Applying ultrasound beamformers to photoacoustic imagingTyler Harrison and Roger J. Zemp

Electrical and Computer Engineering, University of Alberta, Canada

IntroductionPhotoacoustic (PA) imaging can formultrasonically-focused optical contrast images at highframe rates.

Single-element transducers limit imaging speed byrequiring one excitation per A-line.

Ultrasound (US) array transducers can image at up tolaser repetition rate.

Purpose: Use conventional US array systems toform PA images in realtime with minimalhardware modification.

Ultrasound versus photoacoustic imagingUltrasound

Backscattered ultrasound waves used to form images.

Ultrasound propagation is two-way.

Photoacoustic imaging

Optical absorption → heating → thermal expansion →ultrasound wave.

Ultrasound propagation is one-way only.

Figure 1: Fundamental concepts of US and PA imaging.

Ultrasound beamforming

Figure 2: Geometry of beamforming relative to transducer (TX). A samplereconstruction line is shown in red within the imaging area shown in gray.

At each chosen reconstruction point at distance R steered atθ:

Get time of flight (tn) for each element n at distance xn.

Use time of flight to sum across elements.

Data will sum coherently at their origin.

tn(R , xn, θ) =

√(xn − Rsinθ)2 + R2cos2θ

c(1)

Usually just use τn, a delay based on t = 2Rc .

τn(t, xn, θ) = −xnsinθc

+x2ncos

2θ

c2t(2)

Two terms are the steering and the focusing termrespectively.

An apodization width of w = df#

is typically used at a

distance d from the transducer face with f# > 1 to givebetter near-field performance.

Photoacoustic beamformingPA imaging has only one-way ultrasound travel:

τn(t, xn, θ) = −xnsinθc

+x2ncos

2θ

2 c2t(3)

Photoacoustic imaging with ultrasound systemsPhotoacoustic imaging is analagous to ultrasound flashimaging.

Commercial ultrasound systems provide multiple A-scanimages per excitation using custom hardware.

The difference in the focusing term of the PAbeamformer may be overcome by using thespeed of sound parameter (c). We use c = ac0, c0

is the actual speed of sound, a is a user-specifiedparameter.

Scaling c results in a misplacement of pointtargets if there is an error in the steering term.A re-mapping of coordinates to (R ′, θ′), where R ′ = 2

aRand θ′ = sin−1(sinθa ) helps correct for this.

Figure 3: Beamforming flow showing important steps to create a PAimage using US beamformers.

What choice of c gives the best reconstruction?

a = 1 Focusing term incorrect→ poor image resolu-tion.

a =√

2 Ideal when steering angle is zero, other-wise steering term incorrect.

a = 2 Both terms incorrect, but works for unapprox-imated beamformer.

Display changed in software → no hardwaremodification.

Experimental setup

Figure 4: Pump laser (Continuum Surelite III) pumps an opticalparametric oscillator (OPO Plus), providing tunable light, illuminating asample in a bath from the side. Raw data collected by ATL L7-4 lineararray transducer, streamed through a research ultrasound acquisitionsystem (Verasonics VDAS I) to a host PC for reconstruction.

Human hair glued across an acrylic frame as point target.

Transducer was scanned to give point targets in differentlocations in the field of view.

Composite images are formed to investigate a andrescaling options. f# = 1.3 for each reconstruction, butmust be scaled to get the correct depth.

Point-spread results

Figure 5: Point-spread functions from linear-scanning, varying a.

Differing values of a

(a) a = 1 (b) a =√

2 (c) a = 2Figure 6: Linear-scanned US-beamformed PA images, varying a.

Depth-scaling versus re-mapping

(a) PA BF (b) Depth-scaled (c) (r ′, θ′) mapFigure 7: Comparison of PA beamformed (PA BF) sector-scanned imageto US beamformed images with a =

√2 using different image mappings.

DiscussionUS beamformer can only be made to match a PAbeamformer in the linear-scanned case, butsector-scanning possible with worse resolution at higherθ.

Triggering US acquisition likely requires hardwaremodification (system-dependent), but scaling c meansno additional hardware changes.

ConclusionUltrasound beamformers can be used to createphotoacoustic images by scaling the speed ofsound using a factor of

√2 and software image

warping to coordinates R ′ =√

2R andθ′ = sin−1(sinθ√

2).

AcknowledgmentsFunding: NSERC, Alberta Innovates Technology Futures,Canada Foundation for Innovation

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