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  • Help regarding CT phantom - Responses

    Dear Biomch-L subscribers,

    You will find below responses to my post
    regarding methods for creating hydroxyapatite
    calibration phantoms. Thanks to all those who
    responded. Several labs choose to use
    commercially available phantoms to avoid issues
    pertaining to accuracy, precision and
    homogeneity. I have divided the responses up into
    those recommending commercial phantoms and those
    regarding methodology for custom phantoms.



    Original Post:

    Our lab is interested in creating a calcium
    hydroxyapatite calibration phantom with a range
    of densities so that we may relate CT attenuation
    data to equivalent hydroxyapatite density. As an
    initial step, we suspended hydroxyapatite powder
    in a gelatin solution. Unfortunately, the
    hydroxyapatite settled out before the gelatin set
    in. We would greatly appreciate any information
    pertaining to 1) substances for hydroxyapatite
    powder suspension, 2) ensuring uniform density,
    and 3) any other helpful tips or tricks you have
    picked up through experience. References on the topic would be helpful as well.

    Responses (commercial phantoms):

    * I am not very familiar with the technology,
    but I know that Scanco makes pre-fabricated
    phantoms for use in their CT scanners. They have
    a range of densities that I think are equivalent
    to particular densities of hydroxyapatite. Might be worth a look.

    Sean Osis
    Human Performance Lab
    University of Calgary

    * Are you aware of the commercial phantoms
    available this (Image Analysis, Mindways, plus
    many others)? If not, I'd recommend going with
    that route unless you have scientific reasons to
    not use these phantoms. This is because precision
    and stability are important issues in all this
    and these commercial phantoms have evolved over
    the years to address these issues.
    Good luck with your project.
    Tony Keaveny

    * This company makes commercial calcium
    hydroxyapatite phantoms. The old one had 0, 50,
    100 and 200 mg/cc concentrations. New ones might
    be different. I bet they could find an old one on
    ebay from a center that no longer does Coronary
    calcium scans with the phantom underneath the

    Responses (methods for custom phantoms):

    * A good strategy is to keep the gel moving
    during solidification, by using a roller mixer or
    a 3d mixer. In our group
    ( we
    managed to get uniform dispersions of ceramic
    microspheres in gelatin gels by rolling the
    suspensions in a tin until the gelatin
    solidified. It is important to fill the tin
    completely and to roll it slowly to prevent the
    formation of bubbles in the gel. To give you an
    idea we rolled a 0.5 l tin for about 3 hours.

    Best regards,

    * I've never worked with hydroxyapatite
    before, but I have created gels for MRI phantoms,
    which simulate electrical properties of human
    tissue. We use HEC-based gel recipes
    (hydroxyethyl cellulose). I don't know if this
    will give you the type of gel you need, but it
    might be worth looking into. We make liquid gels
    with a viscosity of 12000 - 17000 cP at room
    temperature. HEC powder turns to gel of this
    viscosity quite quickly (within 10 minutes of
    mixing) when mixed with water. Roughly 2% HEC to DI water by weight.

    Kieran Coghlan, BSME, MSES, MSBME

    * You can also use test tubes containing
    different concentrations of di-potassium hydrogen
    phosphate solution (e.g. 64%, 32%, 16% and 4%)
    prior to scanning in order to calibrate the grey
    values of the CT-scan images. The solutions do
    not interfere with the attenuation of X rays of
    the tissues and there is a direct relationship
    between the Hounsfield units and the
    concentration of the dipotassium hydrogen
    phosphate solution. After scanning, the average
    values of the Hounsfield units corresponding to
    each test-tube can be noted. The calibration of
    di-potassium hydrogen phosphate solution will
    make it possible to convert the grey values at
    different locations of the images to Hounsfield
    units, based on the linear relationship.

    Good luck.

    Rajshree Mootanah, Ph.D, MBA
    Visiting Scientist, Leon Root, M.D. Motion Analysis Laboratory
    Hospital for Special Surgery
    Director, Medical Engineering Research Group, Anglia Ruskin University
    535 East 70th Street
    New York, N.Y. 10021
    Lab: (212) 606-1215
    Fax: (212) 774-7859

    * Encountering the same problems as described
    in your mail we used coconut oil instead of
    gelatin solution. Being solid, or at least
    viscous, at room temperature it could be molten
    easily and returns to the solid state after
    cooling, while x-ray absorption is close to zero.
    Careful stirring during the cooling process keeps
    the mixture as homogeneous as possible (depending
    on your HA grain size) and careful centrifugation
    removes remaining voids. We used small Eppendorf
    tubes for each HA concentration so the phantom
    can easily adapted to the scanned material. And
    don't forget to put something no x-ray
    translucent into your Phantom for orientation and
    identification of the test volumina...

    I hope this was a little help for you,


    * Available hydroxyapatite phantoms are
    mostly based on water-equivalent epoxy resins.
    The tricky secret is the way you disperse
    uniformly the powder within the resins, which is
    not so easy. After some experiences in home-made
    resins phantoms for morphometric microCT (Perilli
    E et al, A physical phantom for the calibration
    of three-dimensional X-ray microtomography
    examination, J Microsc. 2006 May;222(Pt
    2):124-34) we decided to buy our densitometric
    phantoms from specialized providers of quality control tools in radiology:
    * European Spine Phantom for clinical CT
    * QRM phantoms for in-vitro microCT ( )
    Phantoms are normally provided with accuracy
    certifications in hydroxyapatite content.

    Hope it helps, Fabio Baruffaldi

    * Here you find work done at our institute
    some while ago: - Nazarian A, Snyder BD,
    Zurakowski D, Muller R. Quantitative
    micro-computed tomography: a non-invasive method
    to assess equivalent bone mineral density. Bone
    43: 302-311 (2008). - Schweizer S, Hattendorf B,
    Schneider P, Aeschlimann B, Gauckler L, Müller R,
    Gunther D. Preparation and characterization of
    calibration standards for bone density
    determination by micro-computed tomography. Analyst 132: 1040-1045 (2007).

    Best regards

    * I carried out some work on the creation of
    high solid content low viscosity hydroxyapatite
    slurries as part of my PhD studies. The work
    involved both the manipulation of particle size
    distribution and the search for an optimum
    dispersant to aid both steric and electrostatic
    repulsion. In particular the dispersant results
    may be of interest to you. In the end we found
    the optimum dispersant for our HA powder was
    Ammonium polyacrylate. The work was published and
    can be found by searching for:
    ‘High-solid-content hydroxyapatite slurry for the
    production of bone substitute scaffolds’.
    Proceedings of the Institution of Mechanical
    Engineers, Part H: Journal of Engineering in
    Medicine, 2009. 223(6): p. 727-737.If this info
    can be of any use and you have difficulty finding
    the paper I can send it to you. If you have any further queries please ask.

    Best regards

    Dr. Eoin Cunningham
    Research Assistant
    School of Mechanical & Aerospace Engineering
    Queen's University
    Ashby Building Stranmillis Road
    BT9 5AH
    Telephone: 02890 974782

    W. Brent Edwards, Ph.D.
    Postdoctoral Research Fellow
    Department of Kinesiology and Nutrition
    University of Illinois at Chicago
    1919 W. Taylor Street
    650 AHSB, M/C 517
    Chicago, IL 60612

    Tel. 312 996 1582