Clinical indications for intravascular ultrasound imaging

High-Frequency Ultrasonic Forceps for the In Vivo Detection of Cancer During Breast-Conserving Surgery

A major aim in the surgical management of soft tissue cancers is to detect and remove all cancerous tissues while ensuring noncancerous tissue remains intact. Breast-conserving surgery provides a prime illustration of this aim, since remaining cancer in breast margins results in multiple surgeries, while removal of too much unaffected tissue often has undesirable cosmetic effects. Similarly, resection of benign lymph nodes during sentinel lymph node biopsy can cause deleterious health outcomes. The objective of this study was to create an intraoperative, in vivo device to address these challenges. Instant diagnostic information generated by this device could allow surgeons to precisely and completely remove all malignant tissue during the first surgery. Surgical forceps based on Martin forceps were instrumented at the tips with high-frequency ultrasonic transducers composed of polyvinylidene difluoride, a thickness-sensing rotary potentiometer at the base, and a spring to provide the appropriate restoring force. Transducer wires within the forceps were connected to an external high-frequency pulser-receiver, activating the forceps' transmitting transducer at 50 MHz and amplifying through-transmission signals from the receiving transducer. The forceps were tested with tissue-mimicking agarose phantoms embedded with 58–550 μm polyethylene microspheres to simulate various stages of cancer progression and to provide a range of measurement values. Results were compared with measurements from standard 50 MHz immersion transducers. The results showed that the forceps displayed similar sensitivity for attenuation and increased accuracy for wave speed. The forceps could also be extended to endoscopes and laparoscopes.

Stent-Assisted Angioplasty of Spontaneous Bilateral Extracranial Vertebral Dissections under Intravascular Ultrasound Guidance

The authors here report a case of stent-assisted angioplasty under intravascular ultrasound (IVUS) guidance for the treatment of spontaneous bilateral extracranial vertebral artery (VA) dissection. A 47-year-old woman presented with spontaneous severe posterior neck pain. Examinations revealed bilateral extracranial VA dissection, which was thought to be the reason for her symptom. However, since the pain was gradually worsening even after sufficient medical treatment, she underwent stent angioplasty under IVUS guidance, following which her symptoms improved. We propose that stent placement under IVUS guidance is a safe and feasible method for treating extracranial VA dissections. Since the intravascular environment is seen in real time with IVUS, this technique is useful for confirming a true lumen and evaluating appropriate stent apposition. More clinical experience with this technique is required and mandatory, and devices with smaller diameters with improved trackability are essential for further introduction of IVUS into the field of endovascular neurosurgery.

Characterization of an improved polyimide-etalon all-optical transducer for high-resolution ultrasound imaging

All-optical transduction of ultrasound provides high-frequency (>20 MHz) operation in the absence of electrical noise and distortion that hinders small-scale piezoelectric probes. Although fabrication of an all-optical 2-D array suitable for in vivo imaging remains incomplete, a thin-film structure integrating a polyimide film with a Fabry-Perot (etalon) receiver has been shown to be a viable candidate. We present here incremental improvements in the performance of a polyimide-etalon transducer and demonstrate imaging with an array configuration alternative to our previous study. We first show that a gain of more than 30% in output pressure is achieved when increasing the thickness of a bare polyimide film from 3 to 15 μm. This motivated the choice of polyimide as the etalon medium--a configuration made possible by utilizing a dielectric mirror that transmits wavelengths used for generation of ultrasound (ultraviolet) and reflects those for detection (near infrared). The increased reflectivity of the dielectric mirror resulted in a 2-fold decrease in noise-equivalent pressure to 3.3 kPa over a bandwidth of 47.5 MHz (0.48 Pa/√Hz). The transmit/receive center frequency increased from 37 to 49 MHz with a -6-dB bandwidth of 126%, and a maximum pressure of 213 kPa was produced using a 43 μm UV spot. A 2 x 2 mm synthetic array of 957 transmitters centered on a 1 x 1 mm synthetic array of four receivers was used to image two wire targets. Offline reconstruction indicated lateral resolutions of 70 and 114 μm at depths of 2.4 and 5.8 mm, respectively, with an average axial resolution of 35 μm. Finally, we explore the challenges of imaging in this configuration, which provides the best opportunity for real-time performance pending further development.

Comparison of intravascular ultrasound versus angiography-guided drug-eluting stent implantation: a meta-analysis of one randomised trial and ten observational studies involving 19,619 patients

AIMS

The impact of intravascular ultrasound (IVUS) guided coronary drug-eluting stent (DES) implantation on clinical outcomes remains controversial. A meta-analysis of the currently available clinical trials investigating IVUS-guided DES implantation was undertaken.

METHODS AND RESULTS

We searched Medline, the Cochrane Library and other internet sources, without language or date restrictions, for published articles comparing clinical outcomes between IVUS-guided and angiography-guided DES implantation. Clinical studies with both adjusted and unadjusted data were included. Eleven studies were identified (one randomised controlled trial and 10 registries) and included in the meta-analysis with a weighted follow-up time of 20.7±11.5 months. Compared with angiography guidance, IVUS-guided DES implantation was associated with a reduced incidence of death (hazard ratio [HR]: 0.59, 95% confidence interval [CI]: 0.48-0.73, p<0.001), major adverse cardiac events (HR: 0.87, 95% CI: 0.78-0.96, p=0.008) and stent thrombosis (HR: 0.58, 95% CI: 0.44-0.77, p<0.001). The incidence of myocardial infarction (HR: 0.82, 95% CI: 0.63-1.06, p=0.126), target lesion (HR: 0.90, 95% CI: 0.73-1.11, p=0.316) and target vessel (HR: 0.90, 95% CI: 0.77-1.05, p=0.195) revascularisation was comparable between the angiography and IVUS-guided arms. A repeat meta-analysis of propensity-matched studies only (six studies, n=5,300) yielded broadly similar results in terms of clinical outcomes.

CONCLUSIONS

IVUS-guided coronary DES implantation is associated with a significant reduction in death, MACE and stent thrombosis compared to angiography guidance. Appropriately powered randomised trials are necessary to confirm the findings from this meta-analysis.

Does intravascular ultrasound provide clinical benefits for percutaneous coronary intervention with bare-metal stent implantation? A meta-analysis of randomized controlled trials

BACKGROUND

The role of intravascular ultrasound (IVUS) in percutaneous coronary interventions (PCI) is still controversial despite several previously published meta-analyses. A meta-analysis to evaluate the controversial role of IVUS-guided PCI with bare-metal stenting was performed and a previous published meta-analysis was re-evaluated in order to clarify the discrepancy between results of these studies.

METHODS

A systematic review was performed by an electronic search of the PubMed, Embase and Web of Knowledge databases and by a manual search of reference lists for randomized controlled trials published until April 2011, with clinical outcomes and, at least, six months of clinical follow-up. A meta-analysis based on the intention to treat was performed with the selected studies.

RESULTS

Five studies and 1,754 patients were included. There were no differences in death (OR = 1.86; 95% CI = 0.88-3.95; p = 0.10), non-fatal myocardial infarction (OR = 0.65; 95% CI = 0.27-1.58; p = 0.35) and major adverse cardiac events (OR = 0.74; 95% CI = 0.49-1.13; p = 0.16). An analysis of the previous published meta-analysis strongly suggested the presence of publication bias.

CONCLUSIONS

There is no evidence to recommend routine IVUS-guided PCI with bare-metal stent implantation. This may be explained by the paucity and heterogeneity of the studies published so far.

A novel semiautomated atherosclerotic plaque characterization method using grayscale intravascular ultrasound images: comparison with virtual histology

Intravascular ultrasound (IVUS) virtual histology (VH-IVUS) is a new technique, which provides automated plaque characterization in IVUS frames, using the ultrasound backscattered RF-signals. However, its computation can only be performed once per cardiac cycle (ECG-gated technique), which significantly decreases the number of characterized IVUS frames. Also atherosclerotic plaques in images that have been acquired by machines, which are not equipped with the VH software, cannot be characterized. To address these limitations, we have developed a plaque characterization technique that can be applied in grayscale IVUS images. Our semiautomated method is based on a three-step approach. In the first step, the plaque area [region of interest (ROI)] is detected semiautomatically. In the second step, a set of features is extracted for each pixel of the ROI and in the third step, a random forest classifier is used to classify these pixels into four classes: dense calcium, necrotic core, fibrotic tissue, and fibro-fatty tissue. In order to train and validate our method, we used 300 IVUS frames acquired from virtual histology examinations from ten patients. The overall accuracy of the proposed method was 85.65% suggesting that our approach is reliable and may be further investigated in the clinical and research arena.

Focus on the research utility of intravascular ultrasound - comparison with other invasive modalities

Intravascular ultrasound (IVUS) is an invasive modality which provides cross-sectional images of a coronary artery. In these images both the lumen and outer vessel wall can be identified and accurate estimations of their dimensions and of the plaque burden can be obtained. In addition, further processing of the IVUS backscatter signal helps in the characterization of the type of the plaque and thus it has been used to study the natural history of the atherosclerotic evolution. On the other hand its indigenous limitations do not allow IVUS to assess accurately stent struts coverage, existence of thrombus or exact site of plaque rupture and to identify some of the features associated with increased plaque vulnerability. In order this information to be obtained, other modalities such as optical coherence tomography, angioscopy, near infrared spectroscopy and intravascular magnetic resonance imaging have either been utilized or are under evaluation. The aim of this review article is to present the current utilities of IVUS in research and to discuss its advantages and disadvantages over the other imaging techniques.