been described extensively, the usefulness of this approach in patients who have undergone coronary intervention and subsequent stent placement has only been evaluated in prelim- inary and small studies. EBCT has been used in CTA after balloon angioplasty, but nowadays few patients undergo PCI without implantation of stents (Achenbach 1997).

In-vivo imaging of coronary stents requires scan protocols that are optimized for spatial resolution.

As in imaging of the native coronary vessels, imag-

ing of coronary stents is influenced by cardiac motion,

depending on the heart rate and the location of the

stent, and by the presence of calcified plaque material in the vessel wall. Because a blooming artifact depends on the spatial resolution in relation to the vessel diam- eter, the effect on smaller stented vessels is greater.

While 4-slice CT cannot reliably visualize stented coronary vessels and in-stent lumen, 16-slice CT combined with a sufficiently slow heart rate can provide a reasonably good assessment of coronary artery segments with stents (Fig. 7.43). Although no large comparative studies have been published, there seems to be consensus about the ability of 16-slice CT to accurately assess stent patency and to recognize complete stent occlusion (Hong 2003). Hong et al.

investigated 19 patients with 16-slice CT to follow- up and confirm stent patency 1–3 weeks after stent placement. A total of 26 stents were investigated with different reconstruction techniques and stented lumen diameters were measured. Five patients had heart rates of more than 70 bpm during the scan and the six stents could not be reliably assessed due to motion artifacts. In the remaining 20 stents, stent patency was accurately determined. The use of a spe- cial reconstruction kernel resulted in improved accu- racy of the delineation of the in-stent lumen diame- ter and of the diameter of the vessel lumen proximal and distal to the stent. In contrast to the assessment of coronary arteries without stents, the detection of moderate stenosis in stented coronary vessels with 16-slice CT is regarded as less reliable, due to bloom- ing artifacts that cover the tissue and the lumen near

the stent struts. Blooming artifacts also limit qualita- tive interpretation of the plaque material within the stent. As a result, the role of 16-slice CTA after PCI is limited to the detection of disease progression in the coronary segments without stents, detection of ste- nosis near the edges of the stent, assessment of stent patency, and detection of in-stent occlusion and per- haps high-grade stenosis.

Better stent visualization has become possible with further improvement of the spatial resolution with 64-slice CT by implementation of thinner detec- tor slices combined with a flying focal spot sampling technique in the z-direction (Figs. 7.44, 7.45). Dedi- cated image reconstruction and post-processing techniques have been developed to further reduce stent-related image artifacts and more accurately depict the coronary lumen adjacent to the stent. In a very recent study, Rist et al. used 64-slice CT to examine 44 stented coronary arteries in 23 patients.

The authors evaluated the detection accuracy of sig- nificant pre-stent, in-stent, and post-stent stenosis compared to conventional angiography (Rist 2005).

Conventional angiography revealed four pre-stent, five in-stent, and three post-stent stenoses with

>50% lumen narrowing in the 44 stented segments.

With 64-slice coronary CTA, the sensitivity for pre- stent stenosis was 75%, the specificity 95%, NPV 97%, and PPV 60%; for in-stent stenosis, sensitivity was 80%, specificity 97%, NPV 95%, and PPV 75%;

for post-stent stenosis, sensitivity was 67%, specific- ity 85%, NPV 97%, and PPV 25%. The high speci- ficity and high NPV suggest that 64-slice CT with improved spatial resolution can rule-out significant coronary stenosis also in stented vessels.

Stent manufacturers have introduced stents with thinner struts and less radiopaque material. In partic- ular, the latest-generation drug-eluting stents are usu- ally less radiopaque than conventional stents. These newer stents improve multi-slice CT evaluations in that blooming artifacts are largely reduced (Fig. 7.46).

7.7.5 Conclusion

While the diagnostic accuracy of coronary CTA in non-stented vessels has reached the stage of clinical applicability, the use of multi-slice CT after coro-

RCA

LCA

Fig. 7.43. Low-opaque stent (arrow) in the ostium of the RCA, visualized with curved MPR. There is practically unhindered interpretability of the vessel lumen, which was imaged with 16-slice CT. LCA Left coronary artery

Fig. 7.44a–d. Patent stent in the LAD, examined with 64-slice CT. Curved MPR (a) and cross-sectional MPR (b) reveals a patent lumen without the pres- ence of stenosis. VRT re- construction demonstrates the run-off of the native vessel (c). Conventional angiography confi rms the 64-slice CT fi ndings (d).

(Case courtesy of Gros- shadern Clinic, University of Munich, Germany)

LV

LV Ao

RVOT

LV RV

LAD

Fig. 7.45a,b. 64-slice CT examination of a patient with long extend- ing stents of the LAD and RCA. VRT reconstruction with a special setting to mimic radiography demonstrates the stents and the strut structure (a). Curved MPR reveals a moderate in-stent restenosis in the LAD stent (b). (Case courtesy of Toyohashi Heart Center,

Tokyo, Japan) a b

a b

c d

nary intervention with stent placement is still chal- lenging. With 16-slice CT scanners, assessment is restricted to determining stent patency and total stent occlusion; however, the latest 64-slice CT scan- ners holds promise to enable significant stenosis of stented coronary segments to be reliably excluded.

Fig. 7.46a,b. 64-slice CT examination of a patient after placement of a drug-eluting stent in the proximal LAD. The low-opaque stent can be visualized with curved MPR (a) and curved MIP (b), with practically unhindered interpret- ability of the patent vessel lumen. (Case courtesy of Sarawak Heart Center, Kuching, Malaysia)

a b

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