We developed a new dual-modality intravascular imaging system based on fast time-gated fluorescence intensity imaging and spectral domain optical coherence tomography (SD-OCT) for the purpose of interventional detection of atherosclerosis. A pulsed supercontinuum laser was used for fluorescence and OCT imaging. A double-clad fiber (DCF)- based side-firing catheter was designed and fabricated to have a 23 μm spot size at a 2.2 mm working distance for OCT imaging. Its single-mode core is used for OCT, while its inner cladding transports fluorescence excitation light and collects fluorescent photons. The combination of OCT and fluorescence imaging was achieved by using a DCF coupler. For fluorescence detection, we used a time-gated technique with a novel single-photon avalanche diode (SPAD) working in an ultra-fast gating mode. A custom-made delay chip was integrated in the system to adjust the delay between the excitation laser pulse and the SPAD gate-ON window. This technique allowed to detect fluorescent photons of interest while rejecting most of the background photons, thus leading to a significantly improved signal to noise ratio (SNR). Experiments were carried out in turbid media mimicking tissue with an indocyanine green (ICG) inclusion (1 mM and 100 μM) to compare the time-gated technique and the conventional continuous detection technique. The gating technique increased twofold depth sensitivity, and tenfold SNR at large distances. The dual-modality imaging capacity of our system was also validated with a silicone-based tissue-mimicking phantom.
Endovascular treatment of cerebral aneurysms with radioactive coils may prevent recanalization after embolization. This strategy requires an accurate estimation of the volume of the mass of coils to evaluate the intervention dosimetric success. The purpose of this work is to develop a computer-aided method to estimate the coil volumes using only two orthogonal angiographic projections. The originality of the method resides in the direct reconstruction of two 3-D contours of the mass of coils and the following variational interpolation of the 3-D surface in order to estimate its volume. Validated by simulations, the reconstruction algorithms could estimate the enclosed volumes with an average error of 2.9% and a variability of 2.5%. In addition, the feasibility of the method was also demonstrated using clinical images. Results showed that this reconstruction technique could quickly generate an accurate and realistic 3-D shape of the mass of coils without interfering with an ongoing clinician procedure.
Non-invasive ultrasound elastography (NIVE) was recently introduced to characterize mechanical properties of superficial arteries. In this paper, the feasibility of NIVE for the purpose of studying small vessels in humans and small animals is investigated. The experiments were performed <i>in vitro </i>on vessel-mimicking phantoms of 1.5-mm lumen diameter and 1.5-mm wall thickness. Polyvinyl alcohol cryogel (PVA-C) was used to create double layer vessel walls. The stiffness of the interior portion of the vessels was made softer. The vessels were insonified at 32 MHz with an ultrasound biomicroscope. Radial stress was applied within the lumen of the phantom by applying incremental static pressure steps with a column of a flowing mixture of water-glycerol. The Lagrangian speckle tissue model estimator was used to assess the 2D-strain tensor, and the composite Von Mises elastograms were then computed. The two-layer vessel walls were clearly identifiable. Strain values close to 3% were measured for the interior portion, whereas strains around 1% were noted for the stiffer outside layer. In conclusion, the feasibility of NIVE for small vessel elasticity imaging was demonstrated <i>in vitro</i>.
The objective of the project was to design a vascular phantom compatible with X-ray, ultrasound and MRI. Fiducial markers were implanted at precise known locations in the phantom to facilitate identification and orientation of plane views from the 3D reconstructed images. They also allowed optimizing image fusion and calibration. A vascular conduit connected to tubing at the extremities of the phantom ran through an agar-based gel filling it. A vessel wall in latex was included to avoid diffusion of contrast agents. Using a lost-material casting technique based on a low melting point metal, complex realistic geometries of normal and pathological vessels were modeled. The fiducial markers were detectable in all modalities without distortion. No leak of gadolinium through the vascular wall was observed on MRI for 5h of scan. The potential use of the phantom for calibration, rescaling, and fusion of 3D images obtained from the different modalities as well as its use for the evaluation of intra and inter-modality comparative studies of imaging systems were recently demonstrated by our group (results published in SPIE-2003). Endovascular prostheses were also implanted into the lumen of the phantom to evaluate the extent of metallic imaging artifacts (results submitted elsewhere). In conclusion, the phantom can allow accurate calibration of radiological imaging devices and quantitative comparisons of the geometric accuracy of each radiological imaging method tested.
The aim of this work was to compare the geometric accuracy of X-ray angiography, MRI, X-ray computed tomography (XCT), and ultrasound imaging (B-mode and IVUS) for measuring the lumen diameters of blood vessels. An image fusion method also was developed to improve these measurements. The images were acquired from a realistic phantom mimicking normal vessels of known internal diameters. After acquisition, the multimodal images were coregistered, by manual alignment of fiducial markers and then by automatic maximization of mutual information. The fusion method was performed by means of a fuzzy logic modeling approach followed by a combination process based on possibilistic logic. The data showed (i) the good geometric accuracy of XCT compared to the other methods for all studied diameters; and (ii) the good results of fused images compared to single modalities alone. For XCT, the error varied from 1.1% to 9.7%, depending on the vessel diameter that ranged from 0.93 to 6.24 mm. MRI-IVUS fusion allowed variability of measurements to be reduced up to 78%. To conclude, this work underlined both the usefulness of the vascular phantom as a validation tool and the utility of image fusion in the vascular context. Future work will consist of studying pathological vessel shapes, image artifacts and partial volume effect correction.
Several strategies, known as clutter or wall Doppler filtering, were proposed to remove the strong echoes produced by stationary or slow moving tissue structures from the Doppler blood flow signal. In this study, the matching pursuit (MP) method is proposed to remove clutter components. The MP method decomposes the Doppler signal into wavelet atoms that are selected in a decreasing energy order. Thus, the high-energy clutter components are extracted first. In the present study, the pulsatile Doppler signal s(n) was simulated by a sum of random-phase sinusoids. Two types of high-amplitude clutter signals were then superimposed on s(n): a time-varying low frequency component (type 1), covering systole and early diastole, and short transient clutter signals (type 2), distributed within the whole cardiac cycle. The Doppler signals were modeled with the MP method and the most dominant atoms were subtracted until the signal-to-clutter (S/C) ratio reached a maximum. For the type 1 clutter signal, the improvement in the S/C ratio was 19.0 +/- 0.6 dB, and 72.0 +/- 4.5 atoms were required to reach this performance. For the transient type 2 clutter signal, exactly 10 atoms were required and the maximum improvement in S/C ratio was 5.5 +/- 0.5 dB. These results suggest the possibility of using this signal processing approach to implement clutter rejection filters on ultrasound commercial instruments.