Characterisation of coronary atherosclerotic morphology by spectral analysis of radiofrequency signal: in vitro intravascular ultrasound study with histological and radiological validation

Abstract

Objective: To determine whether spectral analysis of unprocessed radiofrequency (RF) signal offers advantages over standard videodensitometric analysis in identifying the morphology of coronary atherosclerotic plaques.

Methods: 97 regions of interest (ROI) were imaged at 30 MHz from postmortem, pressure perfused (80 mm Hg) coronary arteries in saline baths. RF data were digitised at 250 MHz. Two different sizes of ROI were identified from scan converted images, and relative amplitudes of different frequency components were analysed from raw data. Normalised spectra was used to calculate spectral slope (dB/MHz), y-axis intercept (dB), mean power (dB), and maximum power (dB) over a given bandwidth (17-42 MHz). RF images were constructed and compared with comparative histology derived from microscopy and radiological techniques in three dimensions.

Results: Mean power was similar from dense fibrotic tissue and heavy calcium, but spectral slope was steeper in heavy calcium (-0.45 (0.1)) than in dense fibrotic tissue (-0.31 (0.1)), and maximum power was higher for heavy calcium (-7.7 (2.0)) than for dense fibrotic tissue (-10.2 (3.9)). Maximum power was significantly higher in heavy calcium (-7.7 (2.0) dB) and dense fibrotic tissue (-10.2 (3.9) dB) than in microcalcification (-13.9 (3.8) dB). Y-axis intercept was higher in microcalcification (-5.8 (1.1) dB) than in moderately fibrotic tissue (-11.9 (2.0) dB). Moderate and dense fibrotic tissue were discriminated with mean power: moderate -20.2 (1.1) dB, dense -14.7 (3.7) dB; and y-axis intercept: moderate -11.9 (2.0) dB, dense -5.5 (5.4) dB. Different densities of fibrosis, loose, moderate, and dense, were discriminated with both y-axis intercept, spectral slope, and mean power. Lipid could be differentiated from other types of plaque tissue on the basis of spectral slope, lipid -0.17 (0.08). Also y-axis intercept from lipid (-17.6 (3.9)) differed significantly from moderately fibrotic tissue, dense fibrotic tissue, microcalcification, and heavy calcium. No significant differences in any of the measured parameters were seen between the results obtained from small and large ROIs.

Conclusion: Frequency based spectral analysis of unprocessed ultrasound signal may lead to accurate identification of atherosclerotic plaque morphology.

Figure 1

Scan converted RF image (upper left), raw ultrasound data (middle left), normal videoimage (lower left), histology, and radiology from corresponding vessel site (right).

Figure 2

Spectral analysis. (Upper left) Raw, unprocessed radiofrequency data. (Lower left) Scan converted image of the area of interest. (Right) Power spectrum from two regions of interest.

Figure 3

Morphometric features of sonolucent plaques. (A) Normal media, smooth muscle cells of the normal media (m) are separated from a thin intima (i) by an internal elastic lamina (arrow). (B) Loose fibrotic tissue, atheromatous plaque composed of loose, collagenous fibrous tissue with small numbers of cells. (C) Moderately fibrotic tissue, atheromatous plaque showing an increase in numbers of fibroblasts and smooth muscle cells, and increased amounts of stainable collagenous connective tissue. (D) Fatty tissue, atheromatous plaque with foam cells, free lipid, and cholesterol crystals (arrow).

Figure 4

Morphological features of fibrotic and calcified plaques. (a) Loose fibrotic tissue. (b) Moderately fibrotic tissue. (c) Dense fibrotic tissue. (d) Microcalcification, areas of microcalcification with plaques visualised following staining for calcium with alizarin red. (e) Calcified plaques, heavy calcification identified with haematoxylin and eosin. (a-d) Original magnification ×50, bar = 100 µm; (e) original magnification ×12.5, bar = 200 µm.

Figure 5

The validation of histological correspondence was achieved by comparing ultrasound images with the comparative histology derived from microscopy (histology slices were taken every 100 µm from the suture mark corresponding to the beginning of the motorised pullback of the IVUS catheter set to record data every 200 µm) and radiology techniques (radiograph of the vessel before histology to localise the clockwise and horizontal position of heavy calcium).

Figure 6

Identification and localisation of calcium with longitudinal and cross sectional radiographs of coronary arteries from heart 15: left anterior descending (6437; 0) and three sites from the right coronary artery (6439; 1, 2, 3).

Figure 7

Power spectrum of lipid, loose (LFT), moderate (MFT), and dense fibrosis (DFT), heavy calcium (CA), and microcalcification (mCa).