Imaging modality
Resolution
Cap thickness
Lipid core
Calcium
Thrombus
Macrophage
Neovascularization
IVUS
100 μm
+
+
++
+
−
−
OCT
10 μm
+++
++
++
++
+
++
VH
100 μm
+
+
++
+
−
−
NIRS
−
+
+++
−
−
−
−
Angioscopy
−
+
+
−
+++
−
−
9.1 Basic Mechanism
Spectroscopy is well established and widely accepted method to identify unknown chemicals in a variety of industries and scientific studies. Basically, spectroscopy employs the mechanism that light reflection (scattering) and absorption vary at different wavelengths according to each chemical component or substance [1, 2]. Organic component in the atheromatous plaque (collagen, cholesterol, etc.), when near-infrared (wavelength 780–2500 nm) light is shed on them, provides unique spectral signature (there are particular and specific peaks and trough patterns according to each chemical substances) that can be used as “chemical thumbprint ” [3]. All these information are integrated with grayscale intravascular ultrasound (IVUS) images and displayed into a single picture (Fig. 9.1).
Fig. 9.1
Representative case of near-infrared spectroscopy (NIRS) in patient with acute coronary syndrome. The coronary angiogram shows significant stenosis at the proximal segment of the left anterior descending artery (white arrow) (a). NIRS shows large lipid burden within coronary artery wall (b). The cross-sectional image of NIRS clearly reveals lipid accumulation is present from 7 o’clock to 10 o’clock (white arrow), while concomitant intravascular ultrasound (IVUS) image demonstrates the presence of plaque rupture (black arrow) at the same location. In this case the identification of lipid by IVUS image is not feasible
9.2 Validation
NIRS system was rigorously validated with 84 human autopsy hearts in a prospective and double-blind manner to assess the accuracy in detecting the lipid core plaque (LCP) [4]. In order to develop quantitative index for the validation, an LCP of interest was defined as a lipid core >60° in circumferential extent, >200 μm thickness, and with a mean fibrous cap thickness <450 μm. The algorithm of NIRS system prospectively identified LCP with a receiver-operator characteristic area of 0.80 (95% confidence interval [CI]: 0.76–0.85). The lipid core burden index detected the presence or absence of any fibroatheroma with an area under the curve of 0.86 (95% CI: 0.81–0.91). This study successfully demonstrated good agreement between NIRS system and histopathology in coronary autopsy specimens. Clinical verification of NIRS system was performed by SPECTACL (Spectroscopic Assessment of Coronary Lipid) study. This study showed that spectral data obtained from patients by NIRS system were similar with those from autopsy specimens [5]. Furthermore, high reproducibility of NIRS system for the detection of LCP was demonstrated by Garcia et al. [6].
9.3 NIRS System and Measurement
NIRS system (TVC®, InfraReDx, Burlington, MA, USA) consists of 3.2F catheter, which uses 0.014-in. coronary guidewire system and pullback devices (Fig. 9.2). Mechanical pullback and rotation are performed at a speed of 0.5 cm/s and 240 rotation/m. The NIRS system acquires approximately 1000 NIRS measurement/12.5 cm of artery scanned and determines the presence of lipid core plaque (LCP) at each interrogated location in the artery using a predictive algorithm. The calculated data are displayed in a two-dimensional map of the vessel (“chemogram ”) (Fig. 9.3a). The x-axis of the chemogram represents pullback position in millimeter scale, and the y-axis represents circumferential position in degrees (0–360°); a color scale from red to yellow indicates increasing probability that a LCP is present.
Fig. 9.2
Near-infrared (NIR) spectroscopy system (TVC®, InfraReDx, Burlington, MA, USA). The system consists of a console (a), a mechanical rotation pullback device (b), and a 3.2F imaging catheter (c). The disposable imaging catheter uses traditional 0.014-in. monorail system and contains an optical fiber to deliver NIR light from a console as well as intravascular ultrasound (IVUS) imaging system. The console integrates NIR information with IVUS image using predictive algorithm
Fig. 9.3
An example of chemogram and block chemogram . (a) The color of chemogram from red to yellow indicates the increasing probability that a lipid core plaque (LCP) is present at this location. (b) Each color of the block chemogram is determined by 90th percentile value of the chemogram within a 2-mm segment. Four colors of the block chemogram represent chance of a LCP at this location (red, p < 0.57; orange, 0.57 ≤ p ≤ 0.84; tan, 0.84 ≤ p < 0.98; yellow, p ≥ 0.98)
The block chemogram is a summary measurement of the probability that a LCP of 2-mm pullback interval is analyzed and displayed in a color map (Fig. 9.3b). The block chemogram uses the same color scale as the chemogram, but the display is summed up to four discrete colors to facilitate visual interpretation (red, p < 0.57; orange, 0.57 ≤ p ≤ 0.84; tan, 0.84 ≤ p ≤ 0.98; yellow, p > 0.98, algorithm probability that a LCP is present in that 2-mm block). Lipid core burden index (LCBI ) is defined as the fraction of valid pixel in the chemogram that exceed a LCP probability of 0.6, multiplied by 1000 (Fig. 9.4). LCBI provides a summary measurement of the LCP presence in the entire scanned segment. The maxLCBI4mm is defined as the maximum value of LCBI for any of the 4-mm segment in the interrogated region and used as the index representing the size of the LCP (Fig. 9.5).
Fig. 9.4
Lipid core burden index (LCBI ) . LCBI is defined as cholesterol-positive signals which exceed an LCP probability of 0.6 within the region of interest divided by total valid signals multiplied by 1000 (‰)
Fig. 9.5
maxLCBI4mm . maxLCBI4mm is defined as the maximum value of lipid core burden index for any of the 4-mm segment. It represents the angular size of the LCP
9.4 Clinical Studies
9.4.1 Prediction of Periprocedural MI
NIRS is able to identify high risk of periprocedural myocardial infarction (MI). Goldstein JA et al. observed 62 patients with stable cardiac biomarker who underwent coronary stenting [7]. Periprocedural MI was observed in 50% of patients with a maxLCBI4mm ≥ 500. On the other hand, periprocedural MI occurred only in 4.2% of patients with maxLCBI4mm < 500 (p = 0.0002). Quantification of LCP measured as maxLCBI4mm ≥ 500 was associated with increased risk of periprocedural MI, which is completely in accordance with traditional studies with IVUS or virtual histology (Fig. 9.6). The CANARY (Coronary Assessment by NIR of Atherosclerotic Rupture-Prone Yellow) study [8] enrolled 85 stable angina patients in a prospective and multicenter manner. NIRS performed prior to PCI showed maxLCBI4mm was significantly higher (481.5 vs. 371.5, p = 0.05). However, among the randomized lesions with maxLCBI4mm ≥600, there was no difference of periprocedural MI with vs. without the use of distal protection filter (35.7% vs. 23.5%, respectively; relative risk, 1.52; 95% confidence interval: 0.50–4.60, p = 0.69). It is unclear whether this result is due to the limitation of NIRS predicting periprocedural MI or that of distal protection device preventing periprocedural MI. Further investigations will be needed to clarify this issue.
