Bone volume density (BV/TV) – Light version

The usual suspects

Episode 1

BV/TV, Tb.Th., Tb.Sp. and the other usual suspects: what do they really mean?

If you are asking yourself this question, you should definitely read this! Coming soon in the same series: Trabecular Thickness (Tb.Th) and Trabecular Spacing (Tb.Sp), Bone surface density (BS/TV), Structure Model Index (SMI), Connectivity Density (Conn. D), and others. The “Light version” series is intended to those who are just approaching the field, or to those who are looking for a simple way to explain these concepts to others. If you are a professional, please insert your comments to help us making this post more complete and interesting.

So let’s start from BV/TV!

Bone volume density

BV/TV stands for Bone Volume over Total Volume.

It indicates the fraction of a given volume of interest (VOI, i.e. the Total Volume TV) that is occupied by mineralized bone (Bone Volume). BV/TV is usually reported as a % value. This index is probably the most known and the most used output of a microCT analysis performed on bone.

To make it simple, if the glass is your TV, the glass with no water has 0% water volume density, the middle glass has 50% water volume density, and the full glass has 100% water volume density (ok, maybe 98%….). I guess you never wish a 100% bone volume density though… It is clear that in the calculation of BV/TV, the appropriate selection of the TV plays a key role (read the post Choose the right Volume of Interest!).

Why is BV/TV useful?

It can be used to evaluate relative changes in bone volume density following a given treatment.

Let’s take some examples:

EXAMPLE 1: Evaluation of the effects of an anti-osteoporosis drug

Your preclinical study will most likely comprise a negative control group (e.g. OVX) and a treatment group to whom the drug is administered. One way to determine the efficacy of the drug is to compare BV/TV in defined VOIs in the control vs treatment group. You would expect that if your drug works, the treatment group shows higher BV/TV values than the control group. In the figure below a similar case is presented where one group was ovariectomized and the other one underwent sham control (the image is property of b-cube)

EXAMPLE 2: Integration of implant in bone

You have a great idea for a new coating that should promote the implant integration in bone. To prove the concept, you run a preclinical study where a group is treated with the non-coated and another group with the coated implant. A possible way of evaluating the efficacy of the coating is to analyze BV/TV in a VOI surrounding the implant. Hopefully, you will see that the non-coated implant group has lower BV/TV than the coated implant one. There is much more you could look at, like for example implant/bone contact surface, but we will talk about that in another post. In the figure here below we are showing the case of a dental implant (blue) and the bone/implant contact surface (brown) (the image is property of b-cube).

What are typical values for BV/TV?

BV/TV depends on the species (human, mouse, sheep, etc.), on the type of bone, on the location within a bone, and on the health status of the subject. You can get a good idea of typical values for human bone from this publication.

Qualitative vs quantitative analysis:

We all like a nice graph showing statistical difference between two bars. Mission accomplished, next task.

But…it is almost never that easy. BV/TV and microCT indices alltogether are only a piece of the whole picture. And microCT “numbers” are only a piece of a piece of a picture.

Sometimes the value of images is underestimated. Looking at representative 2D slices of the microCT scan can actually reveal lots of interesting information that in some cases can be even more useful than the straight BV/TV number. The images may give you qualitative input on the bone healing mechanism of action, on how a spinal fusion looks like, on where bone is localized within the VOI, on how microCT images compare with histology sections. Sure, it is more work, but it is worth if you really want to understand “what’s going on”.

Not always straight-forward:

For microCT professionals this may sound obvious, however it is important to point out that the “magic” BV/TV number spit out by the scanner can vary significantly with the methods used to perform the analysis. So, for different measurements on the same sample, BV/TV will vary if you select one or another Volume of Interest, if another material present in the VOI creates x-ray absorption artifacts, if different scanning settings have been used, etc. The best way to reduce the risk of result misinterpretation, is to use consistent methods, and possibly an automated analysis software that eliminates the bias of the operator. When you compare results from different studies, make sure you check how the results were obtained and if a comparison makes sense.

Questions?

We hope you have found this post useful. By posting a question, suggestion, or criticism you’ll help us and the other readers. Thanks!

18 thoughts on “Bone volume density (BV/TV) – Light version

  1. Pingback: Choose the right Volume Of Interest – Light version |

    1. Phil Salmon

      Not much. In fact some backscatter SEM even suggests trabecular bone is very slightly more dense than cortical. However if you see a lower voxel grey level in trabecular than cortical bone in a micro-CT image, it is likely to mean that, due to the partial volume effect and the limit of resolution, the density of trabecular bone is under-reported in the image. Quite high resolution (< about 3-4 microns voxel) is be needed to show the true density of mouse or rat trabecular bone.

      This is one of the reasons why it is customary to measure BMD in trabecular bone averaged together with the marrow soft tissue. Conversely cortical bone density, thick enough to be unaffected by partial volume effect, is measured for the mineralised tissue alone, excluding the surrounding tissue, and has recently (Bouxsein et al 2010) been given the term TMD (tissue mineral density.)

      Reply
    2. Stacey Russell

      Of course! Specific gravity is just the ratio of the material density to the density of a reference material (usually water). Since cortical and trabecular bone have very different densities, they will have different specific gravities.

      Reply
    3. Enrico
      Post author

      Ben, does your question refer to bone tissue itself or more to the bony structure, i.e. to a representative volume including trabeculae, marrow, etc.?
      Also, is there a rationale for talking of specific gravity instead of density?
      Thanks!

      Reply
  2. Phil Salmon

    I like the article and the concept behind it, thanks!

    BV/TV stands for Bone Volume over Total Volume.

    Actually according to the ASBMR nomenclature for histomorphometry (Parfitt et al 1987) the TV stands for “tissue volume” since much histomorphometry was done on clinical biopsies. But it is generally more helpful to use “total volume”.

    The volume of interest (VOI) that you describe around the metal bone implant screw is not exactly for beginners I guess – it involves several steps of binary mask dilation combined with Boolean operators. Or do you just set it manually?

    Reply
    1. Enrico
      Post author

      Thanks for the reference on TV Phil!
      I will let my colleagues who actually did the measurement on the bone/implant interface answer on that.

      Reply
    2. Martin

      Dear Phil

      I agree that the segmentation of a screw implant in bone is not an easy task and needs advanced image processing steps. We consider writing a post to explain how we do it.

      In this post, we were using this example to explain that the VOI is not necessarily a simple geometrical volume but can have a complex shape.

      Reply
  3. Pingback: The surface of an object. Bone surface (BS) and specific bone surface (BS/BV) |

  4. Pingback: Trabecular Thickness (Tb.Th), Trabecular Spacing (Tb.Sp), Trabecular Number (Tb.N) |

  5. Holly

    Hi Phil,

    I’ve only just discovered this website, and wanted to ask you about one of your follow-up comments regarding trabecular bone possibly being slightly more dense than cortical bone. Do you have any literature references for this? As I find it very interesting.

    Thanks and BW,

    Holly

    Reply
  6. Dr Dimitar Peev MD

    Hello, my name is Dr. Dimitar Peev – Medical Doctor from Bulgaria.
    Colleagues, intresuvam of software trabecular bone density osteodensitometar DEXXUM-T /OSTEOSYS/ Korea.
    Please direct me to a company that offers compatible with my handset software TBS bones.
    Thanks in advance!
    Happy Holidays!

    Reply
    1. Wafaa Fathi

      if possible to send me a software of trabecular bone density to help me measure the samples of an experimental bone for my PHD research. please direct me to a company that assist me to measure bone density BV/TV, trabecular thickness and seperation, trabecular number .
      with great thanks

      Reply
  7. Rodolfo Puche

    I would greatly appreciate receiving the reference of the paper that estate the the relative mineral density of cortical bone ranges between 0.7 and 1.0 and that a trabecular structure (plates) is circa 0.2. I thank you for this courtesy,

    Reply
  8. Dr. wafaa khalil

    Hi,
    pleases if possible to help me to gain a software to measure bone density and volume with trabecular number, thickness and separation or to direct me to a company that provide such a software to assist me to measure the experimental bones related to my phd research.
    with my best regard

    Reply
  9. Neilab

    This episode is extremely helpful! I’m using MicroView to analyze some mouse femurs. I am wondering whether BV/TV always means trabecular BV/TV or if there is a total bone BV/TV parameter as well. Any help would be appreciated.

    Reply
    1. Martin

      Dear Neilab

      In general, BV/TV refers to bone volume inside a given region of interest; the total volume (TV). The total volume can be anything and thus is not restricted to trabecular bone. However, if it is expanded to other compartments it is better to use other terms to avoid confusion. For the full mouse femur we typically use AVD (apparent volume density), which is BV/TV with TV representing everything inside the outer boundary of the femoral bone. For the cortical compartment we typically use %BV (cortical volume density).

      Please check out the publication below for standardized terms to be used in microCT in the bone field:
      http://www.ncbi.nlm.nih.gov/pubmed/20533309

      Martin

      Reply
  10. Hila Chase

    Question: I understand the concept of BV/TV fairly well but am new to Avizo and for the life of me cannot figure out how to compute it using the software, since I’ve been learning it pretty on-the-fly.
    Any help? (using uCT scans of trabecular bone).

    Thanx =]

    ~Hila~

    Reply

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