U.S. patent application number 12/595567 was filed with the patent office on 2010-05-27 for method and apparatus for noninvasive quantitative detection of fibrosis in the heart.
This patent application is currently assigned to Oregon Health & Science University. Invention is credited to Sumeet S. Chugh, Michael Jerosch-Herold.
Application Number | 20100129292 12/595567 |
Document ID | / |
Family ID | 39864327 |
Filed Date | 2010-05-27 |
United States Patent
Application |
20100129292 |
Kind Code |
A1 |
Jerosch-Herold; Michael ; et
al. |
May 27, 2010 |
METHOD AND APPARATUS FOR NONINVASIVE QUANTITATIVE DETECTION OF
FIBROSIS IN THE HEART
Abstract
Embodiments provide a noninvasive quantitative method for
detecting extent and/or types of fibrosis in the heart. In
embodiments, information pertaining to the extent and/or types of
fibrosis may aid in the diagnosis of specific cardiac diseases and
heart failure and/or may assist in determining suitable treatment
options. Embodiments provide methods and apparatuses for
determining the extent of fibrosis in viable and nonviable
myocardium, which may then be correlated to heart disease and
failure. Thus, in an embodiment, a method of screening individuals
for the purpose of heart disease or heart failure prevention may be
provided using the detection methodology described herein.
Inventors: |
Jerosch-Herold; Michael;
(Lexington, MA) ; Chugh; Sumeet S.; (Los Angeles,
CA) |
Correspondence
Address: |
Schwabe Williamson & Wyatt;PACWEST CENTER, SUITE 1900
1211 SW FIFTH AVENUE
PORTLAND
OR
97204
US
|
Assignee: |
Oregon Health & Science
University
Portland
OR
|
Family ID: |
39864327 |
Appl. No.: |
12/595567 |
Filed: |
April 11, 2008 |
PCT Filed: |
April 11, 2008 |
PCT NO: |
PCT/US2008/060020 |
371 Date: |
October 12, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60911110 |
Apr 11, 2007 |
|
|
|
Current U.S.
Class: |
424/9.3 ;
424/9.4; 435/29 |
Current CPC
Class: |
A61B 6/03 20130101; G06T
2207/30048 20130101; G06T 2207/10088 20130101; A61B 5/0044
20130101; G06T 2207/30024 20130101; A61B 6/503 20130101; A61B
5/0035 20130101; A61B 5/0263 20130101; G06T 2207/10076 20130101;
G06T 7/0012 20130101; G01R 33/5602 20130101; G01R 33/281 20130101;
A61B 5/0275 20130101; A61B 6/504 20130101; A61B 5/055 20130101;
A61K 49/06 20130101 |
Class at
Publication: |
424/9.3 ; 435/29;
424/9.4 |
International
Class: |
A61B 5/055 20060101
A61B005/055; C12Q 1/02 20060101 C12Q001/02; A61K 49/04 20060101
A61K049/04 |
Claims
1. A method of detecting fibrosis in heart tissue, comprising:
selecting a sample of viable or nonviable heart tissue; contacting
the sample with an extracellular contrast agent; obtaining one or
more measurements of the contrast uptake of the sample with an
imaging apparatus to determine an extent of expansion of
extracellular volume in the sample as an indicator of diffuse
interstitial and/or replacement fibrosis.
2. The method of claim 1, wherein contacting the sample with an
extracellular contrast agent comprises contacting the sample with a
Gadolinium-containing contrast agent.
3. The method of claim 1, wherein contacting the sample with an
extracellular contrast agent comprises contacting the sample with
gadodiamide.
4. The method of claim 1, wherein contacting the sample with an
extracellular contrast agent comprises contacting the sample with
an extracellular collagen-binding contrast agent.
5. The method of claim 1, wherein contacting the sample with an
extracellular contrast agent comprises contacting the sample
in-vivo with an extracellular contrast agent.
6. The method of claim 1, wherein contacting the sample with an
extracellular contrast agent comprises contacting the sample
in-vitro with an extracellular contrast agent.
7. The method of claim 1, wherein obtaining one or more images of
the sample comprises obtaining one or more images using magnetic
resonance imaging.
8. The method of claim 7, wherein obtaining one or more images
using magnetic resonance imaging comprises performing a plurality
of T1 relaxation time measurements in blood and in the sample, both
before and after contact with one or more contrast agents to
determine the tissue sample partition coefficient for the
extracellular contrast agent.
9. The method of claim 8, wherein the relaxation times (T1) are
converted into relaxation rates with R1=1/T1, and wherein each R1
rate determined for the sample is linearly regressed against a
determined R1 rate in the blood.
10. The method of claim 8, further comprising obtaining a blood
hematocrit, and wherein the sample partition coefficient and the
blood hematocrit are used to calculate the extracellular volume in
the tissue sample.
11. The method of claim 7, wherein obtaining one or more images
using magnetic resonance imaging comprises performing rapid imaging
before, during and after contrast agent contact to measure contrast
enhancement in the sample and in blood resident in a ventricular
cavity or one or more large vessels.
12. The method of claim 11, wherein dynamics of contrast
enhancement are analyzed with a two-space model to determine the
extracellular volume in the sample.
13. The method of claim 1, further comprising creating parametric
maps of the extracellular volume as a marker of fibrosis.
14. The method of claim 1, further comprising determining a global
diffuse fibrosis burden in the sample.
15. The method of claim 1, further comprising classifying an extent
of diffuse fibrosis in a patient from which the sample is selected
to determine risk of heart failure or heart disease.
16. The method of claim 1, further comprising determining and
distinguishing between an extent of interstitial and an extent of
replacement fibrosis in the sample.
17. The method of claim 1, wherein obtaining one or more images of
the sample comprises obtaining one or more images using computed
x-ray tomography.
18. The method of claim 1, further comprising determining the
presence of, location of, and/or extent of interstitial and
replacement fibrosis in a tissue sample or whole heart and
correlating the determination with a heart condition, disease, or
associated risk of a particular heart disease or failure.
19. The method of claim 18, wherein correlating the determination
with a heart condition, disease, or associated risk of a particular
heart disease or failure is represented as a numeric or textual
risk factor.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
Patent Application No. 60/911,110, filed Apr. 11, 2007, the entire
disclosure of which is hereby incorporated by reference in its
entirety.
TECHNICAL FIELD
[0002] Embodiments relate to the field of medical diagnostics and
monitoring, and, more specifically, to a method and apparatus for
noninvasive quantitative detection of both diffuse and focal
fibrosis in the heart.
BACKGROUND
[0003] Myocardial fibrosis is a morphologic change common to
multiple cardiac disease conditions. In addition to replacement
(scar) fibrosis, there is increasing recognition of interstitial
(reactive) fibrosis being an important player in structural
remodeling of the diseased heart, as well as the genesis of fatal
arrhythmia leading to sudden cardiac death. Currently, fibrosis is
quantified by histochemical analysis of tissue samples obtained by
surgical biopsy. The availability of a non-invasive test to
quantify replacement and interstitial fibrosis, which may be
correlated with the collagen volume fraction (CVF), would be a
significant advance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] Embodiments will be readily understood by the following
detailed description in conjunction with the accompanying drawings.
Embodiments are illustrated by way of example and not by way of
limitation in the figures of the accompanying drawings.
[0005] FIG. 1 illustrates inversion recovery signal intensity
curves. For determination of the contrast agent partition
coefficient, T1 was quantified for user-defined regions of interest
in the heart. A software program was used to load the DICOM-format
MRI images for a series of 12 to 17 inversion times, and fit
inversion recovery signal intensity curves, as shown for two
regions in FIG. 1, were generated to determine T1. This analysis
was performed twice, first for measurements of T1 before contrast
enhancement, and then after incubation of the samples in a
gadolinium contrast agent solution for 24 hours. The graph shows
differences in regional T1 after placing the sample in the contrast
agent solution, and the area with the shorter T1 (posterior lateral
region) corresponds to a region with marked fibrosis.
[0006] FIGS. 2A and 2B illustrate heart slice images. FIG. 2A shows
a post-mortem T1-weighted MRI image of a heart slice (patient with
cardiomegaly, and extensive atherosclerosis) after 24 hour
incubation in contrast agent (gadodiamide) solution. The inversion
time of 300 ms resulted in a contrast, such that tissue with higher
gadodiamide distribution volume appears brighter. FIG. 2B shows a
histological image of the same myocardial slice stained and showing
extensive fibrosis in posterior-lateral areas, matching those with
increased signal intensity on the MRI image.
[0007] FIGS. 3A and 3B illustrate tissue samples stained and viewed
under polarized light and classified as normal, or showing
interstitial or replacement fibrosis, respectively. The measured
gadodiamide distribution volumes, and CVFs for these three
classifications, are shown (boxes show 25% and 75% percentile
limits). The points represent the measured values, and the
gadodiamide distribution volume. Open circles represent the samples
that remained unthawed for 60 hours. Closed circles represent
samples that remained unthawed for 32 hours.
[0008] FIG. 4 provides a graph showing that the gadodiamide
distribution volume in myocardial tissue correlated closely
(measured at 32 hours after thawing of the heart sample) with the
CVF (r=0.727; p=0.017). For the graph on the right, the correlation
between gadodiamide distribution volume and CVF (r=0.987; p=0.012)
is represented. The two graphs show different slopes based on the
time lag between thawing of the sample and the post-contrast MRI
measurement. Longer lag times increase cell membrane breakdown and
therefore gadodiamide distribution volume is larger for the series
of measurements with longer sample incubation times. The dotted
lines in the graphs show the 95% confidence limits.
[0009] FIG. 5 is a flowchart outlining methods of measuring
extracellular-volume fractions in accordance with embodiments.
[0010] FIGS. 6A and 6B provide output graphs of methods of
performing rapid imaging before, during and after contrast agent
contact to measure contrast enhancement in the sample and in blood
resident in a ventricular cavity or one or more large vessels in
accordance with an embodiment.
DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS
[0011] In the following detailed description, reference is made to
the accompanying drawings which form a part hereof, and in which
are shown by way of illustration various embodiments. It is to be
understood that other embodiments may be utilized and structural or
logical changes may be made without departing from the intended
scope. Therefore, the following detailed description is not to be
taken in a limiting sense, and the scope of embodiments is defined
by the appended claims and their equivalents.
[0012] Various operations may be described as multiple discrete
operations in turn, in a manner that may be helpful in
understanding embodiments; however, the order of description should
not be construed to imply that these operations are order
dependent.
[0013] The description may use perspective-based descriptions such
as up/down, back/front, left/right, and top/bottom. Such
descriptions are merely used to facilitate the discussion and are
not intended to restrict the application of the embodiments.
[0014] For the purposes of the description, a phrase in the form
"A/B" or in the form "A and/or B" means "(A), (B), or (A and B)".
For the purposes of the description, a phrase in the form "at least
one of A, B, and C" means "(A), (B), (C), (A and B), (A and C), (B
and C), or (A, B and C)". For the purposes of the description, a
phrase in the form "(A)B" means "(B) or (AB)" that is, A is an
optional element.
[0015] The description may use the phrases "in an embodiment," or
"in embodiments," which may each refer to one or more of the same
or different embodiments. Furthermore, the terms "comprising,"
"including," "having," and the like, as used with respect to
various embodiments, are synonymous.
[0016] In various embodiments, methods, apparatuses, and systems
for determining the presence and extent of fibrosis in myocardial
tissue are provided. In exemplary embodiments, a computing system
may be endowed with one or more components of the disclosed
apparatuses and/or systems and may be employed to perform one or
more methods as disclosed herein.
[0017] Embodiments provide a noninvasive quantitative method for
detecting the extent and/or types of fibrosis in the heart. In
embodiments, information pertaining to the extent and/or types of
fibrosis may aid in the diagnosis of specific cardiac diseases and
heart failure and/or may assist in determining suitable treatment
options. Embodiments provide methods and apparatuses for
determining the extent of fibrosis in viable and nonviable
myocardium, which may then be correlated to heart disease and
failure. Thus, in an embodiment, a method of screening individuals
for the purpose of heart disease or heart failure prevention may be
provided using the detection methodology described herein.
[0018] In an embodiment, a method allows for the detection and
quantification of cardiac fibrosis using contrast enhanced magnetic
resonance imaging (MRI) (or another imaging method such as computed
tomography) as an alternative to histological evaluation. In an
embodiment, a method provides for measurement of the myocardial
partition coefficient for an extracellular contrast agent, showing
the relative distribution volume of the contrast agent, which may
be used to quantify diffuse, reactive, interstitial, or replacement
fibrosis, conditions that may be inadequately detected by current
methods. All forms of fibrosis lead to an expansion of the
extracellular matrix, which in turn increases the volume accessible
to extracellular contrast agents, such as gadiodiamide in the case
of MRI. Embodiments may be applied to in-vitro and in-vivo
evaluation.
[0019] MRI of delayed contrast enhancement (DCE) with an
extracellular contrast agent, such as gadodiamide-DTPA, has become
a desirable method to depict non-viable myocardium with high
spatial resolution. DCE MRI relies on the detection of contrast
enhancement relative to other remote areas in the same heart. In
cardiac diseases such as non-ischemic dilated cardiomyopathies with
evidence from histology of diffuse fibrosis, current non-invasive
tests, including those based on cardiac MRI, may fail in detecting
this diffuse, generalized fibrosis. Focal areas of delayed contrast
enhancement may be either absent, or only provide a partial measure
of fibrosis extent and burden. Embodiments disclosed herein provide
a novel approach, whereby the relative distribution volume of
contrast agent, and/or the myocardial partition coefficient for the
contrast agent, is determined to obtain a quantitative measure of
the extracellular volume fraction as a marker of fibrosis in viable
or nonviable myocardium. In viable myocardium, the myocardial
partition coefficient is proportional to the extracellular volume
fraction. In an embodiment, the extracellular volume fraction may
be quantified with MRI by determining the change of R1 relaxation
rate constants in tissue and blood, respectively, both before and
after administration of the contrast agent. Alternatively, in an
embodiment, one may employ a dynamic imaging method to measure
signal changes observed with a heavily T1-weighted pulse sequence
between the pre-contrast and post-contrast states.
[0020] In an embodiment, the measurement of the myocardial
partition coefficient for certain contrast agents, such as Gd-DTPA,
is suitable for differentiating extent of myocardial fibrosis on a
continuous scale, spanning the range from normal myocardium,
through diffuse interstitial and replacement fibrosis in viable
myocardium, to non-viable fibrotic scar tissue.
[0021] For quantification of the tissue partition coefficient, the
myocardial partition coefficient for a contrast agent is defined as
the ratio of the contrast agent concentrations in tissue and blood,
at equilibrium. At equilibrium, the concentration of an
extracellular contrast agent within the interstitial space should
equal the plasma concentration. In terms of the specific volumes
(volumes normalized by tissue weight, expressed as ml/g of tissue)
of the interstitial (v.sub.isf) and plasma spaces (v.sub.p) one may
express the partition coefficient for an extracellular contrast
agent as:
.lamda. = v isf + v p ( 1 - Hct ) , ( 1 ) ##EQU00001##
where Hct is the blood hematocrit. An MRI contrast agent is
typically detected through its effect on the .sup.1H MR signal. The
signal from the blood constituents, plasma, and erythrocytes, is
characterized by a single relaxation rate due to fast exchange of
water between plasma and erythrocytes--the intracellular lifetime
of water inside erythrocytes is .about.10 ms under physiological
conditions.
[0022] Various strategies have been devised to measure the
myocardial partition coefficient for extracellular contrast agents
with MRI, which were focused almost exclusively on studies in
healthy volunteers, or in patients with ischemic heart disease and
myocardial necrosis. The myocardial partition coefficient may be
calculated from the change of R1 (R1=1/T1) in tissue, divided by
the change of R1 in the blood pool. Alternatively, one may measure
signal changes with pulse sequences producing strong
T1-weighting.
[0023] In an embodiment, the distribution volume of certain
contrast agents may be elevated in myocardium with even mild
interstitial fibrosis. In addition, in an embodiment, the
distribution volume may be linearly related to the CVF, an
established histological quantification of myocardial fibrosis.
[0024] In an embodiment, a suitable contrast agent may be an
extracellular contrast agent or a collagen binding agent. In an
embodiment, a suitable contrast agent may be Gadolinium or a
Gadolinium-based compound, such as gadodiamide.
[0025] In an embodiment, an in-vitro MRI technique is provided for
comparison of contrast agent distribution volume as a measure of
both replacement and interstitial myocardial fibrosis, with a
histologically determined myocardial CVF, the current gold standard
for quantification of myocardial fibrosis. In an embodiment, the
MRI measures of fibrosis may correlate significantly with CVF
supporting the suitability of the new methodology. Using ex-vivo
myocardial tissue samples, it may be observed that gadodiamide MRI
differentiates between normal myocardium, and interstitial
(reactive) fibrosis, and between normal myocardium and replacement
(scar) fibrosis. In accordance with an embodiment, the contrast
distribution volume and CVF also vary significantly across fibrosis
categories, namely normal, interstitial and replacement fibrosis,
allowing for types of fibrosis to be distinguished as well.
Embodiments use MRI to derive a quantitative measure of fibrosis,
which correlates with CVF in myocardial tissue.
[0026] Following acute myocardial infarction, MRI of delayed
contrast hyper-enhancement with gadodiamide-contrast reflects the
breakdown of the cell-membrane, which increases the volume of
distribution of gadodiamide-contrast relative to viable myocardium.
In areas of myocardial infarction, a dense collagen matrix leads to
focal hyper-enhancement relative to areas with viable myocardium.
In a canine model of chronic myocardial infarction with dense
collagen matrix within the infarct zone, it has been shown that
contrast hyper-enhancement was similar to the hyper-enhancement
observed in acute infarcts. Other causes of increased myocardial
fibrosis have also been shown to lead to increased
gadodiamide-contrast uptake compared to normal tissue.
[0027] Currently, total CVF is the most widely used measure of
fibrosis burden. Essentially two methods exist to determine total
CVF: an absolute measure using hydroxyproline to bind collagen and
segregate it from other tissue constituents, and a measurement of
CVF that uses tissue staining and photometry. Embodiments herein
provide contrast-enhanced MRI as a semi-quantitative method of
measuring CVF that extends beyond traditional CVF and histological
analysis. The correlation between CVF and MRI described herein
allows for comparison between data observed between the two
methods. In addition, due to the non-invasive nature of MRI,
various disclosed embodiments may reduce the need for endocardial
biopsies.
[0028] Although the contrast distribution volume estimate from MRI
and CVF from a photometric assay correlate well, the underlying
methods have some significant differences worth noting. The
photometry-based determination of CVF requires the selection of
small (approximately 40 mm.sup.2) areas under a microscope for the
pixel count, and the resulting CVF estimate may not be
representative of a wall segment. With MRI, the region of interest
is user-defined, at a much lower magnification scale and
signal-averages for arbitrarily-sized regions are readily
calculated for each image. This difference between the CVF
measurement and the MRI method is analogous to a biopsy-based
measurement compared to an imaging-based measurement. While the
first represents an often arbitrary and restricted choice of tissue
within the heart, the latter may be freely specified by an
observer, assuming good image quality and spatial resolution.
[0029] As an example of the above-described methodology, eight
samples of human myocardium were obtained postmortem and a fast
spin-echo sequence (3 Tesla) with non-slice selective inversion
pulse performed before and after immersion in a gadodiamide saline
solution for determination of the gadodiamide partition
coefficient. T1 values were calculated from the inversion recovery
signal curves. The same samples were fixed in formalin, and the
collagen volume fraction was determined by the picrosirius red
method using a semi-automated, polarized, digital microscopy
system. The results showed that both gadodiamide distribution
volumes as well as CVF values were significantly different in
normal myocardium vs. interstitial fibrosis (p=0.001), and normal
vs. replacement fibrosis (p=0.015). Moreover, there was a
significant positive correlation between the two methods, across
all three histological categories of myocardial fibrosis (r=0.73;
p=0.017). Thus, these findings indicate an expanded potential for
contrast enhanced MRI as a novel, non-invasive alternative to
histological evaluation, for the quantification of both
interstitial and replacement myocardial fibrosis.
[0030] In accordance with an exemplary embodiment as outlined
briefly above, eight samples of myocardium were obtained from
deceased patients. Each sample of ventricular myocardium was 1 to 2
cm in thickness, and cut at the mid-level of the ventricular septum
to include left and right ventricular free walls. All samples were
stored at -80.degree. C. until time of analysis.
[0031] A pre-contrast MRI was performed approximately 10-12 hours
after thawing each sample. The samples were brought to room
temperature before each MRI measurement. The longitudinal
relaxation time T1 of myocardial tissue and saline was measured
with a spin-echo MRI pulse sequence with non-slice-selective
inversion pulse, and for 12-15 inversion delays (inversion times in
the range from T1=50 to 2000 ms). The other sequence parameters
were: repetition time (TR)=2500 ms, echo-time (TE)=9.5 ms, slice
thickness of 2.5 mm, receiver bandwidth=190 Hz/pixel, and image
matrix=256 by 256. For each sample, a beaker with the tissue sample
immersed in saline was placed in a small radio-frequency coil
designed for wrist imaging, and all images were acquired at 3 Tesla
(Siemens Trio, Siemens Medical Solutions, Malvern, Pa.).
[0032] After the first MRI scan, the sample was incubated at
3-4.degree. C. in a gadodiamide-saline solution for 24 hours
(initial gadodiamide concentration in saline before immersion of
tissue slice was .about.3 mM; saline R1 at 3 Tesla after 24 hours
incubation: 3.9.+-.0.3 s.sup.-1). The gadodiamide contrast agent
(Omniscan; GE-Healthcare, Princeton, N.J.) has an osmolality at
37.degree. C. of 789 (mOsmol/kg water). A second post-contrast MRI
was then performed, with identical sequence parameters as for the
first measurement. The effects of duration of thawing were also
evaluated. Two tissue samples remained thawed for 60 hours before
the second MRI was performed. All other tissue samples were kept in
a temperature range from 3-4.degree. C. during gadodiamide
incubation and room temperature for 32 hours before the second MRI.
Thawing time is potentially important because the integrity of the
cell membranes degrades during the time the tissue is not frozen.
Accordingly, these two sets of samples with different durations of
thawing were analyzed separately.
[0033] For image analysis, a custom software program was written to
load the images for different inversion time values in DICOM
format, and determine T1 from the inversion recovery signal curves
for myocardial tissue and saline, both with and without gadodiamide
contrast, by use of a non-linear least-squares fitting algorithm
(Matlab version 6.5, The Mathworks, Natick, Mass.). Changes in the
relaxation rate R1 (R1=1/T1) are proportional to the local
concentration of gadodiamide in tissue. The gadodiamide-contrast
partition coefficient was calculated as:
.lamda. = 1 / R 1 tissue ( post - contrast ) - 1 / R 1 tissue ( pre
- contrast ) 1 / R 1 saline ( w / Gd ) - 1 / R 1 tissue ( w / oGd )
( 2 ) ##EQU00002##
[0034] For ex-vivo measurements of tissue in saline, the measured
partition coefficient was equated to the relative distribution
volume, assuming that the gadodiamide contrast concentration
reached an equilibrium state after 24 hours incubation of tissue
slices in gadodiamide solution. FIG. 1 is an example of an MRI
image of a sample.
[0035] After MRI analysis, the samples were preserved in formalin.
The formalin fixed tissue was then processed, embedded in paraffin
and sections prepared of the entire surface area of the sample
including septum, left ventricular free wall and right ventricular
free wall. The sections, 5 microns in thickness, were stained with
picrosirius red and viewed under polarized light at 40.times.
power.
[0036] Two investigators independently analyzed all 8 samples under
low (10.times.) magnification. Samples were classified as either
entirely normal, or containing areas with abnormal distribution and
content of collagen. Abnormal samples were further subdivided into
those having interstitial (reactive) fibrosis, or as having
replacement fibrosis. In samples with areas of fibrosis, a remote
region without fibrosis or abnormal collagen distribution was also
identified. 15 regions of interest were identified within the 8
samples, with one completely normal sample only containing one
region. The areas identified were viewed under 40.times.
magnification and CVFs were determined. FIG. 2 shows a stained
sample and a corresponding MRI image, with matching areas
identified by arrows.
[0037] Areas of interest, as mentioned above, were identified
histologically on the paraffin-embedded slices. Matching locations
were identified on the MRI images by using anatomical landmarks
such as the insertion of the right ventricle into the left
ventricle, or papillary muscles. Given the differences in tissue
size and shape between fresh tissue and paraffin embedded tissue,
in an embodiment, locations are estimated to be matched with an
accuracy of approximately .+-.1 cm in the circumferential
direction, and approximately .+-.0.5 cm in the radial
direction.
[0038] CVF is a computer assisted quantification of myocardial
fibrosis in histological sections widely used for analysis of
myocardial collagen content. In an embodiment, a modified version
of the photometric assay was employed in this study, by using
picrosirius red instead of a trichrome stain. Picrosirius red
exclusively polarizes collagen allowing for more objective
identification of collagen. This benefit is reflected in the
calculated intra- and inter-observer concordance (r=0.99 and 0.99
respectively).
[0039] Once the preserved myocardial slices were stained, the
sections of interest were identified. Each area of interest was
subdivided into quadrants. Within each quadrant, 16 digital photos
were taken under 40.times. magnification. Each photo represented
2.5 mm.sup.2; therefore, 40 mm.sup.2 were analyzed from each
section. This allows for a representative sampling of CVF.
[0040] Collagen, easily identified by polarizing light, was
manually traced by a mouse pad. This step was repeated for stained
muscle and the area determined in a similar fashion. CVF was then
obtained for each section by the following equation:
CVF=Connective tissue pixel count in 16 fields/Total pixel count in
16 fields (3)
[0041] Analysis of variance was used for comparison of CVF and the
myocardial partition coefficient across tissue sample
classifications. Tukey's Honest Significant Difference for multiple
comparisons was used to evaluate significance for differences
between individual data points. Linear regression analysis was used
to determine the association between the myocardial partition
coefficient and CVF. All statistical analysis was performed with R
(R Foundation for Statistical Computing, Vienna, Austria. ISBN
3-900051-07-0, URL http://www.R-project.org). A p-value of 0.05 was
used as a cut-off to determine statistical significance.
[0042] The mean values of CVF were significantly different based on
histological classification i.e. normal myocardium vs. interstitial
fibrosis vs. replacement fibrosis (p=0.015). Analysis of variance
with adjustment for multiple comparisons indicated significant
differences in CVF between normal samples and those with
replacement fibrosis (p=0.012); but not for normal vs. interstitial
(p=0.45), and also not for interstitial vs. replacement (p=0.19,
FIG. 3B). Specifically, the mean values with corresponding 95%
confidence intervals for replacement fibrosis, interstitial
fibrosis and normal myocardium were 4.1%+/-0.23; 2.3%+/-0.23; and
1.3%+/-0.26, respectively.
[0043] Similarly, mean values of gadodiamide distribution volume
were significantly different based on histological classification
i.e. normal myocardium vs. interstitial fibrosis vs. replacement
fibrosis (p=0.001, FIG. 3A). Specifically, mean values of
distribution volume with corresponding 95% confidence intervals for
replacement fibrosis, interstitial fibrosis, and normal myocardium
were 0.46+/-0.05, 0.43+/-0.22 and 0.17+/-0.05, respectively.
Analysis of variance with adjustment for multiple comparisons
indicated significant differences in gadodiamide distribution
volume between normal samples vs. those with replacement fibrosis
(p=0.003); between normal vs. interstitial fibrosis (p=0.007); but
no significant difference for interstitial fibrosis vs. replacement
fibrosis (p=0.90).
[0044] Measured values for CVF and gadodiamide distribution volume
were found to be closely correlated (FIG. 4). The two graphs in
FIG. 4 represent the results for each of two batches of samples.
Each batch had a different thawing time. The left panel in FIG. 4
for samples thawed for 32 hours shows the correlation between
gadodiamide distribution volume and CVF (Pearson correlation
r=0.73; p=0.017). Likewise, the graph on the right, for samples
thawed for 60 hours, shows a correlation of r=0.99 (p=0.012).
Interestingly, the slopes of the two are different based on the
time lag between thawing of the sample and the post-contrast MRI
measurement.
[0045] A strong positive correlation between CVF and
gadodiamide-MRI is shown in FIG. 4. While the findings were
consistent with either thawing time, longer times likely increased
gadodiamide distribution volume due to increased cell membrane
breakdown, thus increasing the slope of the regression line. A more
uniform distribution of gadodiamide in the tissue samples after 60
hours may account in part for the better correlation of the
apparent gadodiamide distribution volume with the CVF, compared to
the samples incubated in gadodiamide-saline solution for 32
hours.
[0046] FIG. 5 is a flowchart outlining various methods of measuring
extracellular-volume fractions in accordance with embodiments. In
embodiments, measurements may be obtained of an extracellular
volume fraction in a tissue sample with an MR contrast agent. In an
embodiment, multiple pre- and post-contrast injection T1
measurements may be obtained. Such an operation may be performed
over an exemplary period of approximately 15-20 minutes. Further,
the partition coefficient may be calculated from the change of
R1=1/T1 in tissue and blood. Then, in an embodiment, the
extracellular volume fraction (V.sub.ec) may be calculated from the
partition coefficient V.sub.ec=.lamda.(1-Hct), using hematocrit
(Hct) of the patient. Thus, in an embodiment, a plurality of T1
relaxation time measurements may be performed in blood and in the
sample, both before and after contact with one or more contrast
agents to determine the tissue sample partition coefficient for the
extracellular contrast agent. The relaxation times (T1) may be
converted into relaxation rates with R1=1/T1, and each R1 rate
determined for the sample may be linearly regressed against a
determined R1 rate in the blood. In an embodiment, a blood
hematocrit may be obtained, and the sample partition coefficient
and the blood hematocrit may be used to calculate the extracellular
volume in the tissue sample.
[0047] In an alternative embodiment, the time course of signal
enhancement in blood and tissue may be determined during a first
pass. Such an operation may be performed over an exemplary period
of approximately 4-5 minutes. A model-based analysis of tissue
contrast enhancement may be performed, and then an extracellular
volume calculated from best-fit model parameters. Thus, in an
embodiment, rapid imaging before, during and after contrast agent
contact may be performed to measure contrast enhancement in the
sample and in blood resident in a ventricular cavity or one or more
large vessels. In an embodiment, the dynamics of contrast
enhancement may be analyzed with a two-space model to determine the
extracellular volume in the sample.
[0048] FIGS. 6A and 6B provide output graphs of methods of
performing rapid imaging before, during and after contrast agent
contact to measure contrast enhancement in the sample and in blood
resident in a ventricular cavity or one or more large vessels in
accordance with an embodiment. For such a method, images of the
heart may be acquired rapidly during the first pass of an injected
extracellular contrast agent, resulting in signal intensity changes
in the blood pool of the left ventricular cavity ("arterial input")
and in myocardial tissue as shown in FIG. 6A. An initial peak in
the arterial input may be observed during the first pass of the
contrast agent, followed by recirculation and approximation to a
semi-equilibrium state. During this later phase of
semi-equilibrium, one may take the ratio of signal intensities for
the myocardial tissue and the arterial input to determine the
partition coefficient. Applying this calculation to the data in
FIG. 6A, results in the first (more variable) curve in FIG. 6B. For
the partition coefficient calculation, one may also take a running
average (the second (smooth) curve in FIG. 6B) and estimate the
partition coefficient when concentration in the blood pool is in
semi-equilibrium. A window representing a suitable semi-equilibrium
is outlined by the small box in FIG. 6B. In an embodiment, one may
also use a tracer kinetic model and correct the estimate of the
partition coefficient for any variations of the arterial input.
[0049] There are several important heart diseases/conditions for
which qualitative and quantitative measurements of myocardial
fibrosis would be valuable for diagnosis and/or risk assessment,
including (1) the broad category of heart failure that results from
a variety of conditions ranging from familial disorders to
myocardial infarction, (2) patients who will suffer sudden cardiac
arrest in the future, which again may result from a spectrum of
heart conditions, and (3) congenital heart disease, comprising
several distinct disorders eventually having a component of
myocardial fibrosis. In both ischemic and non-ischemic heart
diseases, fibrosis often plays an important role. Even with healthy
aging, diffuse fibrosis may be the underlying cause of stiffening
of the ventricles which may be an important contributor toward
diastolic dysfunction.
[0050] Prior methods utilize biopsies to obtain an indication of
fibrosis, but biopsies are simply localized samples that do not
provide a reliable indication of the overall (global) fibrosis
burden. Thus, embodiments herein provide an imaging-based test to
analyze diffuse fibrosis to determine the global fibrosis burden.
Such an analysis provides important information to help in planning
suitable treatment.
[0051] In accordance with an embodiment, a positive correlation
between the methodology presented herein and the amount(s) of
myocardial fibrosis for both interstitial as well as replacement
fibrosis has been shown. In another embodiment, the present methods
may be helpful in risk stratification for sudden cardiac death as
well as disease severity of the conditions identified above.
[0052] Thus, embodiments may be used to correlate a determination
of the presence of, location of, or extent of fibrosis in a tissue
sample or whole heart with a heart condition, disease, or
associated risk of a particular heart disease or failure. In an
embodiment, a determined amount and/or location of myocardial
fibrosis may be correlated to the risk of heart disease or failure.
In an embodiment, a risk factor, such as a numeric or textual risk
factor, may be assigned reflecting the extent and/or location of
myocardial fibrosis (i.e., a scaled number, a percentage, a textual
indicator such as high, medium, low, etc.). For example, in an
embodiment, a higher amount of fibrosis (such as represented as a
percentage of fibrotic tissue to healthy tissue) may result in a
higher risk factor. In embodiments, other factors may be included
in the analysis, such as age of the patient, other health
conditions, etc.
[0053] Any one or more of various embodiments as previously
discussed may be incorporated, in part or in whole, into an
apparatus or system. In various embodiments an apparatus or system
may comprise a server or other computing device, a storage medium
and a plurality of programming instructions stored in the storage
medium. In various ones of these embodiments, the programming
instructions may be adapted to program an apparatus to enable the
apparatus to perform one or more of the previously-discussed
methods. For example, the programming instructions may be adapted
to program an apparatus to enable the apparatus to perform image
acquisition and/or analysis.
[0054] Although certain embodiments have been illustrated and
described herein for purposes of description of the preferred
embodiment, it will be appreciated by those of ordinary skill in
the art that a wide variety of alternate and/or equivalent
embodiments or implementations calculated to achieve the same
purposes may be substituted for the embodiments shown and described
without departing from the intended scope. Those with skill in the
art will readily appreciate that embodiments may be implemented in
a very wide variety of ways. This application is intended to cover
any adaptations or variations of the embodiments discussed herein.
Therefore, it is manifestly intended that embodiments be limited
only by the claims and the equivalents thereof.
* * * * *
References