Patent Description:
Contrast agents are microparticles detectable by imaging. The term "imaging" refers to detection using an imaging device, examples include but are not limited to, ultrasound or ultrasonic (US) imaging, magnetic resonance imaging (MRI), scintigraphy, single photon emission computed tomography (SPECT), positron emission tomography (PET), computed tomography (CT), X-ray imaging/fluoroscopy, fluorescence imaging, bioluminescence imaging, microscopy, optical methods, or multi-modal variants thereof.

Suitable contrast agents for (contrast enhanced) imaging depends on the nature of the imaging modality proposed, and vice versa. For example, gas-containing microparticles such as microbubbles may be used as contrast agents in US imaging; microparticles containing radionuclides (e.g., technetium-<NUM>, thallium-<NUM>, iodine-<NUM>, iodine-<NUM>, gallium-<NUM>, indium-<NUM>, fluorine-<NUM>, carbon-<NUM>, nitrogen-<NUM>, oxygen-<NUM>, rubidium-<NUM>) may be used as contrast agents in scintigraphy, SPECT or PET; microparticles containing paramagnetic, superparamagnetic or ultrasuperparamagnetic materials (e.g., gadolinium (Gd), iron oxide, iron, platinum, manganese) may be used as contrast agents in MRI; microparticles containing radio-opaque materials (e.g., iodine, barium, metal) may be used in CT or X-ray imaging/fluoroscopy; microparticles containing fluorophores or fluorescent dye (e.g., fluorescein-<NUM>-isothiocyanate, rhodamine, <NUM>,<NUM>'-dioctadecyl-<NUM>,<NUM>,<NUM>',<NUM>'-tetramethylindocarbocyanine perchlorate (DiI), <NUM>,<NUM>'-dioctadecyloxacarbocyanine perchlorate (DiO)) may be used in fluorescence imaging/microscopy; microparticles containing enzyme substrates (e.g., that for luciferase) may be used in bioluminescence imaging. Contrast agents may be detectable by more than one imaging modality. For example, a microbubble with/without paramagnetic material may be detected by US imaging, MRI and microscopy.

The term "imaging signal" refers to the received signal in imaging, that is identified to represent that of a contrast agent or contrast agent plus another element (e.g., tissue or blood). The received signal includes, but is not limited to, the raw, radiofrequency or front data, data before/after coding or processing, image pixel data (or image), or the number/density/concentration of microparticles observed visually under microscopy.

The term "signal intensity" refers to the intensity or strength of the imaging signal, it may be used synonymously with similar terms such as (but not limited to), the signal amplitude, signal strength, signal power (eg signal voltage squared, signal audio loudness), signal decibel (dB), signal videointensity, signal videodensity (e.g., pixel intensity on an image in grey or other colour scale), or the number/density/concentration of microparticles observed visually under microscopy. Where appropriate, the image videodensity (e.g., pixel intensity) may be substantially linearised using a suitable function (e.g., decompression using an anti-log function).

Contrast agents which comprise molecular binding elements can be used in appropriate imaging modality/modalities for molecular imaging, for the detection of molecules of interest (target molecules). For example, US molecular imaging can be achieved using targeting microbubbles as contrast agents. Microbubbles are formed of a shell encapsulating a gas. The shell can be made of a lipid, protein or polymer. Microbubbles oscillate within an acoustic field producing signals appearing as bright spots on an US picture, thereby effecting US contrast enhancement. The microbubbles are sufficiently small to flow without obstruction through small blood vessels, rather like the way in which red blood cells flow. Targeting microbubbles have shells containing molecular binding elements, which bind to molecules of interest one wishes to detect. Thus, for example, when targeting microbubbles are introduced into the bloodstream, they circulate with the blood and attach and accumulate on and around the molecules of interest, detectable using US imaging. Non-targeting microbubbles can also be imaged using US molecular imaging.

The molecule of interest (target molecule, targeted molecule or molecular target) may be, but is not limited to, a molecule, protein, receptor, particle or cell (including that present on artificial/implanted materials, e.g., metal, polymer or drug on a coronary stent, prosthetic heart valve or closure device). The molecule of interest may be present on the surface of cells.

The molecule of interest may exist de-novo or may be introduced artificially into a subject or system.

The terms "contrast agent", "microparticle", "targeting microparticle", "microbubble", or "targeting microbubble" may be used synonymously.

"Targeted microbubble contrast enhanced ultrasonography" (targeted MCU) is a name given to such a technique whereby targeting microbubbles are introduced into a subject or system, and the microbubbles are imaged using an US device. Examples of a suitable device includes, but are not limited to, the Siemens Acuson Sequoia <NUM> ultrasound system (using, for example, its contrast pulse sequencing (CPS) imaging mode), Phillips HDI5000 ultrasound system (using, for example, pulse inversion imaging mode), Phillips Sonos <NUM> ultrasound system (using, for example, power modulation imaging mode), or VisualSonics Vevo <NUM> (using for example linear imaging mode) or Vevo <NUM> (using, for example, non-linear imaging mode).

A short period of time after microbubble administration, part of the microbubble population will have adhered to the molecules of interest, and are described as retained microbubbles, while others may remain free, described as free microbubbles. Retained microbubbles are microbubbles retained or accumulated in a tissue or system (for example a flow chamber system) due to adherence to the molecule(s) of interest. Retained microbubbles may also be retained in a tissue or system due to other mechanisms including, but not limited to, non-specific adherence or cellular-uptake. Free microbubbles are microbubbles that circulate freely in a tissue or system. Both retained and free microbubbles decrease in number over time owing to their elimination.

Targeted MCU has been used to determine the concentration of a molecule of interest (a target molecule), by measuring the retained microbubble signal intensity after a certain time following the introduction of the microbubbles such that the free microbubbles in the body or system have decreased through elimination to a relatively low level (for example when the signal caused by free microbubbles has become low, insignificant, minimal or undetectable). However, I have found this method of imaging signal analysis lacks sensitivity and has a low degree of quantification, as evidenced by it being poor at detecting low concentrations as well as small changes in the target molecule concentration. Furthermore, it is prone to inaccuracies, inconsistencies and wide variations. Alternative imaging signal analysis methods suffer from attenuation and/or saturation of microbubble ultrasound signals when microbubbles are at high or moderate concentrations, making signal analysis for the determination of the target molecule concentration very difficult or inaccurate. While these problems can be mitegated to some degree by reducing the number of microbubbles administered into the body or system so that signal attenuation and/or saturation either does not occur or is minimised, I have found that this reduces the number of target molecules that can be detected as well as the accuracy and degree of quantification of the target molecule concentration.

The foregoing identifies a major obstacle limiting the development of targeted MCU towards human application.

Stokes et al: "Characterization of restricted diffusion in uni- and multi-lamellar vesicles using short distance iMQCs" discloses how the composition, lamellar structure, vesicle size, and concentration affects the iDQC signal between coupled water spins at very short separation distances. The iDQC signal is used to monitor dynamic changes in the lamellar structure as temperature-sensitive liposomes released their contents.

Accordingly, it is an aim to attempt to provide a method of imaging signal analysis which is robust and highly quantitative, and thus suitable for use in contrast imaging (molecular imaging) of human subjects as well as animals, in particular for targeted MCU. The method may have one or more advantages, including but not limited to: higher sensitivity, accuracy and degree of quantification for the molecular targets; more robust and reproducible, higher dose of contrast agents (e.g., targeting microbubbles in targeted MCU) can be used, which can be administered as a bolus (continuous infusion may also be used); other useful physical properties can be obtained simultaneously, such as the retained or free contrast agent half-life; tissue fractional vascular volume may also be obtained. For example, the method may quantify a wider range of target molecule concentrations as well as detecting smaller changes in them.

According to an aspect of the invention there is provided a method of quantifying the characteristics of an object according to claim <NUM>.

According to another aspect of the invention there is provided apparatus for quantifying the characteristics of an object according to claim <NUM>.

Further aspects of the invention are disclosed in the dependent claims.

As mentioned, examples of the disclosure have a number of advantages, including a higher degree of quantification of molecular targets. As different types, severity and stages of disease may express different combinations of molecules and in different concentrations, embodiments of the present invention provide a more robust method which increases the degree of quantification of molecular imaging, such as in US molecular imaging.

The plurality of imaging signals may be captured once the concentration of contrast agent is less than the point where attenuation and/or saturation of the signal intensity is significant.

The processing of the imaging signals allows larger numbers of microbubbles to be used without imaging signal saturation and/or attenuation causing the problems set out above. The bi-exponential function allows separation of the retained microbubble signal from that of the freely circulating ones. Images can be processed after the event, meaning that post processing can be used remotely.

A plurality of ultrasound images may be captured in step (a) once the concentration of microbubbles is less than the point where saturation and/or attenuation of the signal intensity occurs or becomes significant.

According to one arrangement, the biexponential equation may further comprise an additional component(s), such as (but not limited to) a constant(s), a scaling factor(s), a factor(s) or an exponential term(s). This may be used to account for factors such as (but not limited to) background signal, system noise, or cellular internalisation of the contrast agent. For example, an additional exponential term may be used in the latter; while a constant may be used for one or both of the former two. Thus, the bi-exponential function may comprise a constant (K), e.g., <MAT>.

One may obtain Ar and Af using, Ar = Ar' - K[Ar'/(Af'+Ar')] and Af = Af' - K[Af'/(Af'+Ar')], respectively.

Contrast agents may be administered as a bolus or infusion. One aspect of the invention may allow high contrast dose (to allow saturation of contrast-to-target molecule binding for accurate/reproducible quantification of target molecule expression level/concentration) causing signal saturation and/or attenuation to be used. For bolus administration, this may be given over upto a few seconds. For administration as an infusion, the contrast agent may be administered as a continuous infusion over a sufficient period of time (eg several minutes) to allow saturation of contrast agent-to-target molecule binding or contrast agent retention. The infusion is then stopped. Contrast agent signal intensities will then start to decrease, a plurality of imaging signals may then be captures and processed in the same way as described.

The present invention will now be described by way of example only with reference to the drawings in which:.

The use of US scanners to image organs within a body non-invasively is well known. It is also known to enhance the images that are obtained by introducing targeting microbubbles intravenously (iv), the microbubbles being contrast agents which are visible to US imaging apparatus, the shell of the bubble being designed to adhere to molecules of interest (target molecules) in the tissue or organ to be imaged in a process called "targeted microbubble contrast enhanced ultrasonography" (targeted MCU).

In one embodiment of the present invention, targeting microbubbles are used as a contrast agent and US imaging is used for their detection. Targeting microbubbles are introduced into the body to adhere to target molecules, and images of a region of interest are collected spaced over a period of time. A point in the region of interest (ROI) can be selected, the pixel on each of the US images corresponding to that point can be identified, and the bubble US signal intensity of those pixels are measured and curve fitted to a bi-exponential function. The bi-exponential function comprises two exponential terms, one of which is related to the elimination of retained targeting microbubbles which have adhered to target molecules, and the other being related to the elimination of free targeting microbubbles which have not adhered and circulate freely. Once the curve has been identified, various characteristics of the targeting microbubbles and targeted molecules to which they attach can be identified.

This process is repeated for a number of other points in the region of interest from the same images in order to obtain the various characteristics for a number of points within the region of interest. The values of those various characteristics can then be averaged in order to obtain a very accurate quantification of the characteristics for that region of interest.

For example, one characteristic of importance might be the concentration of retained microbubbles because this quantifies the concentration or expression level of target molecules in the region of interest. The concentration of target molecules in the region of interest might be indicative, for example, of the presence or extent of a disease.

Another characteristic might be the half-life of the free and/or retained microbubbles. These can be determined in the same way, by identifying the curve to which the measured signal intensity of a point or region of interest shown in the images best fits.

In this way, important characteristics of a point or region of interest can be quantified. This can also be done for other points or regions of interest.

It might also be useful to create an image made up of a particular characteristic at each of the points of interest which have been quantified.

It is important to understand the significance of the bi-exponential curve mentioned above. A short time after the microbubbles are introduced into the body/system, a minor proportion of those microbubbles will attach themselves to target molecules, but many will remain unattached. The concentration of the free microbubbles decreases quite quickly. The concentration decreases through a number of different elimination mechanisms, including phagocytosis in the liver and spleen, deflation by gaseous diffusion in the bloodstream. This elimination is shown quite clearly in <FIG>. In <FIG>, the elimination of the free microbubbles is faster than the elimination of the retained microbubbles. In <FIG>, the elimination rate constant of free bubbles is a substantially straight line having a steeper gradient than that of retained bubbles.

The concentration of microbubbles in the elimination phase can be defined by the following bi-exponential function: <MAT> where Af is the maximum concentration of free microbubbles in the elimination phase, Ar is the maximum concentration of retained microbubbles in the elimination phase, λf is the elimination rate constant of free microbubbles, and λr is the elimination rate constant of retained microbubbles.

Of course, you would expect the detected concentration in an US image for a particular point to correspond to the actual concentration of microbubbles. As described above, the detected concentration of microbubbles becomes inaccurate (under-estimated) due to signal saturated and/or attenuated at high or moderate concentrations. However, for accurate and reproducible quantification of molecular targets, a relatively high dose of targeting microbubbles (to allow saturation of bubble-to-target molecule binding) is required. Signal intensities may be curve fitted after the point at which saturation and/or attenuation is low or no longer a factor. This means that one may wait for a few minutes after the microbubbles have been introduced into the body before imaging and using the signal intensity values for curve fitting. Looking, for example, at the curves shown in <FIG>, imaging signals may be obtained from US imaging after about <NUM> to <NUM> post bolus administration of the microbubbles into the body. Curve fitting the signal intensities after this point has the effect that the exponential function for the retained microbubbles are extrapolated back to the initial (maximum) concentration Ar. This also applies for the free microbubbles. In this way, the disadvantages of the prior art are overcome.

Below is an example of how one embodiment of this invention is applied in quantifying the characteristics of the heart of a mouse treated with a contrast agent. This method can, of course, be applied to quantifying the characteristics of other tissues or organs of a mouse, or indeed, the tissues or organs of other animals, including humans.

In this example, an inflammatory response was stimulated to cause the heart to express E-selection (Esel) to which the microbubbles adhere. Wild type (WT) mice were first injected with lipopolysaccharide (LPS) to induce systemic inflammation. The heart's inflammatory response was to express Esel (an endothelial adhesion molecule expressed on activated endothelium during inflammation). Microbubbles were introduced into the cardiovascular system which adhered to Esel molecules (target molecules), and the accumulation of bubbles attached to Esel targets in the heart allowed the heart to be quantitatively analysed (e.g., for Esel concentration or expression level indicative of the degree of endothelial activation or inflammation) from ultrasound (US) images, using one aspect of the invention.

MES-<NUM> monoclonal antibody (mAb), a rat IgG2a,k against mouse Esel, and its F(ab')<NUM> fragment (MES-<NUM> F(ab')<NUM>) was provided by Dr D Brown (UCB Celltech, UK). Reduced MES-<NUM> F(ab')<NUM>, containing <NUM> thiol groups per F(ab')<NUM>, were prepared by tris(<NUM>-carboxyethyl)phosphine hydrochloride (TCEP) reduction. <NUM> mAb, rat IgG2a,k against mouse PECAM-<NUM> (BD Biosciences). Rat IgG2a,k isotype negative control mAb (BD Biosciences). Biotinylated rabbit mAb against rat IgG2a (secondary antibody) (Vector Laboratories).

Maleimide-functionalised lipid-shelled octafluoropropane (C<NUM>F<NUM>) microbubbles were prepared by sonication of a gas-saturated aqueous suspension of <NUM>,<NUM>-distearoyl-sn-glycero-<NUM>-phosphocholine (DSPC; Avanti Polar Lipids, AL), <NUM>,<NUM>-distearoyl-sn-glycero-<NUM>-phosphoethanolamine-N-(maleimide(polyethylene glycol)-<NUM>) (DSPE-PEG2000-Mal; Avanti Polar Lipids), mono-stearate poly(ethylene)glycol (PEG40 stearate; Sigma-Aldrich), and fluorescent dye <NUM>,<NUM>'-dioctadecyl-<NUM>,<NUM>,<NUM>',<NUM>'-tetramethylindocarbocyanine perchlorate (DiI; Molecular Probes) at <NUM>:<NUM>:<NUM>:<NUM> molar ratio, in the presence of C<NUM>F<NUM>. Approximately <NUM>×<NUM><NUM> TCEP reduced MES-<NUM> F(ab')<NUM> molecules per bubble were incubated for <NUM> at <NUM>, near neutral pH, under C<NUM>F<NUM> atmosphere with constant gentle agitation; the reaction was terminated by adding excess N-Ethylmaleimide (Sigma-Aldrich) to quench any unreacted thiol. Microbubbles were washed with cold degassed normal saline using multiple cycles of centrifugation flotation under C<NUM>F<NUM> atmosphere at <NUM> before and after microbubble conjugation, to remove unincorporated components and particle fragments. Freshly prepared Esel targeting microbubbles were immediately divided into <NUM>-50µl aliquots, capped and sealed with parafilm (American National Can), then snap frozen in liquid nitrogen and stored at -<NUM> until use. The concentration and size of subsequently thawed Esel targeting microbubbles were, respectively, <NUM>-<NUM>×<NUM><NUM> bubbles/ml and diameter <NUM>(mean) ±<NUM>(SEM) µm (<NUM>% or <NUM>% of the bubbles were under <NUM> or <NUM> in diameter, respectively). The Esel targeting microbubbles were sufficiently echogenic, stable, lacked non-specific binding, and produced no immediate adverse effects in vivo.

Wild-type (WT) mice: adult male C57Bl6/Jax (Charles River, UK). Esel knock-out (KO) mice: adult male Esel homozygote KO on C57B16 background, bred locally from mice donated by Dr K Norman and Prof P Hellewell (University of Sheffield, UK). All the animal work was carried out under Project Licences and Personal Licences granted by the Home Office under the Animals (Scientific Procedures) Act <NUM>; ethical approval was additionally obtained from the local Ethical Review Panel.

WT and Esel KO mice were pre-treated with 50µg LPS from E Coli <NUM>:B4 (Sigma-Aldrich), made up to 200µl volume in normal saline, by intraperitoneal (ip) injection to induce systemic inflammation. Systemic administration of LPS by ip injection produces systemic inflammation, which includes induction of Esel expression in multiple organs including the heart and kidneys.

Immunohistochemistry was performed on acetone-fixed cryosections of freshly harvested hearts of WT (with/without LPS pre-treatment) and Esel KO (pre-treated with LPS) mice. After blocking non-specific binding sites with 100µl of <NUM>:<NUM> rabbit serum (Sigma-Aldrich) for <NUM> hour (h) at room temperature (rt), sections were incubated for <NUM> at rt with 100µl of <NUM>/ml primary antibody: MES-<NUM> (for Esel), MEC13. <NUM> (for PECAM-<NUM>, endothelial marker) or isotype negative control. Each section was then incubated with 100µl of <NUM>/ml biotinylated secondary antibody for <NUM> at rt. After blocking of endogenous peroxidase with <NUM>% H<NUM>O<NUM> methanol for <NUM>-<NUM> at rt, the horseradish peroxidase-based detection system, Vectastain ABC kit (Vector Laboratories), was used with <NUM>,<NUM>'-Diaminobenzidine solution (SIGMAFAST™ DAB tablet, Sigma-Aldrich) as the chromagen substrate. Sections were counterstained using Harris Modified Hematoxylin Solution (Sigma-Aldrich) and <NUM>% NaHCO<NUM>, then dehydrated through <NUM>-<NUM>% ethanol, dried and mounted with Histomount (VWR), and examined under light microscopy. The duration between the time of LPS pre-treatment and sacrifice of the animal for immediate tissue harvesting was noted as the LPSTime.

Reverse Transcriptase - Real Time Quantitative Polymerase Chain Reaction (RT-qPCR). WT mice were pre-treated with LPS as described above. The duration between the time of LPS pre-treatment and sacrifice of the animal for immediate tissue harvesting was noted as the LPSTime. Freshly harvested tissues were kept in RNAlater® solution (Ambion) to preserve ribonucleic acid (RNA) in-situ; total RNA was subsequently extracted using TRIzol reagent (Invitrogen) according to the manufacturer's instructions. First-strand complementary deoxyribonucleic acid (cDNA) synthesis was then performed using the Qiagen Omniscript® Reverse Transcription kit (Qiagen) according to the manufacturer's instructions. This was followed by real-time qPCR with the SYBR®Green detection method for Esel and hypoxanthine phosphoribosyltransferase-I (HPRT-I), carried out on a <NUM>-well plate in the iCycler™ (iCycler iQ Real-Time PCR Detection System, Bio-Rad) according to the manufacturer's instructions. All PCR reactions were carried out in triplicate wells on the same plate. The primer sequences were: Esel forward primer <NUM>'-CTCATTGCTCTACTTGTTGATG-<NUM>', Esel reverse primer <NUM>'-GCATTTGTGTTCCTGATTG-<NUM>', HPRT-I forward primer <NUM>'-ATTAGCGATGATGAACCAG-<NUM>', HPRT-I reverse primer <NUM>'-AGTCTTTCAGTCCTGTCCAT-<NUM>'. For data analysis, the threshold cycle (Ct) was determined from the amplification plot using the iCycler™ iQ Optical System Software Version <NUM>. 0a (Bio-Rad). As PCR efficiency of the Esel and HPRT-I primer pairs differed by ≤<NUM>% (<NUM> ±<NUM>% and <NUM> ±<NUM>% (mean ±SD), respectively; n = <NUM> each), comparative Ct method was used to estimate the amount of Esel messenger (mRNA) relative to that of HPRT-I, using the formula: Esel mRNA (%HPRT - I) = <NUM>-ΔCt, where ΔCt = CtEsel - CtHPRT-I, subscripts refer to the gene of interest. Mean of the replicates was used and plotted against LPSTime for each animal.

Further details of the methodology is as follows. The yield of total RNA from the mouse heart was typically ≈1µg pure RNA per <NUM> tissue, kept at concentrations over ≈<NUM>/ml in molecular grade (RNase-free) H<NUM>O (Sigma-Aldrich). The RT reaction mixture for first-strand cDNA synthesis consisted of 1µg total RNA, 2µl 10x buffer RT, 2µl deoxyribonucleotide triphosphate (dNTP) mix (<NUM> each <NUM>'-deoxyadenosine <NUM>'-triphosphate (dATP), <NUM>'-deoxycytidine <NUM>'-triphosphate (dCTP), <NUM>'-deoxyguanosine <NUM>'-triphosphate (dGTP), <NUM>'-deoxythymidine <NUM>'-triphosphate (dTTP), 1µ (<NUM> units) Omniscript reverse transcriptase, 2µl (1µg) oligo(dT)<NUM>-<NUM> primer (Invitrogen) and molecular grade H<NUM>O made up to a total reaction volume of 20µl, incubated for <NUM> at <NUM>. qPCR was carried out in a 25µl-reaction volume in each well of a <NUM>-well <NUM> thin-wall PCR plate (Bio-Rad) covered with an Optical Quality Sealing Tape (Bio-Rad). The qPCR reaction mixture consisted of 5µl cDNA template (<NUM>:<NUM> water dilution of the finished RT reaction), <NUM>. 5µl (<NUM>) each of the forward and reverse primer for the respective gene (see text for primer sequence; the primers were custom ordered from Invitrogen), <NUM>. 5µl molecular grade H<NUM>O and <NUM>. 5µl iQ™ SYBR® Green Supermix (Bio-Rad). The qPCR cycling condition was: initial <NUM> denaturing step at <NUM> (Well Factor analysis in first <NUM>); then <NUM> cycles of <NUM> at <NUM>, <NUM> at <NUM>; melt-curve analysis in <NUM> steps (<NUM> denaturation at <NUM>, <NUM> reset at <NUM>, then <NUM> cycles of <NUM> at <NUM> with <NUM> increment for each cycle); final cooling step at <NUM>. Esel and HPRT-I were amplified on the same plate for each animal; no-template negative control using molecular grade H<NUM>O in place of cDNA template for both primer pairs were included in all plates. For data analysis, wells with abnormal amplification plot or melt-curve were excluded.

WT and Esel KO mice were all pre-treated with LPS, tail vein cannulated and anaesthetised with xylazine/ketamine mixture as described above. The chest, abdomen and pelvis were then shaved and the animal placed supine. ECG electrode pads (Ambu® Blue Sensor P, Ambu) were applied to the paws and connected to the US machine (Acuson Sequoia® <NUM> system, Siemens, CA) equipped with `Small Animal ECG Filter'. A layer of warm gel (Gel for ultrasonic & electrical transmission, Henleys Medical) was coupled between the skin and US transducer (15L8-s linear array transducer, foot print <NUM>, Siemens). US settings used were: <NUM> (P14MHz, spatial resolution ≈<NUM>) Contrast Pulse Sequencing (CPS) mode (a non-linear imaging mode specific for microbubbles), transmission power 9dB giving low mechanical index (MI) <NUM>-<NUM> estimated by the scanner, dynamic range <NUM>dB, time gain <NUM>%, CPS gain <NUM>, fundamental 2D gain 15dB, colour map M:<NUM> (bubble signal presented in heated object scale (`CPS-contrast only' images), tissue signal in grey scale ('B-mode' images)), TEQ was not used. Before bubble injection, baseline parasternal short axis (PSA) view at the papillary muscle level, parasternal long axis (PLA) and apical <NUM>-chamber (A4C) views of the heart with and without `regional expansion selection' (RES; giving magnified images with enhanced resolution) were recorded as <NUM>-digital clips. Thereafter, imaging was maintained in the PSA view with the transducer fixed in position using a free standing clamp. A stopwatch was then started and a high dose of <NUM>×<NUM><NUM> Esel targeting bubbles (in 100µl volume made up with normal saline) injected at <NUM> via the tail vein catheter as a rapid iv bolus over <NUM>-<NUM>, followed by a 100µl-normal saline flush over <NUM>-<NUM> at <NUM>. Continuous US insonation was applied without pausing from time <NUM>-<NUM> <NUM> on the stop-watch, then paused, then resumed only for <NUM> each time for digital image acquisition. <NUM>-digital clips (RES activated) of the heart containing several consecutive cardiac cycles were recorded at <NUM> and <NUM>, then at <NUM> intervals from <NUM>-<NUM> <NUM>, then at <NUM> intervals from 2min20s-10min20s, then at <NUM> intervals from 12min20s-30min20s, then at <NUM> intervals until 60min20s on the stopwatch (image acquisition was stopped earlier if particle contrast enhancement in the left ventricular (LV) cavity (central blood pool) was no longer visible). Unmagnified (non-RES) images of the thorax containing the heart in the PSA view and surrounding tissues were recorded at ≈<NUM> intervals. Other views of the heart (PLA and A4C views) were acquired at the end. In some animals, <NUM> (P7MHz, spatial resolution ≈<NUM>) CPS imaging at MI <NUM> (gain and other settings kept the same as <NUM> imaging) was also acquired at baseline and the end of the <NUM> imaging study. When switching from <NUM> to <NUM> CPS imaging, the transmit power was first reduced from -9dB to -19dB before reducing the US frequency, to avoid an increase in MI (up to ≈<NUM>) causing inadvertent particle destruction. All animals received only one dose of bubbles to avoid carry-over effect from previous bubble dosing (e.g., blocking of Esel binding sites by previously administered Esel targeting bubbles). At the end of imaging, animals were sacrificed and tissues immediately harvested for frozen section immunohistochemistry and qRT-PCR as described above.

Videodensitometric method was used to measure particle signal intensity off-line, using the YABCO© software (LLC Charlottesville, Virginia). End-diastolic image frames of the heart in the PSA view (`CPS-contrast only' images) were selected and aligned, those that could not be aligned (e.g., due to large movement artefact) were excluded. Regions of interest (ROIs) were placed on the mid-anterior wall of the myocardium (M) and adjacent region in the LV cavity (C), as shown in <FIG>. These regions were chosen because they were consistently least or minimally affected by US attenuation in all animals. The video signal intensities (VI) were 'linearised' by log-decompression using the formula: Linearised <MAT>, where dB is the dynamic range (<NUM>dB in this study). Linearised VI (I) was expressed in arbitrary acoustic units (AU). I of several end-diastolic image frames within the <NUM>-recording period at each time point were averaged, then corrected for background noise by subtracting away average I of the baseline images (images before particle administration) in the respective animals. TICs of the myocardium (tissue) and LV cavity (central blood pool) were constructed by plotting background-subtracted I of the myocardium and LV cavity, respectively, against time post bubble administration.

The LPSTime for US molecular imaging was taken as the duration between the time of LPS pre-treatment and administration of the targeting bubbles. The mean heart rate (HR) for each animal was calculated from all HRs recorded at different time points during cardiac imaging.

Pearson correlation, linear or non-linear regression analysis was performed as indicated. Student's t-test or ANOVA with Turkey's post-hoc analysis was used for significance testing where indicated, with p <<NUM> taken as significant.

Frozen section immunohistochemistry showed that Esel was expressed in the heart of all WT mice pre-treated with LPS (n = <NUM>, LPSTime = <NUM>-<NUM>). The spatial distribution was essentially uniform throughout the myocardium but limited to the post-capillary venules and capillaries. Esel was not detectable in the negative controls: WT mice not treated with LPS (n = <NUM>), and Esel KO mice pre-treated with LPS (n = <NUM>, LPSTime = <NUM>-<NUM>), <FIG>.

Quantification of Esel expression by qRT-PCR. RT-qPCR showed that the concentration of Esel mRNA in the heart decreased exponentially with time after ≈<NUM> post LPS pre-treatment, reaching very low levels by ≈<NUM> (n = <NUM>, LPSTime = <NUM>-<NUM>), <FIG>. This trend was similar to that of the cell-surface Esel protein concentration (expressed as % injected dose of radioactivity/g tissue (%ID/g tissue)) determined using iv radio-labelled mAb by Eppihimer et al<NUM> using the same strain, sex, and age of mice, as well as the same dose and route of LPS administration (n=<NUM>), <FIG>.

From the best-fit curves of Esel mRNA concentration vs. LPSrime (mRNA (in %HPRT - I) = 3600e-<NUM>. 7LPSTime(in hours), R<NUM> = <NUM>) and that of Esel cell-surface protein concentration vs. LPSTime (protein (in % ID/g tissue) = <NUM>. 13LPSTime(in hours) + <NUM>, R<NUM> = <NUM><NUM>), using LPSTime as the common denominator, an empirical formula describing the relationship between the concentration of Esel mRNA and cell-surface protein could be derived as: <MAT> <FIG>. Thus the Esel cell-surface protein concentration was predictable from its mRNA concentration in the heart, in this LPS mouse model. Within the mRNA or protein concentration range (<NUM>-<NUM>% HPRT-I or <NUM>-<NUM>% ID/g tissue, respectively) used for US quantification of Esel expression in this study, the relationship between the concentration of mRNA and cell-surface protein was approximately linear, allowing direct use of the mRNA concentration as a surrogate quantifier for the cell-surface protein concentration (the latter being the actual target of the targeting microbubbles).

US attenuation caused by overlying bone, lung air ± retained-bubbles located proximally in the US path, affected certain parts of the myocardium depending on the imaging scan plane - this caused pseudo-loss of targeted bubble signal for Esel in the WT animals (e.g., there was artefactual loss of retained-bubble signal in the mid-posterior, -inferior, -posteroseptal and - anteroseptal walls of the LV (anti-clockwise from <NUM>-<NUM> o'clock positions, respectively) in the PSA view with <NUM> CPS imaging). However, by changing the scan plane (e.g., from PSA to PLA or A4C view) to alter the relative position of overlying entities, or by lowering the US frequency to increase its penetrative depth (e.g., from <NUM> to <NUM>), these attenuation effects could be overcome with good recovery of the retained-bubble signals, Figure Sb. The global expression of Esel in the WT myocardium was thus demonstrated on US imaging, consistent with the immunohistochemistry data.

In the above situation, US detection of molecular target was limited by late distal attenuation from overlying bone/air and retained-bubbles located proximally in the US path. However, it was found that such attenuation could be overcome by using lower frequency US (greater penetrative depth) or a different imaging angle. From the human imaging perspective, where the use of lower frequency US (e.g., <NUM>-<NUM>) and multi-plane imaging are the norm, and the footprint of the transducer is much smaller relative to the body size (making it easier to achieve optimal probe position/angle and avoid overlying bone/air), these attenuation issues are likely less important/significant. Nevertheless, refinements in the machine's attenuation correction algorithm may further minimise the attenuation artefacts. (<NUM>) Adverse effects. No death or significant adverse events attributable to the Esel targeting bubbles were observed. No significant bubble-medicated intravascular obstruction in the myocardium, causing loss of regional myocardial perfusion manifest as regional wall motion abnormality, was detected.

Three phases with distinct characteristics were discernable from the TICs following iv bolus administration of the targeting bubbles, <FIG>:.

The elimination rate constant of the retained-bubbles in the myocardium (λr) decreased with increasing Ar, the latter representing the maximum concentration of retained-bubbles in the myocardium, <FIG>. The relationship was non-linear and could be empirically fitted to an exponential function (λr = <NUM>. 45e-<NUM>Ar, R<NUM> = <NUM>) or sigmoidal function ( <MAT>, R<NUM> = <NUM>), n = <NUM> WT & <NUM> KO. This suggested that the half-life of the retained-bubbles was shorter the lower the maximum retained-bubble concentration (the latter correlated with the target molecule concentration, see above). In another word, for the retained-bubbles in the elimination-phase, the higher their maximum concentration at the beginning when bubble accumulation at the target site was complete (Ar), the lower its elimination rate constant (λr) or the longer its half-life ( <MAT>). The relationship appeared non-linear. While I do not intend to be bound to any particular theory, I found that one reason for this may be that as the concentration of the retained-bubbles increases, the distance between neighbouring bubbles decreases, which may result in: (i) increased acoustical interaction between adjacent bubbles causing mechanical responses such that the net diffusion of gas out of the bubble population is reduced; and/or (ii) reduced concentration gradient for gas diffusion out of the bubbles due to increased gas saturated micro-environment surrounding the bubbles.

The videodensitometric method is used to process the US images. Above, it is described how a number of three seconds clips of images are acquired and recorded, and when they are processed, the corresponding images from each recorded clip are compared pixel by pixel or by region of interest so that signal intensities at those points of interest can be compared. Since the number of microbubbles reduces over time, the intensity of a point of interest will reduce over time. Different points of interest may behave differently with respect to the presence/absence/concentration of molecular target, flow and microparticle elimination characteristics, artifact (e.g., alignment/motion/machine/attenuation artifacts, which may/may not be intermittent). Using the bi-exponential function described above, the intensity detected in the images can be curve fitted so as to identify which bi-exponential curve fits best for each point. Once the curve is known, it is possible to obtain the parameters for the point of interest. Ar indicates the maximum retained bubble concentration at the point of interest in the elimination phase. This process can be repeated for multiple points of interest across a region of interest. For each point of interest, pixels from each of a series of images are compared and then curve fitted. Once a number of points have been curve fitted, an average can be taken to obtain an accurate quantification of the values of the parameters for that region of interest. Alternatively, the averaging can be done before curve fitting takes place, averaging the intensities of the pixels in a particular image corresponding to a region of interest with a plurality of points of interest, and then comparing average intensity values for each image. The average values can then be curve fitted instead.

It will be appreciated that, for a particular point in the heart, the measured response for that point from an image taken at ≥ three time points after for example ≈ <NUM>-<NUM> minute post bubble administration (e.g. at <NUM>, 3and <NUM> minutes or more time points; or at <NUM>, <NUM> and <NUM> minutes or more time points; or at <NUM>, <NUM> and <NUM> minutes or more time points; or at <NUM>, <NUM> and <NUM> minutes or more time points post bubble administration) will give a series of values which can be fitted to a curve comprising a bi-exponential function: <MAT>.

The part of the function: Afe-λft represents the elimination of free microbubbles, and the part of the function Are-λrt represents the elimination of retained microbubbles. The bi-exponential function predicts the elimination of free and retained microbubbles and allows the calculation of the concentration of retained and/or free microbubbles at any point.

Once the concentration of retained or free microbubbles for a series of points in a region of interest have been calculated, it is also possible to create an image to show the variation of these concentrations across that region of interest. Further, any of the other characteristics which can be derived from the curve could also be formed into an image, if desired. An image showing the variation of the half-life of the retained microbubbles within the region of interest might be formed, for example.

It will be appreciated that, since more than one image is collected, image processing takes place later because it requires a comparison to be carried out across the plurality of images that have been collected. Thus, the quantification takes place as a postprocessing operation.

In another arrangement, it is possible to quantify characteristics even where contrast agents are not retained, for example in the blood pool of a heart cavity or blood vessel. For example, microbubbles can be administered and US imaging used, as described above, to take multiple images of a tissue and nearby blood pool (e. g, heart cavity or blood vessel). Points of interest in the tissue and nearby blood pool can be analysed, as described above, and the intensities curve fitted to the bi-exponential equation. In the blood pool, where microbubbles are not retained, the exponential term of the bi-exponential function relating to retained microbubbles will be zero or approach zero. The characteristics of the free microbubbles in the blood pool is determined from its respective exponential term. Further, the characteristics of free microbubbles within the tissue and free microbubbles within the blood pool can be compared to obtain useful information. For example, the ratio of tissue Af or tissue A UCf with that of the blood pool as the denominator allows one to obtain the fractional vascular volume of the tissue. Thus, an important clinical characteristic can be obtained using this method. Specifically, this method uniquely allows determination of the tissue fractional vascular volume using large bolus dose of microbubbles without imaging signal saturation and/or attenuation causing problems described above (works best when retained microbubble concentrations are minimal/low/moderate), in contrast to prior-art methods which require small (ie non-attenuating/saturating) microbubble dose to be used. Using this method also has advantages in quality control and data interpretation. For example, unexpected deviation of tissue Af, AUCf or λf from that of the blood pool may alert one to particular disease states such as circulatory obstruction; unexpected relationships amongst tissue λf, blood pool λf and tissue λr may alert one to anomalies in the US setting, bubbles or host.

In another arrangement, the quantified characteristics can be obtained for non-targeting contrast agents or targeting contrast agents (the latter administered to a subject where the target molecule is known to be absent). In these situations, the contrast agent (depending on its nature) may be retained in a tissue/system due to non-specific mechanisms, such as (but not limited to) non-specific binding, non-specific cellular uptake, mechanical obstruction, or flow stasis. Here, the exponential term previously representing retained (targeted) contrast agent (as described above) can be used to represent non-specifically retained contrast agent in the analysis. Thus Ar represents the degree of non-specific contrast agent retention in the tissue/system, which may be used to determine the dosage of contrast agent bearing therapeutic material being delivered to the tissue/system by non-specific retention.

<FIG> illustrates a system <NUM> used for implementing the method for quantifying the characteristics of an object using imaging as described herein.

The system <NUM> comprises an imaging system <NUM> and a data processing system <NUM>. The imaging system <NUM>, which may be a system such as an ultrasonic imaging system or an MRI system, is arranged to image an object <NUM>. The imaging system <NUM> then sends captured image data to the data processing system <NUM>. The data processing system <NUM> receives the captured image data at communications unit <NUM>, which sends any data received from external systems to processor <NUM>. The processor <NUM> then stores the received image data in memory <NUM>. The processor <NUM> is then able to perform the various processes and analysis discussed throughout this document. The results of the processing performed by the processor <NUM> are then sent to a graphics card <NUM> for display on a monitor <NUM>. The data processing system <NUM> also comprises a user interface module <NUM> arranged to receive instructions for controlling the operation of the data processing system <NUM> by a user via a user interface <NUM>.

While the above system <NUM> is described as comprising a number of distinct components it will be appreciated that all of the components, or a subset of the components, shown in <FIG> may be integrated within a single unit. Furthermore, it will be appreciated that aspects of the system <NUM> may also be distributed components. For example, the processing may be performed by a distributed network of data processing systems. In addition, the processing may be carried out in a cloud computing environment.

Claim 1:
A method carried out on a data processing system (<NUM>) to quantify the characteristics of an object (<NUM>) using ultrasound imaging, the object being treated with microbubbles used as contrast agent which has been introduced into a body of the object, comprising:
(a) receiving, from an imaging system (<NUM>), a plurality of imaging signals of the object (<NUM>) captured over a period of time, the imaging signals being obtained after about <NUM> to <NUM> minutes post administration of the microbubbles into the body;
(b) measuring, at the data processing system (<NUM>), the signal intensity in each of the plurality of imaging signals corresponding to a point on the object (<NUM>);
(c) curve fitting, at the data processing system (<NUM>), the measured signal intensity of the point on the object (<NUM>) to a bi-exponential function comprising a first and a second exponential term, the first exponential term representing the decrease in concentration of the contrast agent which is free over time and the second exponential term representing a decrease in concentration of the contrast agent which is retained on the object (<NUM>) over time, with the curve fitting including an extrapolation back to the initial (maximum) concentration of retained contrast agent in the elimination phase of the contrast agent.