Navalgund Rao

Investigation of a pulse compression technique for medical ultrasound: A Simulation study

Pulse compression techniques that are capable of producing a large signal-to-noise (SNR) enhancement, have been used successfully in many different fields. For medical applications, frequency-dependent attenuation in soft tissue can limit the usefulness of this method. In the paper, this issue is examined through model-simulation studies. Frequency-modulation (FM) chirp, considered in the study, is just one form of pulse coding technique. Pulse propagation effects in soft tissue are modelled as a linear zero phase filter. A method to perform simulations and estimate the effective time-bandwidth product K is outlined. K describes the SNR enhancement attainable under limitations imposed by the soft-tissue medium. An effective time-bandwidth product is evaluated as a function of soft-tissue linear attenuation coefficient αo, scatterer depth z and the bandwidth of the interrogating FM pulse, under realistic conditions. Results indicate that, under certain conditions, K can be significantly lower than its expected value in a non-attenuating medium. It is argued that although limitations exist, pulse compression techniques can still be used to improve resolution or increase penetrational depth. The real advantage over conventional short-pulse imaging comes from the possibility that these improvements can be accomplished without increasing the peak intensity of the interrogating pulse above any threshold levels set by possible bio-effect considerations.

Multispectral Photoacoustic Imaging of Prostate Cancer: Preliminary Ex-vivo Results.

Objective: The objective of this study is to validate if ex-vivo multispectral photoacoustic (PA) imaging can differentiate between malignant prostate tissue, benign prostatic hyperplasia (BPH), and normal human prostate tissue. Materials and Methods: Institutional Review Boards approval was obtained for this study. A total of 30 patients undergoing prostatectomy for biopsy-confirmed prostate cancer were included in this study with informed consent. Multispectral PA imaging was performed on surgically excised prostate tissue and chromophore images that represent optical absorption of deoxyhemoglobin (dHb), oxyhemoglobin (HbO2), lipid, and water were reconstructed. After the imaging procedure is completed, malignant prostate, BPH and normal prostate regions were marked by the genitourinary pathologist on histopathology slides and digital images of marked histopathology slides were obtained. The histopathology images were co-registered with chromophore images. Region of interest (ROI) corresponding to malignant prostate, BPH and normal prostate were defined on the chromophore images. Pixel values within each ROI were then averaged to determine mean intensities of dHb, HbO2, lipid, and water. Results: Our preliminary results show that there is statistically significant difference in mean intensity of dHb (P < 0.0001) and lipid (P = 0.0251) between malignant prostate and normal prostate tissue. There was difference in mean intensity of dHb (P < 0.0001) between malignant prostate and BPH. Sensitivity, specificity, positive predictive value, and negative predictive value of our imaging system were found to be 81.3%, 96.2%, 92.9% and 89.3% respectively. Conclusion: Our preliminary results of ex-vivo human prostate study suggest that multispectral PA imaging can differentiate between malignant prostate, BPH and normal prostate tissue.

Preliminary Results of Ex Vivo Multispectral Photoacoustic Imaging in the Management of Thyroid Cancer

OBJECTIVE The purpose of this study was to validate whether ex vivo multispectral photoacoustic imaging can be used to differentiate malignant tissue, benign nodules, and normal human thyroid tissue. SUBJECTS AND METHODS Fifty patients undergoing thyroidectomy because of thyroid lesions participated in this study. Multispectral photoacoustic imaging was performed on surgically excised thyroid tissue, and chromophore images that represented optical absorption of deoxyhemoglobin, oxyhemoglobin, lipid, and water were reconstructed. After the imaging procedure, the pathologist marked malignant tissue, benign nodules, and normal regions on histopathologic slides, and digital images of the marked histopathologic slides were obtained. The histopathologic images were coregistered with chromophore images. Areas corresponding to malignant tissue, benign nodules, and normal tissue were defined on the chromophore images. Pixel values within each area were averaged to determine the mean intensities of deoxyhemoglobin, oxyhemoglobin, lipid, and water. RESULTS There was a statistically significant difference between malignant and benign nodules with respect to mean intensity of deoxyhemoglobin (p = 0.014). There was a difference between malignant and normal tissue in mean intensity of deoxyhemoglobin (p = 0.003), lipid (p = 0.001), and water (p < 0.0001). A difference between benign nodules and normal tissue was found in mean intensity of oxyhemoglobin (p < 0.0001), lipid (p < 0.0001), and water (p < 0.0001). The sensitivity, specificity, and positive and negative predictive values of the system tested in differentiating malignant from nonmalignant thyroid tissue were 69.2%, 96.9%, 81.8%, and 93.9%. CONCLUSION The preliminary results of this ex vivo human thyroid study suggest that multispectral photoacoustic imaging can be used to differentiate malignant and benign nodules and normal human thyroid tissue.

Combining pulse compression and adaptive drive signal design to inverse filter the transducer system response and improve resolution in medical ultrasound

An adaptive inverse filtering technique has been incorporated into a medical ultrasound B-scan scheme that used a linear frequency modulated pulse for imaging. Resolution improvement is demonstrated with imaging experiments on wire targets and tissue-mimicking phantoms.

Experimental Point Spread Function of FM Pulse Imaging Scheme

In this paper, we have examined the possibility of incorporating pulse compression techniques into a conventional medical B-scan imaging scheme. Linear frequency modulation fm, one form of pulse coding among many others, has been used in this study. With this approach, one can overcome current peak intensity limitations. A theoretical framework that includes medium propagation effects, transducer bandwidth and diffraction effects is presented, which could be used to examine the system point spread function under this imaging scheme. A prototype experimental set-up and signal processing are described and used for simple imaging tasks in attenuating and nonattenuating media. Analysis of the experimental point spread functions shows that resolution similar to conventional short pulse imaging can be achieved. However, the existence of large range side lobe levels usually associated with pulse compression processing can degrade contrast resolution in medical ultrasound. We have considered various different factors that can affect the range side lobe levels and examined their effect either experimentally or through simulations. The technique has the potential for improving signal-to-noise ratio (SNR), maximum penetration depth and resolution without exceeding peak intensity limitations. Some possible applications are discussed that merit further evaluation. Our work demonstrates the feasibility of this technique and presents a theoretical framework that can be used in future studies aimed at evaluating image quality, system performance, and possible artifacts under such an imaging scheme.

Independent Component Analysis Applied to Ultrasound Speckle Texture Analysis and Tissue Characterization

Analysis of ultrasound speckle texture will provide us information about the underlying properties of tissue, could find applications in early lesion detection and tissue characterization. Traditional first and second order statistics based approaches ignore the higher order statistics information in the texture. On the other hand, conventional multichannel filtering or multiresolution analysis approaches rely on the predefined analytical bases which are not fully adaptive to the data being analyzed. In this paper independent component analysis (ICA), which is based on higher order statistics, is proposed to deal with the ultrasound speckle texture analysis problem. ICA image bases obtained from the training images are applied as a filter bank to the testing images. Then the independent features containing higher order statistics information can be extracted from the marginal distributions of the filtered images. ICA is used here as a dimensionality reduction tool to overcome the difficulty of estimating high dimensional joint density of texture. Support Vector Machine (SVM) is then used as a classifier to classify the tissues. By using the digitally simulated tissues and corresponding B-scan images, we can further correlate the change of tissue microstructure or change of imaging conditions with the change of the ICA feature vectors. Our numerical simulation has shown ICA to be a promising technique for ultrasound speckle texture analysis and tissue characterization compared with some traditional methods such as PCA and Gabor transform.

Photoacoustic Imaging: Opening New Frontiers in Medical Imaging

In todays world, technology is advancing at an exponential rate and medical imaging is no exception. During the last hundred years, the field of medical imaging has seen a tremendous technological growth with the invention of imaging modalities including but not limited to X-ray, ultrasound, computed tomography, magnetic resonance imaging, positron emission tomography, and single-photon emission computed tomography. These tools have led to better diagnosis and improved patient care. However, each of these modalities has its advantages as well as disadvantages and none of them can reveal all the information a physician would like to have. In the last decade, a new diagnostic technology called photoacoustic imaging has evolved which is moving rapidly from the research phase to the clinical trial phase. This article outlines the basics of photoacoustic imaging and describes our hands-on experience in developing a comprehensive photoacoustic imaging system to detect tissue abnormalities.

Time- and frequency-domain descriptions of spatially averaged one-way diffraction for an unfocused piston transducer

Time-domain and frequency-domain expressions describing spatially averaged effects of one-way diffraction in the case of an unfocused piston transmitter and finite receiver are of theoretical and practical interest. Here, a time-domain description based on the arccos diffraction formulation and a frequency-domain description based on the Lommel diffraction formulation are derived. Numerical results obtained from the two descriptions are then compared. It is shown that the two descriptions show satisfactory agreement for finite receivers of practical interest. Two mathematical lemmas are also provided.

Pre-enhancement of chirp signal for inverse filtering in medical ultrasound

In medical ultrasound imaging, the axial resolution is limited by the bandwidth of the transducer. The frequency response of the transducer usually peaks at its center frequency f/sub 0/ but falls off at higher and lower frequencies. In a linear frequency modulation (FM) pulse coding technique for imaging, the frequency can be swept across the entire bandwidth of the transducer. In this technique it is possible to apply at the input an amplitude boost function [1] that is the inverse of the transducer frequency response. Thus the output from the transducer is an equalized FM pulse, with a wider effective bandwidth. Subsequent auto correlation or pulse compression processing preserves this bandwidth and consequently improves the resolution. This concept has been evaluated experimentally in this paper.<<ETX>>

Development of a c-scan photoacoutsic imaging probe for prostate cancer detection

Prostate cancer is the second leading cause of death in American men after lung cancer. The current screening procedures include Digital Rectal Exam (DRE) and Prostate Specific Antigen (PSA) test, along with Transrectal Ultrasound (TRUS). All suffer from low sensitivity and specificity in detecting prostate cancer in early stages. There is a desperate need for a new imaging modality. We are developing a prototype transrectal photoacoustic imaging probe to detect prostate malignancies in vivo that promises high sensitivity and specificity. To generate photoacoustic (PA) signals, the probe utilizes a high energy 1064 nm laser that delivers light pulses onto the prostate at 10Hz with 10ns duration through a fiber optic cable. The designed system will generate focused C-scan planar images using acoustic lens technology. A 5 MHz custom fabricated ultrasound sensor array located in the image plane acquires the focused PA signals, eliminating the need for any synthetic aperture focusing. The lens and sensor array design was optimized towards this objective. For fast acquisition times, a custom built 16 channel simultaneous backend electronics PCB has been developed. It consists of a low-noise variable gain amplifier and a 16 channel ADC. Due to the unavailability of 2d ultrasound arrays, in the current implementation several B-scan (depth-resolved) data is first acquired by scanning a 1d array, which is then processed to reconstruct either 3d volumetric images or several C-scan planar images. Experimental results on excised tissue using a in-vitro prototype of this technology are presented to demonstrate the system capability in terms of resolution and sensitivity.