Patent Application
20120046538
This invention relates to the field of convection-enhanced drug delivery (see U.S. Pat. No. 5,720,720) and estimates the expansion of tissue upon pumping of an infusate into the tissue.
In convection-enhanced delivery of drugs in solution into brain parenchyma in particular, the expansion or edema of the tissue is one of the most important determinants of the distribution of the infusate. The extracellular volume fraction of tissue can rise from a nominal 0.2 to 0.7 or so. This very dramatic increase in the fraction of fluid containing extracellular space can increase the hydraulic conductivity by orders of magnitude, thus directing flow of fluid into such edematous spaces in favor of other channels which are much more restricted because of smaller relative extracellular volume.
It is thus important to predict which regions of tissue, in particular brain tissue, are expanded and by how much, for given flow rates of infusate. As important as predicting the volumes of the nervous system that would readily expand, is to actually predict those places that cannot expand due to anatomic or physiological reasons. Those places that cannot expand would likely act as a barrier to fluid flow since other regions around them may become more conducive to the fluid flow.
The present invention provides anatomic-based methods to predict expansion coefficients in different regions of tissue.
The present invention as specified in this description and, in particular, as defined by the appended claims, affords advantages over prior art methodologies. A method according to the invention may comprise any of the features herein disclosed, and any sensible combination of one or more of such features.
For moderate infusion rates (less than 5 microliters per minute for example), the grey matter regions of the brain do not show edema, while the major white matter tracts away from the corpus callosum do show edema. This invention makes such knowledge more quantitative, and defines the expected extracellular space under such conditions of infusion.
For the special application of delivering cells to tissue, it has long been the question where cells would stop migration and settle down. The information about the presence of crossing nerve fibers could be used to deduce and compute those locations and hence be able to estimate the pathway and target of cell migration. The information generated thereby can be relevant for cellular treatments of any kind, including the treatment with stem cells, and including the development and execution of therapies for neoplastic and neurodegenerative diseases as well as epilepsy.
Similarly, the information can be used to derive the migration pathways and the target of viruses or viral vectors present within or delivered to tissue. For example, the treatment of Progressive Multifocal Leukoencephalopathy, an infection of the brain that occurs primarily in immunosupressed patients, may be significantly improved since from a visible lesion the presences of other likely masses of virus can be predicted.
Another useful embodiment of the invention includes the simulation of not only the migration of tumor cells, but more importantly their location of settlement. For example, brain tumor cells choose the white matter as their routes of distribution within the brain, causing distant recurrences of tumor. The information about the presence of fiber crossings can enhance the existing view to add the points of cancer cell settlement, allowing for a pre-emptive strike against the now foreseeable locations of recurrent tumor.
Especially in terms of identifying locations of tumor recurrence, speed of migration and space play important roles. Speed of migration, as described above, can be deduced by knowing the locations of blockages for migrations. The available space can be computed by using image processing methods such as those herein described, especially extracting the resting pore fraction from the images.
Knowing the resting pore fractions and being able to predict and manage them is another important factor in the treatment of many different diseases. For example, in brain tumors or multiple sclerosis, a disruption of the blood-brain-barrier results in a local increase of interstitial pressure and hence in the formation of edema (swelling) around the volumes that are affected by the disease. By combining medication that attacks the disease with medication that globally or locally reduces the swelling of the brain, one can manage the therapeutic effect of an infused agent for medication, while also actively influencing any spreading and/or settling of the disease in distant volumes and locations.
In terms of the infusion of fluid, it is also obtainable from infusion imaging (for example, imaging of a contrast agent that is infused or co-infused into tissue as in
An exemplary flow chart of an embodiment of a method according to the invention would be as follows:
The attached figures illustrate various aspects of the invention. They represent Pig Experiments with reference to the infusion of contrast agents using Convection-Enhanced Delivery (“CED”).
In the following, methods for predicting a pore fraction are described. In this respect, it is to be noted that such “pore fraction” is a term synonymously used for the term “extracellular volume fraction” as used herein elsewhere.
It appears that the expansion is highly localized to the white matter, with little observed change in gray matter regions. Second, the expansion appears to be very significant within the white matter, with the exception of the tightly-packed fibers of the corpus callosum. It is fairly uniform where it occurs, with a sharp boundary between the edematous and normal white matter regions.
The location of the expansion due to infusion appears to match the area infused, in regions of white matter. It might appear that it is necessary to estimate the infusion extent before the pore fraction expansion can be estimated. However, it is reasonable to estimate a pore fraction expansion everywhere, as if the infusion covered the entire brain. This will be incorrect where there is no infusate, but there will be no subsequent infusion simulation at these locations anyway. Thus, the pore fraction change corresponds to a correction for edema covering the entire brain. The estimation is done as follows, at every voxel in the imaging volume.
1. Estimate the fraction of unexpanded white matter, fWM, Contained in the Voxel.
In one embodiment, the unexpanded white matter fraction can be estimated in an ad hoc fashion, using the fractional anisotropy, FA, of the diffusion tensor. One method is to use a linear ramp: fWM=FA/FAmax for FA=FAmax and fWM=1 when FA>FAmax. In order to allow a smoother transition, the ramp can be replaced with a sigmoid function:
f
WM=1/(1+e−s(FA−FAmax/2))
This function yields a white matter fraction of 0.5 when FA=FAmax/2. On either side of this value, the white matter fraction heads asymptotically towards 0 or 1. Parameter s determines the steepness of the approach. We have used s=20, and FAmax=0.4.
2. Modify the Local Pore Fraction by Proportionally Increasing the Value in Proportion to the Local Unexpanded White Matter Fraction.
In one embodiment, the expanded local pore fraction modification, —e, is obtained as a linear combination of the maximally expanded pore fraction, _max, and the original pore fraction, _, according to a weighted sum expression:
_e=(fWM•_max)+(1−fWM)
We have used a value of 0.6 for _max.
There are a number of “proton density weighted” magnetic resonance imaging sequences in which the acquisition parameters can be adjusted to obtain an image in which the image values are nearly proportional to the density of spins in the imaged region, which is approximately a measure of the amount of water contained in the region. It is possible to create a more accurate estimate of this spin density by taking multiple images with varying TR, TE, or flip angle, in order to compensate for the effects of T1 and T2 weighting. Starting with such a proton density weighted image, one can obtain a map of “water fraction” by dividing the proton density image by the constant value of the proton density measured in pure water.
Typical water fractions, w, in normal brain tissue are approximately 0.8 in gray matter, and 0.7 in white matter largely due the volume of myelin covering white matter axons. Typical pore fraction, _, in normal white and gray matter is approximately 0.2. This extracellular volume is assumed to contain a fraction of water close to 1.0. The remainder of the 0.7-0.8 fraction of water is therefore found in the intracellular space.
The processes of vasogenic edema and infusion-induced edema expand the extracellular volume, thus reducing the density of cells. Assuming the water fills this expanded extracellular space, this water displaces both intracellular water and non-water cellular material proportionally. Thus, the reduction in the intracellular volume fraction should be proportional to the reduction in the non-water fraction. Let w0 and w represent the water fraction before and after the expansion, respectively. These two quantities can be measured (at the appropriate times) from MRI as described above. Let _0 and _ represent the extracellular volume fraction, or pore fraction, before and after expansion, respectively. The proportionality of intracellular volume change to non-water volume change can be described by the following equation:
(1−_)/(1−_0)=(1−w)/(1−w0)
This expression can be re-written to express the change in pore fraction,
Δ_=_−—0=(1−—0)(w−w0)/(1−w0)
This expression requires a measurement of the proton density before and after the expansion. Furthermore, it requires an estimate of the initial pore fraction, _0. In normal, unexpanded white matter this can be assumed to be approximately 0.2.
In some cases, one may not have a pre-expansion measurement of the water fraction, w0, available. Furthermore, there is no direct way to measure the pre-expansion pore fraction, _0, from MRI. But because pore fraction expansion is largely restricted to white matter regions, one can use the nominal normal white matter values, _0=0.2 and w0=0.7 for unexpanded white matter. Then, the pore fraction in white matter can be estimated from a single measurement of water fraction using the expression for change in pore fraction as:
—=1−[(1−—0)(1−w)/(1−w0)]
European Patent Application Nos. 07108374.5 and 08155788.6, respectively filed on May 16, 2007 and May 7, 2008, from which priority is claimed, are hereby incorporated herein by reference in their entireties.
| Number | Date | Country | Kind |
|---|---|---|---|
| 07 108 374.5 | May 2007 | EP | regional |
| 08 155 788.6 | May 2008 | EP | regional |
This application is a continuation of application Ser. No. 12/121,103 filed on May 15, 2008, the contents of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12121103 | May 2008 | US |
| Child | 13241936 | US |
