Paul C. Beard’s research while affiliated with UCL Eastman Dental Institute and other places

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Publications (331)


Non-iterative model-based inversion for low channel-count optical ultrasound imaging
  • Article

November 2024

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19 Reads

The Journal of the Acoustical Society of America

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Eleanor C Mackle

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Ultrasound image reconstruction is typically performed using the computationally efficient delay-and-sum algorithm. However, this algorithm is suboptimal for systems of low channel counts, where it causes significant image artefacts. These artefacts can be suppressed through model-based inversion approaches; however, their computational costs typically prohibit real-time implementations. In this work, the emerging optical ultrasound (OpUS) modality is considered, where ultrasound waves are both generated and detected using light. With this modality, imaging probes comprise very low channel counts, resulting in significant image artefacts that limit the imaging dynamic range. However, this low channel counts offer an opportunity for non-iterative (“direct”) model-based inversion (DMI) on modest computational resources available in a typical workstation. When applied to both synthetic and experimental OpUS data, the presented DMI method achieved substantial reduction in image artefacts and noise, improved recovery of image amplitudes, and–after one-off pre-computation of the system matrices–significantly reduced reconstruction time, even in imaging scenarios exhibiting mild spatial inhomogeneity. Whilst here applied to an OpUS imaging system, DMI can be applied to other low channel-count imaging systems, and is therefore expected to achieve better image quality, reduce system complexity, or both, in a wide range of settings.



Schematic of the freehand OpUS imaging system and concurrent imaging layout. (a) Schematic of the freehand OpUS imaging system and the internal components of the OpUS probe. Pulsed excitation light is focused and coupled sequentially into the discrete tips of the 64 fibers in the fiber optic bundle. In the probe head, the fiber bundle is fanned out and butt-coupled to an array of eccentric optical waveguides to shape the output of the fibers to form eccentric OpUS sources. Ultrasound is generated via the photoacoustic effect in the carbon-loaded PDMS membrane. Reflected ultrasound is detected by a single fiber-optic ultrasound detector mounted centrally in the imaging plane. (b) Schematic diagram of the OpUS acquisition setup for concurrent OpUS-CBCT imaging. The OpUS probe is mounted on an acrylic frame positioned on the patient bed, and imaging is conducted inside the bore of the CBCT scanner. (c) Schematic diagram of the OpUS acquisition setup for concurrent OpUS-MRI. The OpUS imaging console is positioned in the control room of the MRI suite, and the fiber bundle is passed through the waveguide pass-through in the electromagnetic shielding of the MRI scan room.
System setup for concurrent OpUS-CBCT and OpUS-MRI studies. (a) The mobile OpUS imaging console alongside the CBCT imaging system. (b) The mobile OpUS imaging system placed in the control room of the MRI. (c) (Inset) The OpUS probe on an acrylic frame. Annotations: (i) the mobile OpUS imaging console; (ii) the OpUS probe mounted on a CT- and MRI-safe acrylic frame placed on the patient bed; (iii) the CBCT imaging system; and (iv) the MRI imaging system.
Cone-beam CT images of a freehand OpUS probe and a conventional piezoelectric ultrasound probe. (a) and (b) Orthogonal CBCT images of the freehand OpUS probe positioned with the probe face submerged in a water bath. (c) and (d) Orthogonal CBCT images of a conventional piezoelectric ultrasound probe with the probe face submerged in a water bath. All images are presented with both ultrasound imaging devices inactive. ROI regions used for the extraction of water voxel values are indicated. Panels (a) and (c) adapted from Watt et al.2023 IEEE International Ultrasonics Symposium (IUS) IEEE with permission from IEEE.
MRI of a freehand OpUS probe submerged in water bath. (a) MRI image of a freehand OpUS probe submerged in water without imaging the target. The probe was fully submerged, and water was allowed to fill the internal cavities of the probe. (b) and (c) Axial MRI images of the OpUS imaging probe submerged in water at planes through the optical fiber bundle [(b) dotted line] and the optical waveguide array [(c) dashed line].
Concurrent OpUS-CBCT and OpUS-MRI of a tissue-mimicking vessel phantom. (a) Schematic of concurrent OpUS-CBCT imaging of tissue-mimicking wall-less vessel phantom. (b) CBCT image of the vessel phantom, with the position of the phantom and vessel indicated. (c) and (d) OpUS images of the tissue mimicking vessel phantom; (c) in-bore without concurrent CBCT; (d) in-bore with concurrent CBCT. (e) Schematic of concurrent OpUS-MRI imaging of tissue mimicking wall-less vessel phantom. (f) MRI image of the vessel phantom, with the position of the phantom and vessel indicated. (g) and (h) OpUS images of the tissue mimicking vessel phantom; (g) in-bore without concurrent MRI; (h) in-bore with concurrent MRI. Labels: (i) the tissue-mimicking phantom; (ii) the water filled vessel in the phantom; and (iii) the bottom of the tissue-mimicking phantom. All OpUS images are presented on the same logarithmic scale.

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Multimodal optical ultrasound imaging: Real-time imaging under concurrent CT or MRI
  • Article
  • Full-text available

September 2024

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87 Reads

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1 Citation

Optical ultrasound (OpUS) imaging is an ultrasound modality that utilizes fiber-optic ultrasound sources and detectors to perform pulse-echo ultrasound imaging. These probes can be constructed entirely from glass optical fibers and plastic components, and as such, these devices have been predicted to be compatible with computed tomography (CT) and magnetic resonance imaging (MRI), modalities that use intense electromagnetic fields for imaging. However, to date, this compatibility has not been demonstrated. In this work, a free-hand OpUS imaging system was developed specifically to investigate the compatibility of OpUS systems with CT and MRI imaging systems. The OpUS imaging platform discussed in this work was used to perform real-time OpUS imaging under (separately) concurrent CT and MRI. CT and MRI imaging of the OpUS probe was used to determine if the probe itself would induce artifacts in the CT and MRI imaging, and ultrasound resolution targets and background measurements were used to assess any impact of CT and MRI on the OpUS signal fidelity. These measurements demonstrate that there was negligible interaction between the OpUS system and both the CT and MRI systems, and to further demonstrate this capability, concurrent OpUS-CT and OpUS-MRI imaging was conducted of a tissue-mimicking phantom and a dynamic motion phantom. This work presents a comprehensive demonstration of an OpUS imaging system operating alongside CT and MRI, which opens up new applications of ultrasound imaging in electromagnetically challenging settings.

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Towards clinical application of freehand optical ultrasound imaging

August 2024

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62 Reads

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2 Citations

Freehand optical ultrasound (OpUS) imaging is an emerging ultrasound imaging paradigm that uses an array of fibre-optic, photoacoustic ultrasound sources and a single fibre-optic ultrasound detector to perform ultrasound imaging without the need for electrical components in the probe head. Previous freehand OpUS devices have demonstrated capability for real-time, video-rate imaging of clinically relevant targets, but have been hampered by poor ultrasound penetration, significant imaging artefacts and low frame rates, and their designs limited their clinical applicability. In this work we present a novel freehand OpUS imaging platform, including a fully mobile and compact acquisition console and an improved probe design. The novel freehand OpUS probe presented utilises optical waveguides to shape the generated ultrasound fields for improved ultrasound penetration depths, an extended fibre-optic bundle to improve system versatility and an overall ruggedised design with protective elements to improve probe handling and protect the internal optical components. This probe is demonstrated with phantoms and the first multi-participant in vivo imaging study conducted with freehand OpUS imaging probes, this represents several significant steps towards the clinical translation of freehand OpUS imaging.



Fig. 1 Schematic of the dual-modality PA endoscope. EL, excitation laser; SIL, sensor interrogation laser; WL, constant intensity white light source; SM, 2D MEMS scanning mirror; C, CMOS camera. (a) Cross-section of the endoscope tube, which shows white light illumination fibers. (b) Miniature 2D MEMS mirror used to optically scan the FP sensor with an interrogation laser beam. (c) Schematic showing the cross-section of the FP sensor, which comprises a polymer spacer sandwiched between two dichroic mirror coatings.
Fig. 2 Phantom images: (a) x -z cross-section extracted from the reconstructed 3D PAT image of a multi-layer ribbon phantom that shows a cross-section over the probe field of view. (b) A contour plot showing the variation in the lateral spatial resolution, obtained by evaluating the edge spread function from each ribbon feature. Dual-modality images of arbitrarily shaped phantoms. (c), (d) White light images of a synthetic hair knot and leaf skeleton phantom. (e)-(h) Horizontal and vertical projections from the 3D PAT images of the phantoms.
Fig. 3 Dual-modality images of ex vivo mouse abdominal organs in situ. (a)-(c) The top panel shows white light endoscopic images of the spleen, liver, and kidney. (d)-(i) Horizontal and vertical projections from the 3D PAT images showing the underlying vascular anatomy of respective organs. Excitation wavelength: 760 nm. (Video 1, MP4, 704 KB [URL: https://doi.org/10.1117/1 .JBO.29.2.020502.s1].)
Dual-modality rigid endoscope for photoacoustic imaging and white light videoscopy

February 2024

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80 Reads

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2 Citations

Journal of Biomedical Optics

Significance There has been significant interest in the development of miniature photoacoustic imaging probes for a variety of clinical uses, including the in situ assessment of tumors and minimally invasive surgical guidance. Most of the previously implemented probes are either side viewing or operate in the optical-resolution microscopy mode in which the imaging depth is limited to ∼1 mm. We describe a forward-viewing photoacoustic probe that operates in tomography mode and simultaneously provides white light video images. Aim We aim to develop a dual-modality endoscope capable of performing high-resolution PAT imaging and traditional white light videoscopy simultaneously in the forward-viewing configuration. Approach We used a Fabry–Pérot ultrasound sensor that operates in the 1500 to 1600 nm wavelength range and is transparent in the visible and near infrared region (580 to 1250 nm). The FP sensor was optically scanned using a miniature MEMs mirror located at the proximal end of the endoscope, resulting in a system that is sufficiently compact (10 mm outer diameter) and lightweight for practical endoscopic use. Results The imaging performance of the endoscope is evaluated, and dual-mode imaging capability is demonstrated using phantoms and abdominal organs of an ex vivo mouse including spleen, liver, and kidney. Conclusions The proposed endoscope design offers several advantages including the high acoustic sensitivity and wide detection bandwidth of the FP sensor, dual-mode imaging capability, compact footprint, and an all-optical distal end for improved safety. The dual-mode imaging capability also offers the advantage of correlating the tissue surface morphology with the underlying vascular anatomy. Potential applications include the guidance of laparoscopic surgery and other interventional procedures.


(a) Illustration of a PCMR illuminated by a focused Gaussian laser beam, and (b) cavity transfer function (CTF); reflected optical intensity Ir of the PCMR as a function of the laser frequency ν showing a single cavity resonance. At the laser frequency ν0, frequency fluctuations dν are converted to an optical intensity modulation dIr.
Experimental setup used to characterize the frequency noise of laser sources using a PCMR array.
Power spectral density of the frequency noise measured for four different laser sources using the PCMR array frequency discriminator (solid lines) and the commercial frequency noise analyzer (dotted lines).
Laser frequency noise characterization using high-finesse plano–concave optical microresonators

January 2024

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53 Reads

Characterizing laser frequency noise is essential for applications including optical sensing and coherent optical communications. Accurate measurement of ultra-narrow linewidth lasers over a wide frequency range using existing methods is still challenging. Here we present a method for characterizing the frequency noise of lasers using a high-finesse plano–concave optical microresonator (PCMR) acting as a frequency discriminator. To enable noise measurements at a wide range of laser frequencies, an array of PCMRs was produced with slight variations of thickness resulting in a series of discriminators operating at a series of periodical frequencies. This method enables measuring the frequency noise over a wide linewidth range (15 Hz to <100 MHz) over the 1440–1630 nm wavelength range. To assess the performance of the method, four different lasers were characterized, and the results were compared to the estimations of a commercial frequency noise analyzer.


Fig. 1. Schematic illustrating the operation of the multi-view FP sensor-based photoacoustic scanner. PA signals are generated by illuminating the target with a ~2.45cm diameter beam from four sides. The PA signals are detected using a stationary planar FP sensor. Multiview imaging is achieved by rotating the target and acquiring PA signals in 8 views; equivalent to placing 8 FP sensors around the target.
Fig. 2. Multi-view sensor registration: The location of the axis is determined by imaging a registration phantom comprising a single optically absorbing hair strand. The phantom was rotated in 90-degree steps, to acquire PA images in four views. The lines lie on a one-sheeted hyperboloid, the axis of which is the rotation axis. To find this, the unit direction-vectors of the lines were fitted to a circle; the normal vector to the circle gives the direction of the axis (b). Finding any point on the axis will uniquely locate the axis, such as the centre of a circle fitted to the intersection of the four lines with a plane perpendicular to the axis. Equivalent FP sensors are synthesized by rotating the coordinates of the FP sensor around the axis; in the opposite direction to the phantom's rotation. The sensors are assigned time series acquired from the rotated target, in order to reconstruct an image.
Fig. 5. Multi-view PA imaging of an ex-vivo mouse abdomen acquired at an excitation wavelength of 800nm. The images are shown as 2D x-y MIPs of 3D volumes. The reconstructed volume is subdivided into volumes containing the anterior and posterior region of the anatomy. (a) Singleview MIP of the posterior region. Visible anatomical features include ribs (r), right kidney (rk), left kidney (lk), spine (sp). (b) Multi-view MIP of the posterior region. In addition to the features visible in the single-view, the liver (l), spleen (s) and part of the gastrointestinal tract are visible in the multi-view. (c) Single-view MIP of the anterior region. There are no discernible anatomical features despite the detectable contrast, because PA signals are recorded from only one side of the mouse. (d) Multi-view MIP of the anterior region. In contrast to the single-view, anatomic features are accurately reconstructed, including the liver (l) and an extensive network of the gastrointestinal tract (gi). By detecting acoustic waves on all sides of the mouse, the multi-view scanner increases the FOV over which anatomical features can be reconstructed.
Fig. 6. Depth colour coded MIPs of multi-view PA image dataset of a mouse abdomen shown on Fig. 5. The reconstructed volume is subdivided into volumes containing the anterior and posterior region, or left and right lateral of the anatomy. Visible anatomical features include ribs (r), right kidney (rk), left kidney (lk), spine (sp). liver (l), spleen (s) and the gastrointestinal tract (gi). The depth of the features can be referenced to the colour scale on the right of the image, with red being the most superficial and green to turquoise showing deeper lying anatomical features
Fig. 7. Multiview PA imaging: Cross sectional 2D z-y MIP of 3D volumes containing thoracic (a) and abdominal (b) region of a mouse. The thicknesses of the volumes are 3mm and 5mm respectively. The images were acquired at an excitation wavelength of 788nm. Visible anatomical features include Heart (h), ribs (r), spine (sp), liver (l), spleen (s), left kidney (lk), right kidney (rk). The two volumes were taken from a larger 3D dataset with a volume thickness of 14mm. A flythrough movie of the complete dataset, as well as a volume-rendered 3D movie can be viewed online (Supplementary movie 1 and 2).
Three-dimensional Whole-body Small animal Photoacoustic Tomography using a Multi-view Fabry-Perot scanner

January 2024

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48 Reads

IEEE Transactions on Medical Imaging

Photoacoustic tomography (PAT) has the potential to become a widely used imaging tool in preclinical studies of small animals. This is because it can provide non-invasive, label free images of whole-body mouse anatomy, in a manner which is challenging for more established imaging modalities. However, existing PAT scanners are limited because they either do not implement a full 3-D tomographic reconstruction using all the recorded photoacoustic (PA) data and/or do not record the available 3-D PA time-series data around the mouse with sufficiently high spatial resolution ( ~100μm ), which compromises image quality in terms of resolution, imaging depth and the introduction of artefacts. In this study, we address these limitations by demonstrating an all-optical, multi-view Fabry-Perot based scanner for whole body small animal imaging. The scanner densely samples the acoustic field with a large number of detection points ( >100,000 ), evenly distributed around the mouse. The locations of the detection points were registered onto a common coordinate system, before a tomographic reconstruction using all the recorded PA time series was implemented. This enabled the acquisition of high resolution, whole-body PAT images of ex-vivo mice, with anatomical features visible across the entire cross section.


FIG. 3. (a) Images of the beam profiles incident on the FPUS without SLM modulation and after a focus has been formed by WS. (b) ITFs recorded when the FPUS was illuminated by the unmodulated and focused field shown in (a) and in a traditional free-space scanner. The bias wavelength k b is marked. The free space beam was a Gaussian beam with a $50 lm beam waist.
Spatially resolved readout of a Fabry–Perot ultrasound sensor interrogated through a multimode optical fiber using wavefront shaping

November 2023

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69 Reads

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1 Citation

The spatially resolved interrogation of a Fabry–Perot ultrasound sensor using a laser beam focused through a multimode fiber is demonstrated. To scan the beam across the sensor as required to read it out, optical wavefront shaping was employed to compensate for the scrambling of light in the fiber. By providing a means to map ultrasound through inexpensive, lightweight fibers, this could lead to new ultrasonic and photoacoustic imaging systems, such as endoscopes and flexible handheld probes.



Citations (57)


... The generated ultrasound had an average peak pressure of 0.14 MPa, a center frequency of 10.8 MHz, and a bandwidth of 18.6 MHz. 34 The Fabry-Pérot detector in the probe was interrogated by a tunable continuous-wave light source tuned to the wavelength for maximum sensitivity for the cavity 25 Each fiber in the bundle was illuminated sequentially and a scan line was recorded, building up a B-scan array, which was bandpass filtered (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12) to reduce noise in the deeper portions of the field of view. The slight protrusion of the detector beyond the waveguide array face resulted in a significant signal level generated by direct transmission from the sources to the detector, and a time-windowed cutoff was applied to the B-scan to remove this crosstalk. ...

Reference:

Multimodal optical ultrasound imaging: Real-time imaging under concurrent CT or MRI
Towards clinical application of freehand optical ultrasound imaging

... These optical probes find widespread applications in the medical field, thanks to their biocompatibility, compact size, lack of electrical connection, and great sensitivity. Specifically, in ultrasound imaging, there is increasing interest in the realization of minimally invasive devices [4,5]. The sensitive detection of broadband ultrasound waves ranging from hundreds of kHz to tens of MHz is essential for techniques like biomedical photoacoustic and clinical ultrasound high-resolution imaging [6]. ...

Dual-modality rigid endoscope for photoacoustic imaging and white light videoscopy

Journal of Biomedical Optics

... The completed imaging probe, with the full 11 meter optical fibre bundle and protective clamshell, and strain relief pieces, enabled the system to be used in a more versatile freehand manner than previous probes without the risk of fibre damage, enabling the in vivo imaging presented here to be conducted rapidly and repeatedly. The probe also provided the first means of demonstrating OpUS imaging outside of the laboratory alongside other imaging modalities such as cone-beam computed tomography (CB-CT) 29 , in which the probe head was positioned at a significant distance from the imaging console during imaging. ...

Concurrent Optical Ultrasound and CT Imaging
  • Citing Conference Paper
  • September 2023

... These optical probes find widespread applications in the medical field, thanks to their biocompatibility, compact size, lack of electrical connection, and great sensitivity. Specifically, in ultrasound imaging, there is increasing interest in the realization of minimally invasive devices [4,5]. The sensitive detection of broadband ultrasound waves ranging from hundreds of kHz to tens of MHz is essential for techniques like biomedical photoacoustic and clinical ultrasound high-resolution imaging [6]. ...

All-optical multimode fibre photoacoustic endomicroscopy with scalable spatial resolution and field-of-view

... 21 Ultrasound is then transmitted into the imaging volume, and backscattered ultrasound is detected via optical means such as direct optical interferometry [22][23][24] or by monitoring an optically resonant structure. 25,26 These elements have been applied in free-space configurations, [22][23][24]27 miniaturized fiber optic devices for interventional applications, [28][29][30][31] and recently, in real-time freehand imaging systems fabricated on an array of fiber-optic sources and a single fiber-optic detector. 32-34 A free-space OpUS imaging system has previously been used to image radio frequency (RF) ablation with an ablation catheter 35 and did not demonstrate any impact on the OpUS imaging in the presence of strong RF fields. ...

Miniaturised all-optical ultrasound probe for thrombus imaging

... Plano-concave configurations have demonstrated considerable benefits in terms of performance, [16,21,25], mainly from an experimental point of view. However, understanding the impact of a curved surface on the previously introduced optical and acoustic key parameters can be crucial for defining the sensitivity during the design phase, according to the degrees of freedom offered through modern fabrication approaches. ...

All-optical ultrasound catheter for rapid B-mode oesophageal imaging

... Curved structures realized on the tip of optical fiber are already implemented in all-optical ultrasound systems for imaging applications. For example, S. Zhang et al. proposed a fiber-optic system capable of simultaneous laser interstitial thermal therapy (LITT) and real-time in situ all-optical ultrasound imaging for lesion monitoring [19]. Their devices included three optical fibers: one for ultrasound transmission and reception and another for providing thermal therapy light. ...

Miniaturised dual-modality all-optical ultrasound probe for laser interstitial thermal therapy (LITT) monitoring

... Within the realm of photoacoustic tomography, deep learning-based methods primarily encompass post-processing techniques that utilize networks to eliminate artifacts present in images obtained through conventional analytical methods. Researchers are actively exploring various deep learning architectures to enhance image resolution, reduce noise and image artifacts, achieve higher-quality reconstruction imaging, and reconstruct images from limited-view data [16,17]. Currently, detection angles of 70 • [18], 90 • , 120 • [19], and 180 • are commonly used in experimental limited-view settings (Fig. 1). ...

Mitigating the Limited View Problem in Photoacoustic Tomography for a Planar Detection Geometry by Regularized Iterative Reconstruction
  • Citing Article
  • April 2023

IEEE Transactions on Medical Imaging

... In this paper, to overcome the problem of the instable beam propagation in a bulk MPC and address the need to improve the reliability of nonlinear effects, we develop the Gaussian beam propagation model in a bulk MPC, as well as the adjusted optimization method for stable propagations. The model is established based on the resonator principle and the ABCD transfer matrix, which is an established method for studying paraxial beam propagation through optical systems, such as a Fabry-Perot etalons and optical micro-resonators [27][28][29][30]. On this basis, this paper presents a mathematical expression for calculating the eigenvalues of a bulk MPC. ...

ABCD transfer matrix model of Gaussian beam propagation in plano-concave optical microresonators

... 59 Two recent animal studies comparing NIR and SWIR imaging using ICG and IRDye800CW in mice also showed an improvement in contrast to SWIR imaging using the emission tail of these dyes. 45,46,60 The use of SWIR emission tails of NIR dyes, of which some are already FDA-approved, is attractive as it might lead to a swift translation of SWIR imaging to the clinical setting. ...

Shortwave Infrared Imaging Enables High-Contrast Fluorescence-Guided Surgery in Neuroblastoma

Cancer Research