In Vivo Diagnosis of Plaque Erosion and Calcified Nodule in Patients With Acute Coronary Syndrome by Intravascular Optical Coherence Tomography. Jia H, et al. J Am Coll Cardiol 2013; 62: 1748-58.
この論文では、急性冠症候群患者さんに対して OCTで観察し、その病変にはこれまで言われてきた Plaque Ruptureの所見のみならず、Plaque Erosionが多く認められ、さらに高齢者ではCalcified Noduleが多く認められるということを提唱しています。Plaque Ruptureであれば、血管内超音波検査 (IVUS)で明瞭に観察できまずか、Erosionとか、ここでいう calcified noduleというのは微小なものであり、IVUSでは分からない小さなものを指しているようです
それゃあ 原理的に解像度は音波を用いるIVUSよりも、光を用いるOCTの方が波長の長さの半分に比例して良いのですが、本当かな? とも思います
この論文は、128例の急性冠症候群患者さんを対してた観察研究であり、その中で plaque rupture = 43.7%, erosion = 31.0%, calcified nodule = 7.9%で認められた、ということです
Erosionとか Calcified Noduleとか言われればそうななのかな? とも思いますが、論文の中の Figure 3 = Erosionと Figure 4 = Calcified Noduleを見比べてもその違いが全く分かりません だからどうもこの論文の内容にはあまり感心しないのです
以前僕が提出したある雑誌の Editorial Reviewで書いた以下の文章を今でも真実だと思っています
What is essential for the clinical examination? From the standpoint of the information, it must be to retrieve information from the objects as much as possible. Especially for the diagnostic imaging procedures, it is essential to retrieve the image of the objects as accurately as possible.
In order to perceive the image of the objects, we have to use some kind of waves. The 2 most common waves for us are light and sound. By using light wave, we can see the world through our eyes. By using sound wave, we can hear the world through our ears. Aside from these waves common for us, we use microwave in Rader to detect flying airplanes or cruising ships. We also can know where the earthquake was initiated and how strong it was by analyzing the earthquake wave using seismogram. The acquisition of any objects by using waves is generally limited by 2 factors: the wavelength and the sampling rate. In general, if we use the wave with longer wavelength (= lower frequency) to detect the objects, the wave can go through over the obstacles and reach the objects more easily, but the size of the objects which we can recognize is bigger, because the objects smaller than the wavelength can hardly reflect the wave. On the other hand, if we use the wave with shorter wavelength (= higher frequency), we can recognize the smaller objects by the reflected wave from those objects, but the wave can be reflected more easily by the obstacles between us and the objects, and reach the objects with bigger difficulty. In order to detect the reflected waves, we have to sample these waves (Almost all of the information is now processed by digital but not by analog ways). According to the fundamental theorem in the field of information theory: the theorem proposed by Harry Nyquist and Claude E. Shannon (Nyquist-Shannon Sampling Theorem), when sampling a signal, the sampling frequency must be greater than twice the bandwidth of the input signal in order to be able to reconstruct the original perfectly from the sampled version . In other words, we can detect the wave only with the frequency lower than the half of the sampling rate. Thus, the acquisition of the objects is regulated at least 2 factors: the wavelength and the sampling rate.
Since selective coronary angiography was first performed by Mason Sones on October 30, 1958 , it has been the golden standard in the in-vivo evaluation of coronary artery anatomy and pathology for almost 50 years. Gruentzig started percutaneous coronary angioplasty by using his own-made balloon catheters in human patients in 1977. He invented an on-the-wire balloon with double lumens: one lumen was for the balloon inflation and another for the pressure measurement. The pressure lumen opened distal to the balloon so that the distal intra-coronary pressure could be monitored. He utilized the information not only from coronary angiography but also from the intra-coronary pressure distal to the narrowing. However, the measurement of the distal intracoronary pressure had been soon abandoned after the introduction of the steerable guidewire and balloon catheter system, since its clinical significance had not been proved. For many years, coronary angioplasty had been performed based on the information obtained only from coronary angiography.
From early 90's, intracoronary ultrasound (ICUS) examination had been developed and finally achieved the position as another standard to evaluate coronary artery anatomy and pathology especially during coronary angioplasty in mid to late 90's . It is especially useful to evaluate the plaque distribution and morphology while doing directional coronary atherectomy. However, after new-generation stents were put into market, the easiness and efficacy of stent implantation had been gradually pushing away the complicated and troublesome procedures like directional coronary atherectomy. There have been several prospective randomized comparisons to test the usefulness of ICUS examination during coronary angioplasty. In OPTICUS (OPTimization with ICUS to reduce stent restenosis) study, 550 patients being planned to receive coronary stent implantation were prospectively randomized into either ultrasound- (252 patients) or angiography- (269 patients) guided stent implantations, and were followed up for 6 months post procedures. The results were striking to everybody who claimed the clinical significance of ICUS examination during angioplasty. They could not find any significant differences in 6-month restenosis rate, percent diameter stenosis or minimum lumen diameter between 2 groups . The current ICUS system uses 40 MHz ultrasound. The wave speed of ultrasound in the body is defined as 1562.5 m/sec (ANSI standard). Thus, one wavelength equals to 39 microns. The ICUS system emits ultrasound pulses of 3.5 wavelengths in duration (= 137 microns). Thus, theoretical minimum spatial resolution will be 68 microns, which is much better than the minimum angiographic resolution (= 100 to 200 microns). We tend to assume, if the resolution would be higher, it would bring better clinical results. However, it was not true in OPTICUS study.
Since the light used in optical coherent tomography (OCT) has shorter wavelength, the theoretical minimum spatial resolution is much higher than ICUS examination. In this journal , Diaz-Sandoval, et al clearly showed the better imaging ability of OCT in coronary arteries of human patients before and after coronary angioplasty. I am really looking forward to having a nice OCT system in my catheter laboratories. I want to see more precisely how is looking at the lesion, for which I am treating by angioplasty. However, I must be very careful for its real importance in clinical situation, especially in the era of drug-eluting stent. The tremendous clinical effect of drug-eluting stent may cover all of the fine possible effects of OCT in the real-world clinical situations.
 C. E. Shannon, "Communication in the presence of noise," Proc. Institute of Radio Engineers, vol. 37, no.1, pp. 10-21, Jan. 1949.
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 Diaz-Sandoval LJ, Bouma BE, Tearney GJ, Jang IK. Optical coherence tomography as a tool for percutaneous coronary interventions. Catheter Cardiovasc Interv 2005: **; ***-**.