In Vivo photoacoustic imaging solutions

Transcription

1 In Vivo photoacoustic imaging solutions from Endra Life Sciences faster, simpler, and quantitative Endra Life Sciences

2 NEXUS 128 where light meets sound Photoacoustic imaging Short pulses of light absorbed deep within tissue create sound waves that are detected by ultrasound receivers to create an image. This non-invasive approach provides images containing the rich spectral content of optical imaging with the depth and resolution of ultrasound. 3-D Photoacoustic tomography instrument Multi-spectral, quantitative imaging Endogenous contrast: vessels, hypoxia Optional: dye labeled probes Anti-angiogenic therapy in mouse models of cancer Mouse imaged with Nexus 128 Tumor response (Hb content) MCF-7 xenograft Tumor Mouse with implanted MCF-7 tumor (MicroCT shown) Maximum intensity projections of 3D photoacoustic data showing tumor vasculature Whole tumor Hb content (mean and stdev) for anti-angiogenic therapy treated (n=3) and control (n=3) groups

3 Nexus 128 In vivo photoacoustic imaging NEXUS 128 Fast, simple, non-invasive Quantitative vascular imaging of tumors Longitudinal imaging for mouse models of cancer Predefined tumor biology protocols 3D Tumor volume measurement Quantitative Tumor vasculature Hemoglobin Quantitative Tumor hypoxia Sa02

4 36 E N D R A Nexus Dimensions Electrical Requirement Siting Operator Interface Analysis Throughput H x 36 W x 26 D 120 V or 240 V laboratory or inside barrier facility Touch screen LCD imac Workstation 40 animals/day In vivo photoacoustic imaging solutions from Endra Life Sciences Endra Life Sciences info@endrainc.com

5 Nexus 128 Small Animal Photoacoustic Computed Tomography Scanner Application The Nexus 128 is a 3D photoacoustic preclinical imaging system specifically designed for in vivo imaging of mouse models of cancer. The system acquires multi-spectral images of tumor vasculature in order to quantify tumor development and response to therapy. Scanner Components 1) Laser A high power, tunable, pulsed laser provides near infrared illumination. Nd:YAG laser pumped optical parametric oscillator Tuning range: nm Laser pulse repetition rate: 20 Hz 2) Detector The detector is a hemispherical array of individual acoustic receiver elements arranged in a spiral pattern. 128 unfocused ultrasound transducers - 3 mm diameter - 5 MHz center frequency 3) Digital Acquisition System A special purpose, 128 channel digital acquisition system samples all 128 transducers every laser pulse without multiplexing. 4) Detector Drive System A high precision rotation stage and motor control detector rotation through 360 degrees. Continuous rotation acquisition Step-and-shoot rotation acquisition Dynamic, continuous rotation acquisition 5) Animal Handling A specialized animal tray and carrier facilitate repeatable positioning of the mouse in the scanner field of view. Two (2) carriers and two (2) trays are provided. Endra, Inc. 35 Research Drive, Suite 100 Ann Arbor, MI USA

6 6) Preparation Station A preparation station provides a convenient location to prepare one animal while another one is scanned. Additionally, white light images of the animal are acquired with the preparation station for later co-registration with photoacoustic data. 7) Shielding Shielding and interlocks prevent the operator from being exposed to the laser output. Operator Console 1) Standard Acquisition & Reconstruction The standard acquisition & reconstruction interface provides the user with an efficient, streamlined workflow. The user selects one of the pre-configured, application-specific protocols that automatically set acquisition and reconstruction parameters based on the application. 2) Advanced Acquisition & Reconstruction (Research Mode) With Research Mode, the user has access to the scan acquisition parameters and can adjust them based on their unique research requirements. Specifically, the user can select continuous or step-and-shoot detector rotation as well as adjust the wavelength of the laser, number of views acquired, and the number of laser pulses per view (step-andshoot only). Viewing and Analysis Quantitative parametric maps of hemoglobin concentration 3D photoacoustic data overlay on co-registered, 2D white light images Visualization of single slice (2D) or volume (3D) image data Window (contrast) / level (brightness) control of display Zoom, pan, rotate Maximum intensity projections Volume rendering Surface rendering Region of interests - Circles - Rectangles - Polygons Line measurement Image statistics Endra, Inc. 35 Research Drive, Suite 100 Ann Arbor, MI USA

7 System Performance Scan time: 12 seconds per wavelength for continuous rotation acquisitions Field of view: > 20 mm Spatial resolution: < 280 m Physical Specifications Scanner size (W x D x H): 0.9 x 0.7 x 1.1 m Electrical requirements: 110 VAC or 220 VAC Computers: - A multi-core processor Mac is used for viewing and analysis - A multi-core processor PC is used for data acquisition and reconstruction Endra, Inc. 35 Research Drive, Suite 100 Ann Arbor, MI USA

8 A practical photoacoustic CT scanner for preclinical imaging Endra, Inc. 35 Research Drive, Suite 100, Ann Arbor, MI and 222 Berkeley Street, Suite 1040, Boston, MA phone: (617) ABSTRACT We have designed, built, and tested a volume PCT (photoacoustic computed tomography) scanner for preclinical small animal imaging applications. The scanner was designed to optimize both image quality and workflow, minimize animal handling, and maximize throughput and usability. The scanner is capable of acquiring full volume data sets with a scan time of 12 seconds per wavelength. The scanner incorporates a novel detector consisting of a sparse array of acoustic receivers, an OPO near infrared (NIR) laser, and a special purpose 128 channel, low noise, high speed, digital acquisition system (DAS). Evaluation of the PCT scanner shows a limiting spatial resolution of 280 m and a low contrast sensitivity sufficient to detect 350 nm concentration of a NIR-absorbing organic dye embedded in 12.5 mm of soft tissue. The PCT scanner has demonstrated high contrast anatomical mouse imaging of abdominal organs, imaging of hemoglobin concentration and blood oxygen saturation, and targeted molecular imaging with the use of dye labeled probes. The system is used for quantitative tumor imaging in oncology applications. Keywords: photoacoustic tomography, preclinical imaging, oncology, oxygen saturation, hemoglobin, contrast agent, optically absorbing dye, labeled probe Photoacoustic computed tomography (PCT) INTRODUCTION Over the past decade there has been growing interest in small animal imaging in medical research. In 1999 over 90% of research into mammalian models of human disease involved small animals, particularly mice. 1 Nuclear medicine techniques have played a preeminent role in small animal imaging, but increasingly optical imaging techniques are being used to track the distribution of molecular probes engineered to accumulate in targeted tissues. 2 In this regard, fluorescence techniques are most commonly used. 3,4 Multi-spectral, optical imaging techniques, e.g., diffusion optical tomography, have also been proposed for characterizing endogenous tissues on the basis of their spectral absorption properties. 5,6 However the spatial resolution that can be achieved is limited by light scatter. The foregoing optical imaging methodologies share one commonality: optical scatter degrades their spatial resolution significantly as the penetration depth increases. The scatter-induced limitations to spatial resolution 1

9 that plague all purely optical imaging approaches can be mitigated in large part by employing thermo- or photoacoustic signal creation and analytic image reconstruction. Bowen et al. first recognized the potential of inducing thermoacoustic signals in soft tissue in ,8 Kruger et al. reported methodology for forming photoacoustic computed tomography (PCT) images of tissue-mimicking phantoms using a Nd:YAG laser in In that work Kruger demonstrated that 2-D slice images of optically absorbing objects embedded in turbid media could be formed using a Nd:YAG laser and a single focused transducer that was scanned in a circular arc. In 1999 Kruger et al. reported the extension of the methodology to three dimensions, and introduced an efficient, filteredbackprojection algorithm for reconstructing fully three-dimensional images, based on an approximate solution to the Radon transform. 10 In 2003, fully 3D PCT imaging in a mouse was demonstrated. 11 Figure 1: The basic elements of photoacoustic imaging are illustrated. In its simplest form, a short pulse of electromagnetic radiation induces a milli-kelvin rise in temperature within the absorber resulting in a thermal expansion. The rapid heating and cooling results in an acoustic wave that is emitted from the absorber that may be detected at the tissue surface by acoustic receivers. The depth of the absorber may be determined by the time of arrival of the thermo-acoustic signal at the tissue surface. The aim of this work is to use photoacoustic spectroscopy to quantify hemoglobin concentration, oxygen saturation, vascular morphology, and the distribution of molecular probes tagged with organic dyes in small animal models of vascular disease and cancer in mice. PCT SCANNER DESIGN CONSIDERATIONS The design of a new PCT scanner is illustrated in Figure 2. The device uses a 10 cm radius hemispherical detector array with 128 individual piezoelectric transducers arranged in a spiral pattern. Each element of the array captures a radial projection. The animal to be imaged is placed in a tray above the detector array and illuminated from below through a clear aperture at the bottom of the array. Rotating the water-filled array to multiple angular positions allows dense sampling while maintaining uniform angular coverage. We chose the center frequency of the detectors to be 5 MHz with their bandwidth extending from MHz. We chose a 3-mm diameter transducer, which provides a 20-mm-wide field of view, sufficiently wide to cover the width of a typical mouse abdomen. A photograph of the assembled detector array is shown in Figure 2. 2

10 The detector array is integrated into a structural frame housing the necessary system electronics, electromechanical drive system, and the light delivery components. A computer console, located adjacent to the scanner, provides a graphical user console with software for: acquistion control, image reconstruction, viewing, and analysis. (A) Figure 2: (A) The detector array is connected to a computer controlled rotating stage. During an acquisition sequence, the detector array rotates while the animal, or tissue specimen, remains stationary within the tray, providing multiple views of the object being imaged. (B) Photograph of the sparse hemispherical detector array. The scanner incorporates a table-top with a working height of 36. A disposable tray (Figure 3) is thermo-formed from a thin sheet of engineered plastic in a shape that closely approximates the tissue being imaged (e.g. a 20 g mouse). The tray has high optical transmittance and acoustic impedance similar to tissue. The tray is used to position the tissue of interest in the effective field of view of the detector array. The animal (or tissue specimen) is positioned in the tray with a small amount of acoustic coupling media. The tray is then placed onto the table-top of the scanner for the scanning procedure. The tray and animal remain stationary at all times during the scanning procedure. The table-top provides a working surface that may accommodate animal monitoring equipment, gas anesthesia lines, and an infusion pump. The temperature of the fluid filling the detector array is regulated by an external thermo-electric control unit and optimized to comfort of the animal during the scanning procedure. (B) Figure 3: The animal is positioned in the tray for the scanning procedure. The tray is attached to a carrier system with handles to facilitate removal and insertion of the tray into the scanner. The mouse lies prone or supine and does not need to be submerged for the imaging procedure. 3

11 PCT SCANNER PERFORMANCE EVALUATION Spatial resolution We imaged a point absorber fabricated by printing a small dot on transparent film. The actual size of the dot was approximately 200 m. The dot was imaged with the PCT scanner using a tunable OPO laser operated at 800 nm. A cross-sectional image through the PCT image of the dot is shown in Figure 4. Figure 4: PCT image of a 200 m dot (left) and its associated profile plot (right). The full width at half maximum of the profile is 280m. Field of view Figure 5: We fabricated a bilinear array of 400 micron dots printed on transparent film on 2.5- mm centers, and placed it in the PCT scanner without any scattering material present in the optical beam path. The number of dots visible indicated the field of view is 20mm. Low contrast sensitivity A 1.1-mm diameter tube was embedded on the central axis of a 25-mm diameter tube filled with 4% agar + 6% Liposyn II (20% concentration) and imaged with the PCT scanner (64 angles x 8 integrations per angle = 512 laser pulses). The light output from the laser was approximately 6 mj /pulse. The India ink concentration was varied from.005% to 0.8% by factors of two. Three-dimensional images were reconstructed with 0.2 mm voxels. For reference, the absorption of the.005% India ink was equivalent to 350 nm ICG (indocyanine green) at 800 nm. 4

12 Figure 6: The PCT contrast was plotted as a function of India ink concentration. PCT IMAGES AND APPLICATIONS An acquistion sequence consists of laser pulses synchronized with the recording of the resulting thermo-acoustic signals from the tissue while the detector array is rotated one complete revolution. The rotation may be continuous or step and shoot. In both cases, the data are acquired at equally spaced intervals over 360 o. For stepand-shoot acqusition, the rotation is paused while laser pulses are emitted and the thermo-acosutic signals averaged. PCT images are acquired in approximately 12 seconds for a single wavelength. Spectroscopic imaging involves repeating the PCT acquisition sequence at multiple wavelengths (typically 3-5 wavelengths). PCT images acquired with a single wavelength produce high contrast anatomical volume data with only endogenous contrast. Figure 7: Photoacoustic data acquired at 800 nm and reconstructed with 0.1 mm voxels are displayed with maximum intensity projection for a 1.9 mm coronal slab of volume data. Abdominal organs are easily delineated along with larger bone structures. 5

13 PCT data acquired at two wavelengths may be used to calculate quantitative maps of hemoglobin concentration ([Hb]). With the addition of another wavelength, the oxygen saturation state of the hemoglobin within a tissue may be mapped. The quantitative methods based on spectroscopy may be extended to map the concentration of an exogenous contrast agent or molecular probe that has been labeled with an optically absorbing dye by acquiring a fourth wavelength. Figure 8: A single reconstructed slice through a mouse xenograft tumor model (left) with a molecular label for angiogenesis has been imaged in vivo. Multiple wavelengths were acquired and used to calculate maps of [Hb] (C Hbt ), blood oxygen saturation (SaO 2 ), and the concentration of a dye labeled angiogenesis probe (NPR-1). Images courtesy of Dr. K. Stantz. Alternatively, images may be acquired at two wavelengths closely spaced at the peak absorption of an exogenous absorbing dye in order to subtract endogenous contrast. The resulting PCT images may be subtracted to reveal the location of the contrast without the underlying animal tissue contrast. Figure 9: Images of an intact mouse thorax injected with 5 ml of 3 M ICG in the back of mouse were acquired at two wavelengths (805 and 835 nm) and then subtracted. Image data were acquired at 60 rotational positions of the transducer array with the thermo-acoustic signal from 8 pulses of light averaged at each view. (left) A raw reconstructed photoacoustic coronal section acquired at 805 nm. (right) The resulting subtracted image (image at 805nm image at 835nm). Note that the endogenous tissue contrast of the animal is not present in the subtracted image. 6

14 DISCUSSION A volume PCT scanner has been designed, built, and evaluated. The scanner is routinely used in precinical oncology imaging applications. The system produces volume image data with high spatial resolution and high contrast sensitivity, with scan times less than one minute (12 seconds per wavelength). Unlike many of the previous photoacoustic tomographic systems, the PCT scanner does not require the animal to be submerged in water, nor rotated during the scanning procedure. Animal handling is facilitated by a disposable animal support tray. Multiple trays may be used to prepare animals while the scanner is in operation. The combination of minimal animal preparation, optimized work flow and short scan time, provide a throughput capability of 20 animals in 4 hours of scanning. Demonstrated applications for the PCT scanner include quantitative measurements of: hemoglobin, blood oxygen saturation, exogenous molecular probes, and blood flow. Our future objectives include providing this novel PCT imaging technology to collaborators at Stanford and Massachusetts General Hospital in order to assist in the development of novel molecular optically absorbing probes and for applications in preclinical models of cancer. ACKNOWLEDGMENTS Dr. Robert Kruger, OptoSonics Inc., Oriental, NC Dr. Keith Stantz, Photoacoustic imaging laboratory at IUPUI, Indianapolis, IN and Purdue University, West Lafayette, IN REFERENCES 1 Mahmood U and Weissleder R. Some tools for molecular imaging. Acad Radiol 2002; 9: Blasberg RG and Tjuvajev JG. Molecular-genetic imaging: a nuclear medicine-based perspective. J. Molecular Imaging 2002; 1: Achilefu S, Dorshow RB, Bugaj JE, and Rajogopalan R. Novel receptor-targeted fluorescent contrast agents for in vivo tumor imaging. Invest. Radiol. 2000; 35: Licha K, Riefke B, Ebert B, and Grötzinger C. Cyanine dyes as contrast agents in biomedical optical imaging. Acad Radiol 2002; 9:S320-S Cerussi AE, Jakubowski D, Yah N, et al. Spectroscopy enhances the information content of optical mammography. J. Biomed Optics 2002; 7: McBride TO, Pogue BW, Poplack S, et al. Multispectral near-infrared tomography: a case study in compensating for water and lipid content in hemoglobin imaging of the breast. J. Biomed Optics 2002; 7: Bowen T. Radiation-induced thermoacoustic soft tissue imaging. Proc. IEEE Ultrasonics Symposium 1981; 2: Bowen T, Nasoni L, Pifer AE and Sembrosk GH. Some experimental results on the thermoacoustic imaging of soft tissue-equivalent phantoms. Proc. IEEE Ultrasonics Symposium 1981; 2: Kruger RA, Liu P-Y and Fang Y. Thermoacoustic Ultrasound (PAUS) - reconstruction tomography. Medical Physics 1995; 22: Kruger RA, Kiser Jr. WL, Reinecke DR, Kruger GA. Thermoacoustic computed tomography technical considerations. Medical Physics 1999; 26: Kruger RA, Kiser Jr WL, Reinecke DR, Kruger GA, Miller KD. Thermoacoustic optical molecular imaging of small animals. Molecular Imaging 2003; 2(2):