HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Full Text Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Boote, E. J. Search for Related Content PubMed PubMed Citation Articles by Boote, E. J. Related Collections Physics and Basic Science Ultrasound

AAPM/RSNA Physics Tutorial for Residents: Topics in US

Doppler US Techniques: Concepts of Blood Flow Detection and Flow Dynamics¹

¹ From the Department of Radiology, University of Missouri, One Hospital Dr, Columbia, MO 65212. From the AAPM/RSNA Physics Tutorial at the 2002 RSNA scientific assembly. Received March 24, 2003; revision requested May 7; final revision received and accepted June 9. Address correspondence to the author (e-mail: bootee@missouri.edu).

View larger version (30K): [in a new window]   Figure 1.  As an object emitting sound moves at a velocity v, the wavelength of the sound in the forward direction is compressed (s) and the wavelength of the sound in the receding direction is elongated (l). Since frequency (f) is inversely related to wavelength, the compression increases the perceived frequency and the elongation decreases the perceived frequency. c = sound speed.

View larger version (25K): [in a new window]   Figure 2.  The velocity of red blood cells relative to the position and angle of the transducer depends on the angle () between the direction of sound propagation and the motion of the particle.

View larger version (26K): [in a new window]   Figure 3.  In a continuous-wave Doppler device, two separate transducer elements are used. One transmits the acoustic energy, the other receives echoes back. The sensitive region is where the effective beams of the two transducers overlap. Electronics used to detect the Doppler signal are explained in the text.

View larger version (15K): [in a new window]   Figure 4.  A quadrature detector is used to determine the direction (forward or reverse) of the Doppler shift frequencies. The Doppler signal is split into two channels, then mixed with a reference frequency (fref). On one side, this reference signal is shifted by 90° (one quarter of a period). The resulting signals are filtered with a low-pass filter. The relative phases of the two sides are compared to produce forward (fDfor) and reverse (fDrev) flow channels. These two channels may be output to left and right speakers to allow the operator to hear the Doppler shift frequencies in stereo.

View larger version (34K): [in a new window]   Figure 5a.  (a) Doppler frequency spectrum. In this plot, time is along the x axis and Doppler shift frequencies (expressed in units of velocity calculated by using Eq [10]) are along the y axis. The relative intensities of Doppler shift frequencies are depicted by using varying gray scales. This display is inverted: A darker pixel represents a higher relative intensity at that particular frequency and time. s = seconds. (b) Narrow versus broad Doppler spectra. In the narrow spectrum, the frequency content is such that the mean Doppler shift frequency (fmean) is relatively close to the maximum Doppler shift frequency (fmax). The broad spectrum has a frequency content that results in a lower fmean.

View larger version (20K): [in a new window]   Figure 5b.  (a) Doppler frequency spectrum. In this plot, time is along the x axis and Doppler shift frequencies (expressed in units of velocity calculated by using Eq [10]) are along the y axis. The relative intensities of Doppler shift frequencies are depicted by using varying gray scales. This display is inverted: A darker pixel represents a higher relative intensity at that particular frequency and time. s = seconds. (b) Narrow versus broad Doppler spectra. In the narrow spectrum, the frequency content is such that the mean Doppler shift frequency (fmean) is relatively close to the maximum Doppler shift frequency (fmax). The broad spectrum has a frequency content that results in a lower fmean.

View larger version (25K): [in a new window]   Figure 6a.  (a) Schematic of a pulsed-wave Doppler instrument. A single transducer is used in pulse-echo mode. The other boxes indicate the flow of data through the instrument and the stages of Doppler shift processing for pulsed-wave US. (b) Image display from pulsed-wave Doppler US. Red line indicates the axial line used to interrogate a volume contained within the range gate. The angle correction is also indicated.

View larger version (52K): [in a new window]   Figure 6b.  (a) Schematic of a pulsed-wave Doppler instrument. A single transducer is used in pulse-echo mode. The other boxes indicate the flow of data through the instrument and the stages of Doppler shift processing for pulsed-wave US. (b) Image display from pulsed-wave Doppler US. Red line indicates the axial line used to interrogate a volume contained within the range gate. The angle correction is also indicated.

View larger version (13K): [in a new window]   Figure 7.  Sample and hold process of capturing phase shifts in subsequent pulse interrogations within the region of the range gate. Each pulse is launched over the interval in time (t) determined by the pulse repetition frequency (PRF). The phase shifts over time are filtered to produce a Doppler shift frequency.

View larger version (38K): [in a new window]   Figure 8a.  (a) Concept of color flow imaging. In this mode, the US instrument launches interrogating pulses for lines within a region of interest. For each line, the mean Doppler shift is determined for a series of locations along the beam axis. (b) Color flow image shows flow in a vessel of a Doppler phantom. Mean velocities within a pixel are represented in color; a color scale on the image provides the velocity range depicted. These velocities are not corrected for the Doppler angle.

View larger version (82K): [in a new window]   Figure 8b.  (a) Concept of color flow imaging. In this mode, the US instrument launches interrogating pulses for lines within a region of interest. For each line, the mean Doppler shift is determined for a series of locations along the beam axis. (b) Color flow image shows flow in a vessel of a Doppler phantom. Mean velocities within a pixel are represented in color; a color scale on the image provides the velocity range depicted. These velocities are not corrected for the Doppler angle.

View larger version (14K): [in a new window]   Figure 9.  Schematic of the color flow imaging algorithm. A delay is applied to subtract the prior echo signals from the current echo signals. This subtraction reduces undesirable stationary echo signals. The autocorrelator compares the prior echo signals with the current echo signals to determine the mean velocity and the variance in each pixel. Gating circuits and a digital scan converter map these values to the image. Color processing applies a color map of velocities to the data.

View larger version (102K): [in a new window]   Figure 10.  Color flow (top) and power Doppler (bottom) images of the same phantom under the same conditions. The directions of flow toward and away from the transducer are seen in the color flow image (top). The power Doppler image (bottom) displays only the intensity of the Doppler shift.

View larger version (15K): [in a new window]   Figure 11a.  (a) Laminar flow in a vessel of radius r. Flow occurs as a result of pressure differences (P2 - P1) between two points a distance l apart. The velocity of the movement in the center of the vessel is a maximum and follows a parabolic profile to the edge of the vessel wall. (b) Flow in a typical blood vessel follows a blunt profile. The velocity of the flow is relatively uniform over the center of the vessel and falls off at the vessel wall. If a stenosis is present, turbulent flow will result as separation of flow occurs downstream from the stenosis.

View larger version (21K): [in a new window]   Figure 11b.  (a) Laminar flow in a vessel of radius r. Flow occurs as a result of pressure differences (P2 - P1) between two points a distance l apart. The velocity of the movement in the center of the vessel is a maximum and follows a parabolic profile to the edge of the vessel wall. (b) Flow in a typical blood vessel follows a blunt profile. The velocity of the flow is relatively uniform over the center of the vessel and falls off at the vessel wall. If a stenosis is present, turbulent flow will result as separation of flow occurs downstream from the stenosis.

View larger version (18K): [in a new window]   Figure 12.  Pulsed-wave US spectrum displays the maximum, minimum, and average calculated blood flow velocities. The pulsatility index, resistivity index, and systolic-to-diastolic ratio are calculated from these values.