ULTRASOUND PROFUSION IMAGING USING COMPENSATED DIGITAL SUBTRACTION
Invented By: Leng Jiang
Ping Lu Shouping Li
Reference To Prior Copending Application
This application claims the benefit of copending U.S. Provisional
Application No. 60/137,054, filed June 1, 1999.
Field Of The Invention
The present invention relates generally to an ultrasound system,
and more particularly to an improved device and method for ultrasound imaging
using compensated digital subtraction.
Background Of The Invention
Improved ultrasound imaging of myocardial perfusion as well as traditional tissue structures is a long felt need and the subject of constant research. Advances in ultrasound imaging have led to the detection of myocardial perfusion using venous injection of microbubbles. Harmonic imaging has also enhanced ultrasound imaging by means of conventional ultrasound transducers used to transmit a stimulus at a fundamental frequency and to receive a return imaging signal at a harmonic frequency. Although some improvement in imaging blood perfusion, tissue structure, and the identification of microbubbles in myocardial vasculature has
been the subject of much investigation, there still remains a long felt need to improve the imaging of such systems.
It has recently been discovered that transient response imaging or intermittent triggering has significantly increased myocardial contrast detection. Therefore, intravenous myocardial contrast echocardiography (MCE) is an important tool for noninvasive assessment of myocardial perfusion and detection of coronary artery disease at rest and during pharmacological stress.
However, problems still remain in assessment of myocardial perfusion with current MCE methods. Although MCE can produce visually discernible myocardial contrast, visual assessment is subjective and reader- dependent. Accordingly, computer-based videoensitometric methods have been applied to improve the quantitative measurements of MCE. However, intensity curves used with intra-coronary injection have been questioned when applied to intravenous MCE, particularly when there is a significantly prolonged contrast washout phase in the LN.
Currently, the background-subtracted peak myocardial video- intensity and the videointensity ratio between the adjacent zones are used. These methods measure the mean pixel videointensity in a region of interest, requiring repeatedly manual drawing in a relatively small area and having potential problems ofreproducibility.
Digital subtraction techniques by mathematically subtracting the pre and post-contrast images has been applied to MCE, which is particularly important
for assessing intravenous MCE, particularly when there is a low signal to noise ratio and attenuation of the imaging signal in the far field.
The present invention addresses the identified needs and heretofore- unsolved deficiencies, and provides a significant improvement in enhanced video intensity and ultrasound contrast imaging.
Summary of the Invention
The present invention is an ultrasound apparatus and method
employing an algorithm according to CEMVI(BPI) = CEMVI0*e"k*BPI, where logarithmic correction is applied to videointensity to overcome the heterogeneity of contrast-enhanced videointensity improving the digital subtraction and resulting imaging of myocardial videointensity.
Brief Description Of The Drawings For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.
Figure 1 illustrates the relationship between contrast-enhanced myocardial videointensity (CEMNI) and background pixel intensity (BPI) according to the discovery of the present invention.
Figure 2 illustrates the negative exponential correlation between contrast-enhanced myocardial videointensity (CEMNI) and background pixel intensity (BPI) according to the discovery of the present invention.
Figure 3a and 3b illustrate the heterogeneity of contrast-enhanced myocardial videointensity (CEMNI) and background pixel intensity (BPI) according to the discovery of the present invention.
Figures 4a-d illustrate echo images before and after intermittent harmonic imaging according to the discovery of the present invention.
Figure 5 illustrates the result of a pixel-mapping algorithm for digital subtraction of videointensity (VI) and log-corrected videointensity (LUI) and re-scaled VI from LUI according to the discovery of the present invention.
Figures 6-9 illustrate improved 3-deminsional mapping produced according to the discovery of the present invention.
Detailed Description Of The Invention The inventors have discovered, as shown in Fig. 1, that contrast- enhanced myocardial videointensity (CEMNI) is a function of background pixel intensity (BPI), and that CEMVI is negatively exponentially correlated with corresponding BPI. As shown in Fig. 2, there is significant heterogeneity of the CEMVI within and among normally perfused segments. Further, it was discovered that CEMVI, within each segment, is negatively exponentially correlated with corresponding BPI as,
CEMVI(BPI) = CEMVI0*e k*BPI where, k is a constant, CEMVIo is the CEMNI when BPI at a minimum value (r is 0.92 to 0.95, and P< 0.01), producing the result shown in Fig. 2. Significant variations in each segment may be observed in CEMNI, which is higher in anterior
(A), inferior (I), inferior septal (IS) segments, and lower in posterior (P) segments (< .001) on short axis views as shown in Fig. 3a, and higher in the lateral, lower in the apex and basal segments on apical views as shown in Fig. 3b.
Using the relationship of CEMVI(BPI), a significant improvement is achieved in imaging and in accuracy for assessing myocardial perfusion abnormalities on compensated digital subtraction for myocardial contrast echocardiography (MCE) by correcting the CEMNI heterogeneity within and among segments.
Examples of improved imaging using the relationship CEMVI (BPI) are shown in Figures 4a-d. Fig. 4a shows an intermittent harmonic image without MCE, and Fig. 4b shows an intermittent harmonic image with MCE. Fig. 4b illustrates colored coded digital subtraction imaging with compensation, and Fig. 4d illustrates colored coded digital subtraction imaging without compensation. Νon-compensated digital subtraction (ΝC-DS) is shown in Fig. 4d, compensated digital subtraction (C-DS) is shown in Fig. 4b, and perfusion defect (PD) is shown in black. In addition, false perfusion defect (PD) in ΝC-DS images are corrected by the application of C-DS. Accordingly, the present invention presents a significant improvement over existing methods using background-subtracted peak myocardial video intensity and non-compensated digital subtraction for analysis of myocardial contrast echocardiography.
The inventors are the first to theorize and prove that the underlying relationship, where CEMVI is negatively exponentially correlated with
corresponding BPI, is due to a logarithmic conversion of linear ultrasound intensity data (LUI) to gray-scale VI by means of post processing. To confirm this theory, the inventors conducted a 10 sample open-chest dog study with continuous infusion of PESDA and intermittent harmonic imaging. A pixel mapping algorithm was developed and applied for digital subtraction of VI and LUI (log- corrected VI), and for display with the re-scaled VI from LUI.
As shown in Fig. 5, the results of the test demonstrate that digital subtraction of VI produces a negative exponential curve that is best fitted for CE versus BPI, where y=l.l * e"0 093 x, r=0.9, SEE =0.05, PO.0001. In addition, when digital subtraction of LUI is applied, CE is shown to be relatively constant and no longer dependent on BPI, where y=0.0004x + 0.98, ι= 0.001, SEE=0.063, p=0.9. As shown in Fig. 6, by rescaling VI data with LUI, digital subtraction imaging is significantly improved. Therefore, logarithmic correction of VI corrects the negative exponential characteristic inadvertently applied to data collection by digital subtraction of myocardial VI.
The same logarithmic correction techniques can be applied to three- dimensional (3-D) echo reconstruction of myocardial perfusion. This technique has been found to eliminate the need for manually tracing perfusion zones on multiple cross-sectional views which is time-consuming and highly operator dependent. This is accomplished by directly transferring three apical images to a computer, applying digitally subtraction with log-correction, and automatically mapping myocardial perfusion onto a 16-segment model.
To prove this theory, nine open chest dogs with coronary legation (anterior descending 6, circumflex 3) were studied by venous injection of PESDA with harmonic triggered imaging. The percentage of non-perfused (NP) segment by 3D mapping was compared to the actual percentage of NP myocardial mass delineated by blue dye. As shown in Fig.'s 7-9, the study shows that automatic 3-D mapping successfully delineated NP regions in all 9 dogs which was readily shown by a 3-D and topographic display. The percentage of NP segment correlated well with the actual value but with underestimation (y=9.7x + 9.2, r= 0.84, SEE = 5.6%, pO.OOl, mean difference = -2±4.3%). Therefore it was concluded that automatic 3-D perfusion mapping is feasible with intravenous contrast echo and logarithmic-corrected videointensity digital subtraction, and obviates the need for manually defining and tracing perfusion zones, further facilitating the clinical evaluation of myocardial perfusion.
In summary, the current applied background-subtracted peak myocardial video intensity (MVI) and digital subtraction of the pre- and post- contract MVI, are based on the assumption that the contrast enhanced (CEMVI) is homogeneous and is independent on the background pixel intensity (BPI), and that CEMVI of each pixel depends on the amount of microbubbles.
These and other advantages of the present invention will be
apparent to those skilled in the art from the foregoing specification.
Accordingly, it will be recognized by those skilled in the art that changes or
modifications may be made to the above-described embodiments without
departing from the broad inventive concepts of the invention. It should therefore
be understood that this invention is not limited to the particular embodiments
described herein, but is intended to include all changes and modifications that are
within the scope and spirit of the invention as set forth in the claims.