Element-Specific Spectral Imaging of Multiple Contrast Agents: a Phantom

2018 JINST 13 T02001 a February 6, 2018 January 24, 2018 January 23, 2018 : : : November 21, 2017 : Revised Accepted Published Received M. Moghiseh, f C.J. Bateman, https://doi.org/10.1088/1748-0221/13/02/T02001 b , a Published by IOP Publishing for Sissa Medialab R. Aamir, a d J.L. Healy, b and N.G. Anderson e , c S.T. Bell, , b 1 , , a b , a 2018 CERN. Published by IOP Publishing Ltd on behalf of Sissa [email protected] : This work demonstrates the feasibility of simultaneous discrimination of multiple con- c

Medialab. Original content from this work may be used under the terms Corresponding author. 1 Department of Radiology, University of Otago, 2 Riccarton Ave, Christchurch 8140, New Zealand MARS Bioimaging Ltd., 29a Clyde Rd, Christchurch 8140, New Zealand Department of Physics and Astronomy, UniversityChristchurch of 8140, Canterbury, New Zealand Department of Biology, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand European Centre for Nuclear Research (CERN), Geneva,Lincoln Switzerland Agritech Limited, Engineering Drive, LincolnChristchurch University, 7460, New Zealand E-mail: f e c a b d TECHNICALREPORT Element-specific spectral imaging of multiple contrast agents: a phantom study of the Creative Commons Attributionmust 3.0 maintain. licence attribution Any furtherand to distribution DOI. the of this author(s) work and the title of the work, journal citation A.P.H. Butler R.K. Panta, Abstract trast agents based on their element-specific and energy-dependent X-raya attenuation properties pre-clinical using photon-counting spectral CT. WeCT used a scanner photon-counting with based four pre-clinicalconcentrations spectral energy of thresholds iodine to (9, 18 measure8 and the mg/ml) 36 based X-ray mg/ml), attenuation contrast gadolinium (2, agents, propertiesthe 4 calcium spectral of and chloride imaging 8 various (140 performances mg/ml) and of and118 280 different gold kVp, mg/ml) energy (2, based threshold and on 4 schemes water. K-factor and and between signal-to-noiseX-ray We 25 ratio evaluated attenuation to and in 82 ranked keV the at them. K-edge K-factorpreceding was containing energy defined range, energy as expressed range as the a divided percentage.energy by We selection evaluated the the X-ray to effectiveness of attenuation discriminate the inphoton-counting optimised all the spectral three CT contrast using agentssimultaneously four in discriminated energy three a thresholds contrast of phantom agents 27,ous of based concentrations 33, 33 on using 49 iodine, their mm K-edge gadolinium and diameter. and and 81 energy-dependent keV gold X-ray at A attenuation at features 118 vari- in kVp a single 2018 JINST 13 T02001 Computerized Tomography (CT) and Computed Radiography (CR); X-ray detectors; scan. A ranking method to evaluateoptimised spectral to imaging discriminate performance iodine, enabled gadolinium energy andSimultaneous thresholds gold to discrimination contrast be agents of in multiple a contrast singlepossibilities spectral agents of CT in scan. improving a the single accuracy scanbio-markers each of is labelled likely disease with to diagnosis a open nano-contrast by agent. up simultaneously new imaging multiple Keywords: Image reconstruction in medical imaging; Medical-imagecomputer-aided software reconstruction methods and algorithms, 2018 JINST 13 T02001 8 – 1 – [13] or detecting the macrophage content in an atherosclerotic in vivo Many biological processes require simultaneous non-invasive identification and measurement 2.1 Medipix based2.2 spectral CT imaging system: Determination MARS of2.3 scanner2 scanning energy thresholds3 A multi-contrast2.4 phantom6 Spectral6 imaging 2.5 Material decomposition8 3.1 Evaluation of3.2 spectral imaging8 performance Discrimination of3.3 multiple9 K-edges Material decomposition9 of their components.lar Different that cells they and are biomarkers intrinsicallydifferent have indistinguishable nano-contrast X-ray from agents attenuation each at other. properties biologicallytion so relevant Biomarkers in-situ. concentrations simi- can to be Spectral enhance labelled imaging theirthe with sampling differentia- energy-dependent X-ray X-ray data attenuation fromof properties multiple materials X-ray that energy [16, allows ranges discrimination17 ] measures of and interest. quantification The photo-electric effect is the dominant physical interaction plaque [14] or monitoringbiomarkers drug of a delivery disease [15]. will enablerefining personalized tracking The treatment of strategies. ability the progression to or quantify regression of and disease visualize and molecular aid in 1 Introduction The ability to identify disease processes at theremains cellular or a molecular level central for diagnosis goal and treatment chemical for properties pre-clinical of or metallic clinical nanoparticles,contrast research. agents they to are yield Because functional increasingly information of being at7]. the the used Nano-sized cellular as unique X-ray level contrast using targeted physical agents computed can X-ray and tomography deliverof a [1– interest large number to of increase heavy atoms the tointegrated X-ray with a nanoparticle attenuation targeted X-ray region contrast and agents and sensitivity various active of targetingas strategies X-ray spectral (also known detection. molecular imaging) has Spectral shown imaging, in great studies promise of toward providing the quantitative dynamics information ofexample, biological tracking processes and the disease cell within a cell or tissue [6,8–12]. For 3 Results Contents 11 Introduction 2 Materials and2 methods 4 Discussion 10 2018 JINST 13 T02001 – 2 – Material discrimination in spectral imaging is further enhanced by the presence of discontinuity The potential of spectral imaging to discriminate and quantify multiple nano-contrast agents [11, In this paper, we demonstrate the feasibility of simultaneous discrimination of various concen- in the cross section of thephoto-electric photo-electric cross section effect is of called high-Z the contrastthat K-edge. agents. The occurs This K-edge when feature discontinuity is the in characteristic energy the ofelectrons an of in element incident an X-ray atom. photonsthat matches None is to of accessible the the in the binding elements relevantshow energy that diagnostic of distinct naturally energy range. occur K-shell K-edge Exogenous in feature high-Z a withinand contrast human agents the specifically which body quantify diagnostic have elements energy a withinmultiple range K-edge a energy can thresholds biological be (minimum specimen of used [18–21]. twoof to thresholds an are Spectral unambiguously element) required allows imaging capturing to with capture the K-edge the features K-edge of feature several elements simultaneously. 22, 23] within an image voxel allowsopportunity targeting of for multiple more bio-markers simultaneously sophisticated and tissue givesis an characterisation. usually inadequate Detecting for an only accurate atumor diagnosis [24]. single of tumor To cancer improve the marker since accuracy most oftiple are cancer tumor not diagnosis, markers specific it as to is it necessary a could tomultiple particular combine improve tumor the testing of bio-markers diagnostic mul- specificity at, [25 anand26 ]. treatment early monitoring For stage [27, example, in28], detecting which the insurvival clinical turn rates significantly course [29, improves the facilitates30]. treatment diagnosis efficiency and of cancer trations of iodine (I), gadoliniumspecific and (Gd) energy-dependent X-ray attenuation and properties. gold We present (Au) athe based methodology appropriate to contrast determine scanning agents, energy using thresholds in theirspectral an element- CT. energy-resolving Four photon-counting detector energy based thresholdsK-edge features are of used I, to Gd measurebalance the and the trade-off between Au. X-ray energy information attenuation and Optimal signal-to-noise onthe ratio selection (a either level measure of of that side a compares scanning of desired energy signalranking to thresholds method the to is evaluate level the needed of energy background to informationThen noise and in SNR we at the select various measurement). scanning high energy We rankedspecific thresholds. introduce spectral energy a imaging threshold for schemes simultaneous to discriminationa of demonstrate single I, scan. the Gd feasibility and of Au based element- contrast agents in 2 Materials and methods 2.1 Medipix based spectral CT imagingA system: MARS MARS scanner scanner (Marsspectral BioImaging Ltd, imaging Christchurch, system, New comprised(Source-Ray Zealand) of Inc, [31–33] is a Ronkonkoma, a NY, MARS pre-clinical U.S.A.) camera and [34–37], various a mechanical micro-focus components. X-ray source The X-ray mechanism of X-ray photons within diagnostic imagingimately energy proportional range to 30–120 the keV and third itnano-contrast (or is agents fourth) approx- can power facilitate of detection of the biomarkersbiological atomic at processes. number. low concentration The typically present presence in of high-Z 2018 JINST 13 T02001 (2.1) 128 array × 100% × 2 pixel substructure with × of X-ray signal on either side of the ratio , which is subdivided into a 128 2 m), the charge collected from a single photon cm µ – 3 – 1.408 × m. The CdTe sensor layer was configured as a pn-junction diode µ X-ray attenuation at preceeding energy range X-ray attenuation at K-edge-containing energy range m. µ = 50 ≈ K-factor In contrast to the SDNR measurement technique [41], the K-factor method reduces the uncer- The CdTe-Medipix3RX is a high-Z sensor based energy-resolving detector which has four A commonly used technique [41] to select the appropriate width of energy range for measuring In this study, we have introduced the “K-factor” as an indicator of energy information provided In X-ray detectors with small pixels (<500 For this study, we used a MARS camera fitted with a detector module incorporating a tainty of energy information as we are calculating the spectrum-distortion corrected energy thresholdsrealised per by pixel. setting the Physically, the referencethe energy threshold matrix. thresholds of The are the pulse pulse-heightproportional height to comparator obtained the circuit from energy of the of interaction allenergy the and of detected pixels the an applied photon. in energy X-ray thresholds The photon (energy relationship calibration)in with for between this the the the paper energy-resolving sensor detector was measured used is established photon according to Panta et2.2 al. [37]. Determination of scanning energyDiscrimination thresholds of elements inselection of an energy object thresholds. using Thisand is photon-counting signal-to-noise due to spectral ratio the CT (SNR) trade-offenergy-dependent between requires within information, energy-dependent an information judicious such energy as range. K-edgerange, feature the of K-edge A feature an narrow is element,greater energy washed whereas for range out a in preserves due wider a energy to the wideneeds range. data to energy averaging. This be taken contradictory into Conversely, requirement SNR account of when in energy selecting an information the and image width SNR of is energythe ranges. K-edge feature of ansignal element difference is is defined based between reconstructed onfeature. target signal region values difference The on to limitation either noise side ofwithin of ratio this simulation the (SDNR) technique K-edge framework. where is In the thatnoise practice, in it the the has resultant subtraction only image which of been adversely reconstructed affects studied the images using energy amplifies an information. the idealby the detector K-edge of an element: interaction is spread across several neighbouringeffect. pixels. The This Medipix3RX effect chip is implements a known charge aseffect summing the circuit by to charge processing correct sharing the for the charge charge sharing collectedinter-pixel by communication. the This pixel matrix correction using preservesindividual a both photons. 2 the Further spatial details and ofcan spectral working be information of found of Charge elsewhere Summing [38]. Mode (CSM) in Medipix3RX supplied with a negative high-voltage biasof (-600 12-bit V) provides for a collection dynamic of range of electrons. 0–4095 The counts counter per pixel. depth source used for this studyand has focal a spot tungsten of anode, total filtration of 3.8 mm aluminiumMedipix3RX (equivalent) ASIC [38] bump-bondedThe sensitive to area a of each standard chip wasof high 1.408 pixels resistivity with 2 a pitch mm of thick 110 CdTe sensor.

2018 JINST 13 T02001

X-ray intensity (N[keV cm (N[keV intensity X-ray at 1 m 1 at mAs]

-1 2 / X-ray intensity in 6 10 6 3.6 1.2 0 100% × × gold gadolinium iodine X-ray source spectrum – 4 – Energy (keV) 20 40 60 80 100 120

50 30 10 Mass attenuation coefficient (cm coefficient attenuation Mass /g) 2 . X-ray mass attenuation profiles [39] for iodine, gadolinium and gold with corresponding K-edge The spectral camera in a MARS scanner uses a CdTe-Medipix3RX detector [38] which has all energy ranges orthe entire quantum X-ray signal-to-noise spectrum). ratio.ranges The For of higher 27–33 example, keV and in the 33–49 keV Scheme relative provides a Dintensity X-ray maximum for (29 intensity, K-factor (668 iodine, the %) %), the however, relative in higher X-ray selectionBased K-edge of containing on energy K-factor energy and range relative (33–49) X-rayassigned intensity is to each in the of the poorest these K-edge among schemes containing as five energy shown schemes. range, in a table1. rank was four adjustable energy thresholds for correcting spectrum-distortion onfour each energy individual pixel. thresholds These need toAu be for selected capturing judiciously them on onthreshold either spectral scan. side schemes for of1 Table each theshows element examples K-edgesrelative of of and X-ray five I, the intensity different Gd trade-off possible (X-ray and between energy intensity energy in information (K-factor) a and particular energy range Figure 1 at 33.2 keV, 50.2 keV and 80.7tube keV respectively, operating and at X-ray emission 118 spectrumenergy kVp. [40] information obtained The from (depends choice an on ofsource X-ray spectrum). energy mass thresholds attenuation for profiles) K-edge imaging and is signal-to-noise the ratio trade-off between (depends on X-ray K-edge feature. A higherreducing K-factor implies the wash that out higher effect energy(spectroscopic) on information information. the has Similarly, K-edge the been feature. higher preserved the Thedefined by intensity higher as of the X-ray total photons K-factor, the number (X-ray intensity better of is in the photons/area/time) that energy in energy an range energy while range, usingspectrum the (120 the better kVp same and the spectrum 3.8 SNR mm (or of Alfunction radiation thickness), an of dose). and photon image mass energy.1 Figure attenuation K-edges coefficientshows of the of I,tively. I, X-ray Gd Gd The and and X-ray Au Au spectrum are as apparent was a at generatedcoefficient 33.2, of using 50.2 I, SpekCalc and Gd program 80.7 keV and [42] respec- Au and were the generated using mass Photon attenuation Cross sections Database [43]. 2018 JINST 13 T02001 intensity Relative X-ray K-factor – 5 – E 60–85, 85–118 170 3 10 3 E 20–35, 35–60 167E 4 35–60, 60–85 49 79 1 5 23 5 C 21–35, 35–54B 161C 5 35–55, 55–82 35–54, 54–85 139 33B 172 4C 3 3 55–82, 82–118 54–85, 85–118 193 37 130 40 2 4 5 3 12 10 1 3 B 25–35, 35–55 248 2 35 2 D 49–81, 81–118 204 1 12 1 A 32–52, 52–83D 185A 33–49, 49–81 2 52–83, 83–118 287 146 42 1 4 2 46 11 1 2 A 19–32, 32–52D 204 27–33, 33–49 3 668 35 1 2 29 4 E 20–35, 35–60, 60–85, 85–118 3 BC 25–35, 35–55, 55–82, 82–118 21–35, 35–54, 54–85, 85–118 2 4 D 27–33, 33–49, 49–81, 81–118 1 A 19–32, 32–52, 52–83, 83–118 2 Scheme Energy range (keV) Overall rank Gold Iodine Element Scheme Energy range (keV) Value (%) Rank Value (%) Rank . Determination of best energy threshold scheme for simultaneously discriminating the K-edge Gadolinium . Determination of ranks for various potential combination of energy thresholds based on K-factor and Based on the combination of individual rank of each scheme for all elements (I, Gd and Au), (K-edge at 80.7 keV) (K-edge at 50.2 keV) (K-edge at 33.2 keV) an overall rank wasoverall determined rank (1) as because the shown combination of inintensity individual rank ranks table2. based for on all the three For K-factor and elements relative example, is X-ray better Scheme than D that of has the schemes the (A, highest B, C & E). Table 2 features of I, Gddetermined and based Au based on on K-factorrated ranking. schemes and (Scheme relative B Overall and X-raydiscriminate rank Scheme intensity all D) for three are performance various contrast selected agents as schemes for (I, evaluating of shown Gd their and energy spectral in Au) imaging thresholds simultaneously. table1. performance was to Top two Table 1 relative X-ray intensity performance in K-edge containing energy range for I, Gd and Au based contrast agents. 2018 JINST 13 T02001 – 6 – . A schematic of multi-contrast phantom filled with various concentrations of Omnipaque (iodine For evaluating the spectral imaging performance of top two ranked energy threshold schemes (Scheme B and Scheme D in table1)and in Au, simultaneous spectral discrimination imaging of of the a K-edge features multi-contrast of phantom I, was Gd performed. 2.3 A multi-contrast phantom A multi-contrast phantom (diameterspectral of imaging. 33 mm) Using wascare, commercially fabricated Princeton, available for iodine-based NJ), performing Iohexol gadolinium-based (Omnipaque, gadopentateWhippany, element-specific NJ) dimeglumine GE and (Magnevist, 2nm Health- gold Bayer nanoparticles Healthcare, multiple (Aurovist, Nanoprobes, inserts Yaphank, NY) contrast (diameter agents, of36 8 mg/ml), mm) gadolinium were (2, filled 4calcium-chloride with (140 & & various 8 280 mg/ml) concentrations mg/ml) as of andA a surrogate schematic gold iodine of of (2, (9, bone, multi-contrast and 4 phantom 18 water is & as & shown a 8 in surrogate figure2. mg/ml) of based soft2.4 tissue. contrast agents, and Spectral imaging Spectral CT images of awith multi-contrast an phantom energy-resolving detector were (CdTe-Medipix3RX). acquired The usingsummarised settings used in a for table1. MARS spectral scanner imaging The are equipped X-rayspectrum-distortion-corrected tube spectroscopic was mode. operated at Flat 118 field kVp images and (with the no detector object was in operated the in beam) Figure 2 based), Magnevist (gadolinium based) andchloride solutions gold (Not nano-particle to scale). (AuNP) contrast agents, water and calcium 2018 JINST 13 T02001 (2.2) m. µ ) within a circular ROI σ 1000 are linear attenuation coefficient of the × A, 120 ms air 180 µ µ air , × water µ µ − water 180 − µ × , – 7 – water 50 voxel µ mg/ml), gold (2,(140, 4, 180 8 mg/ml) mg/ml), Water, calcium (OSEM) ≈ µ voxel µ = ) 180 3 m) 241 . The slice thickness at isocenter was 241 voxel µ m 3 µ . Technical settings for spectral CT scan. m HU µ m) 110 µ 180 Table 3 m) × µ 180 × is the HU of the voxel, the ) of the reconstructed CT image were transformed to relative X-ray attenuation in 1 300 voxels (N) were measured. The standard error of X-ray attenuation was calculated − ≈ voxel Slice thickness at isocenter ( Energy ranges (keV) for Scheme DFocal spot size 27–33, ( 33–49, 49–81, 81–118 Reconstruction algorithmReconstruction voxel size ( Ordered Subset Expectation Maximization Scan geometryPhantom insertsX-ray detectorCamera mode of operationMagnification factor Cone beam iodineDetector pixel (9, pitch ( 18, 36 mg/ml),X-ray exposure Spectroscopic CSM gadolinium (Charge (2, Summing Mode) 4,Total 8 filtration (Al) CdTe-Medipix3RX Energy ranges (keV) 1.34 for Scheme B 25–35, 35–55, 55–82, 82–118 118 kVp, (1.8 27 mm (inherent) + 2 mm (added)) . For the measurement of the relative X-ray attenuation (HU) of different regions of interest The projection data in each energy range was flat field corrected, processed, and reconstructed Spectral imaging performance of the two highest ranked schemes (Scheme B and Scheme D in For further data analysis, voxel values (absolute X-ray attenuation or linear attenuation co- N σ √ as (ROIs) containing contrast agents, the mean value and standard deviation ( voxel, water and air in the reconstructed image, respectively. comprising where HU simultaneously using Ordered Subsetdimensions Expectation of 180 Maximization (OSEM) technique with voxel table3) were evaluated by comparing the energy informationindividually. (K-factor) and Mass QSNR attenuation for coefficient each for element each high-ZK-factor element of was both calculated schemes. for estimating Flat the fieldcalculate images the (720 SNR images at in total) K-edge-containing energy for ranges each for scheme each were element averaged to individually. efficient (cm were acquired before and afterdetermined scanning by the the chosen multi-contrast scheme, phantom. were collected simultaneously. Spectrally resolved images Hounsfield Units (HU) using equation 2.2: 2018 JINST 13 T02001 (b) Scheme B (25-35,35-55,55-82,82-118 keV) Scheme D (27-33,33-49,49-81,81-118 keV) Signal-to-noise ratio (SNR) iodine gadolinium gold

6 5 4 3 2 1 0 Signal-to-noise ratio (SNR) ratio Signal-to-noise – 8 – (a) K-factor Scheme B (25-35,35-55,55-82,82-118 keV) Scheme D (27-33,33,49,49-81,81-118 keV) iodine gadolinium gold 0

80 60 40 20

140 120 100 K-factor (%) K-factor . Evaluation of spectral imaging performance of two energy schemes (Scheme B (energy range: 25– Figure 3 35, 35–55, 55–82 and 82–118 keV) andare Scheme rated D 2 (energy & range: 1 27–33, respectivelyratio 33–49, as (SNR). 49–81, shown 81–118 in Figure keV) table2) which 3(a) basedshowscontrast on that K-factor agents. (energy Scheme information) Similarly, D figure and signal-to-noise 3(b) hascontrastshows higher agents that than K-factor Scheme Scheme for D B. iodine, has However, Schemeon gadolinium better B K-factor and and SNR has SNR gold for better for gadolinium SNR based all and for contrast iodine gold agents, based based Scheme contrast D agents. has Based marginally better spectral imaging performance. 2.5 Material decomposition Material decomposition (MD) algorithmray in attenuation spectral of CT eachbasis exploits materials voxel in the inside the reconstructed energy the materialconstrained dependence volume. linear object least Material of square from algorithm decomposition [44] X- based was multiple used onenergy on the energy ranges. reconstructed MARS images MARS using ranges MD the algorithm subtracted to splits theattenuation total determine of attenuation coefficient several the into predefined the basis individual X-ray materialsattenuation of for various a materials at given multiple energy. energy ranges, MD Basedmaterial. assigns on the voxel Thus to the the MD pattern corresponding algorithm of generates X-ray the basis material image on a basis. voxel-by-voxel 3 Results 3.1 Evaluation of spectral imaging performance K-factor and SNR were used asscanning metrics energy to thresholds. compare Figure the33–49,3(a) spectralshows imaging 49–81 that performance and K-factor at of 81–118 different keV) Scheme55–82 is D and higher (energy 82–118 range: keV) than for 27–33, thatSNR all of of Scheme three Scheme D elements is B slightly (I,gadolinium (energy higher Gd and than range: gold, that ad of but 25–35, Au). Scheme lower B 35–55, consistent for for qualitatively iodine. Similarly, with K-edge containing figure theoretical The energy spectral 3(b) spectral ranges imaging of imagingshows performanceshows that as performance that spectral presented of imaging in both performance table2. of schemes Scheme This data is D analysis is comprises marginally only better the than spectral Scheme data B. acquired Further using Scheme D. 2018 JINST 13 T02001 300 ≈ 7–35 HU. ± 2500 2000 1500 1000 500 0 -500 -1000 HU 33-49 keV 81-118 keV – 9 – 49-81 keV 27-33 keV . Spectral CT images of multi-contrast phantom containing iodine (9, 18 & 36 mg/ml), gadolinium The performance of the imaging scheme was assessed for different concentrations of each voxels in each ROI. The standard errorX-ray attenuation of of X-ray attenuation all within contrast different agentsenergy ROIs at was range, all in concentrations which is the maximal attenuationrange. for is the greater K-edge-containing X-ray than that attenuation in33–49 is keV, the 49–81 maximum preceding keV and and for succeeding 81–118 all keV energy corresponding respectively. concentrations element. of This The is iodine, maximum K-factors becauserespectively. gadolinium for I, of and These Gd the results gold and K-edge show Au at effect that werecan 157 of each be %, discriminated of the 153 using the % spectral K-edges and CT. for 125 I, % Gd and3.3 Au based contrast agents Material decomposition The basis material imagein was each generated image voxel by as expressing a the linear energy-dependent combination X-ray of attenuation the X-ray attenuation of several predefined basis Figure 4 (2, 4 & 8 mg/ml) andof gold 140 (2, 4 & & 280 8values mg/ml) mg/ml), of based each and contrast voxel agents, soft is and shown tissue-like bone-like in material (water) Hounsfield (calcium-chloride Unit material (HU). at different energy3.2 ranges. X-ray attenuation Discrimination of multiple K-edges Spectral CT images acquired using Schemedistinct D energy are ranges shown are in figure4. shown.iodine (9, The 18 The images & acquired spectral 36 in mg/ml), CTagents, four gadolinium images (2, and 4 of calcium-chloride & multi-contrast 8 (140 mg/ml) phantomfor & and including soft gold 280 (2, tissue mg/ml) 4 as are &energy-dependent 8 surrogate X-ray shown mg/ml) attenuation for based in values. contrast bone, figure4. The and presenceto of inconsistency water of Each ring as signal artifacts CT surrogate collection is in image visible, some these in pixels are during each due scanning. energycontrast range agent. Figures possesses5(a)–5(c) unique present thedifferent mean concentrations values of I, of Gd relative and X-ray Au attenuation contrast agents. (HU) within The mean values were calculated over 2018 JINST 13 T02001 7–35 HU. (d) Gd (2 mg/ml) Gd (4 mg/ml) Gd (8 mg/ml) ± (d) (b) Energy range (keV) 27-33 33-49 49-81 81-118 gadolinium (K-edge at 50.2 keV) gadolinium 0

450 400 350 300 250 200 150 100 Relative x-ray attenuation (HU) attenuation x-ray Relative – 10 – I (9 mg/ml) I (18 mg/ml) I (36 mg/ml) Au (2 mg/ml) Au (4 mg/ml) Au (8 mg/ml) (c) (a) Energy range (keV) Energy range (keV) iodine (K-edge at 33.2 keV) iodine (K-edge gold (K-edge at 80.7 keV) 27-33 33-49 49-81 81-118 27-33 33-49 49-81 81-118 . (a) Relative X-ray attenuation of various concentrations of iodine based contrast agent showing 0 0

500

2500 2000 1500 1000 500 400 300 200 100 Relative x-ray attenuation (HU) attenuation x-ray Relative Relative x-ray attenuation (HU) attenuation x-ray Relative Figure 5 K-edge enhancement at 33–49 keV (b)based Relative contrast X-ray agent attenuation showing of K-edgeconcentrations various enhancement of concentrations gold at based of 49–81 contrast keV gadolinium agent (c) showingin K-edge Relative the enhancement measurement X-ray at of attenuation 81–118 relative keV. X-ray attenuation of The of various various standardMaterial concentrations error decomposed of contrast basis agents is image based onwater, MARS iodine, constrained gadolinium, linear gold least square andthe algorithm calcium same to contrast energy differentiate agents. range selection The as4, in 8 material5(a)–5(c). mg/ml; decomposed yellow: image Purple: gold is at iodine based 2, at 4, on 9, 8 18, mg/ml 36 and mg/ml; White: green: calciummaterials gadolinium at at 140, (water, 2, 180 I, mg/ml. Gd, Au,iodine, gadolinium, Ca). gold and calcium Figure based5(d) attenuation contrastshows properties. agents based the on A basis their different image energy-dependentThe false X-ray which higher color decomposes: the was concentration assigned of water, to materials, each the of brighter the the color decomposed display. materials. 4 Discussion Spectral X-ray imaging isability being to developed provide as specific a and new quantitative material X-ray imaging information. modality This due allows to tissue-type specific its unique 2018 JINST 13 T02001 clinical applications whether as in vivo has been achieved using quantum dots [24, in vivo – 11 – molecular imaging of atherosclerosis or cancer in a mouse model. The importance of in-vivo To readily identify a specific element by using its K-edge feature in spectral imaging, the Near-term applications of element-specific spectral imaging of high-Z contrast agents could Simultaneous imaging of multiple biomarkers energy ranges should be narrow but wellpreserve separated. energy Each information energy (or range should K-factor) be butto-noise narrow should ratio. enough be to wide Moreover, the enoughthe to global minimal provide spectral practical adequate resolution energy signal- of widthdetector a of broadens detector an the is energy energy range. also bordersranges. a and The limiting increases finite factor the global So, for cross-talk spectral forrange effect resolution is K-edge between of a adjacent imaging, a trade-off energy selecting betweenand energy the validated information right a and energy signal-to-noise rankingcombination ratio. thresholds methodology of or to energy This thresholds width study evaluate introduced schemesselecting the of appropriate based energy spectral an thresholds, on simultaneous imaging energy energy discrimination of performance informationI, multiple Gd and K-edge of and features Au of SNR. based various contrast Similarly, agents was by demonstrated.energy This thresholds methodology presented of determining in appropriate this paperbut is can not be limited easily extended to to only other I, element Gd based and contrast Au agents based as contrast well. agents purely diagnostic agents or combined with therapy (theranosticwith regulatory agents) hurdles. is a There difficult are task many nanoparticle and cancerAt fraught therapies least approved three for clinical metal use nanomedicines [46]. have completed clinical trialsnanoparticles [47] are and undergoing iron, clinical silica and trials gold [46]. based agent [48] Gold but is many more the metal most based extensively nanocontrast studiedplatinum, agents are nanocontrast hafnium, being investigated yytterbium, including bismuth, tantulum, andnanoparticles yttrium has [11]. been An pursued effort byagents to approved. some combine This in functionality approach in part has because lediron, to gold, of the fluorophores, the radiotracers, development difficulty of gadolinium, nanoparticles manganese inby are with combinations getting suitable combinations of nanocontrast for of MR, multimodal CT, imaging PET, SPECT, ultrasound and optical imaging [49–51]. 45], a new form ofclinical fluorescent use label. [46]. Translating There metal are nano-contrast agents many to nanoparticle cancer therapies approved for be in imaging. If spectral imaging is integrated withanatomical targeting of tomographic biomarkers, images simultaneous functional in and apossible. single scan, In that this is paper, quantitativefeatures we molecular and demonstrated imaging, energy-dependent simultaneous becomes X-ray discrimination attenuation of featuresbased element-specific of contrast K-edge commercially available agents I, in Gdappropriate a and scanning single Au energy scan. thresholds inbetween an We energy energy information developed resolving and a detector signal-to-noise ranking ratio. by balancing methodology the to trade-off determine the discriminating multiple K-edge features lies in thethe possibility disease to (by simultaneously using identify specific and functionalisedthe locate immune nano-particles response to using target nano-particles the specific bio-markers todelivery the of in immune a cell a disease), (e.g. single macrophages), and scan.agent drug may Furthermore, have clinical the relevance ability in toexample, simultaneous the discriminate entire selective cardio-vascular enhancement more system might of than be enhanced different one with regions.atherosclerotic a high-Z blood-pool plaques contrast For contrast might agent while be identified withbiomarkers a of different high-Z atherosclerotic material lesions nanoparticle like targeted the to necrotic lipid core or intraplaque hemorrhage. 2018 JINST 13 T02001 (2006) 248. 79 Brit. J. Radiol. , (2007) 7661. 129 – 12 – (2008) 83. (2006) 118. (2007) 797. 5 26 42 J. Amer. Chem. Soc. , We demonstrated the feasibility of simultaneous discrimination of Nature Mater. , Invest. Radiol. Gold nanoparticles: a new x-ray contrast agent , Nature Biotechnol. Colloidal gold nanoparticles as a blood-pool contrast agent for X-ray computed , An X-ray computed tomography imaging agent based on long-circulating bismuth In vivo tumor targeting and spectroscopic detection with surface-enhanced raman Antibiofouling polymer-coated gold nanoparticles as a contrast agent for in vivo X-ray tomography in mice sulphide nanoparticles nanoparticle tags computed tomography imaging The key property of a contrast agent that determines the magnitude of X-ray attenuation is the The K-edge discrimination using a spectral imaging system also depends on many parameters [3] Q.Y.K. Cai et al., [1] O. Rabin et al., [5] X. Qian et al., [2] J. F. Hainfeld et al., [4] D. Kim et al., multiple contrast agents by exploiting their element-specificattenuation (K-edges) and properties energy-dependent X-ray in ascanner). single A scan ranking using methodology for a selecting appropriateresolving pre-clinical scanning detector energy photon-counting was thresholds developed and spectral in used an CT to energy measureof (MARS the iodine, K-edge features of gadolinium various and concentrations goldmultiple based contrast agents contrast at agents. the concentrations The relevantnew to ability possibilities biomedical to for application imaging is simultaneously multiple likely discriminate to functional open processes up in a single scan. Acknowledgments The authors would like to acknowledge Medipix2 and Medipix3 collaborations. References atomic number (Z). The highergreater the the Z differential of attenuation, aincluding the contrast other easier agent, contrast to the agents. separate greater thegadolinium the (Z Three contrast = X-ray high-Z 64) agent attenuation. and elements from gold used The other (Zat = low in materials 79). concentrations, this sufficient A study to contrast agent enhance are:the should image ideally contrast K-edge iodine have within high of the (Z X-ray attenuation imaging = iodine energylower 53), range. (at energy Since border 33.2 keV of where thefor X-ray human human attenuation imaging diagnostic because increases energy of range, abruptly)region. photon iodine lies starvation Similarly, is (causes relatively near few not poor photons to signal-to-noise anthe are ratio) the ideal 80.7 produced at keV contrast K-edge by the of X-ray agent K-edge gold, sources which usedhand, reduces in human the human CT sensitivity CT of has above K-edge awhich discrimination. increases relative On the abundance the sensitivity of other of photons K-edge discrimination. above the Gadolinium K-edge ofsuch 50.2 as keV spectral characteristics andradiation on noise the performance measured of X-ray spectrum, anand atomic X-ray energy number tube resolving of settings detector, and sensor filtration. scattered materials,factors size To need provide of to sufficient the be sensitivity sample, taken at into the account. molecular level, these Summary and conclusion. 2018 JINST 13 T02001 8 , 14 Nano Lett. 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