Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.

(FFT) The fast Fourier transformation is a particularly fast and efficient computational method of performing a Fourier transformation, which is the mathematical process by which raw data is processed into a usable image.
The fast Fourier transform analyzer is a common device that performs spectral analysis in ultrasound instruments. In this case, it displays different quadrature Doppler frequencies or reflector velocities when a sample volume cursor is used along time. The Doppler frequency is proportional to the spectral reflector velocity.

See also Proportionality Constant, and Sampling Rate.

Peripheral veins are easily tested using a 5 to 10 MHz transducer. The venous walls are smooth, thin, and compressible. Venous ultrasound imaging requires the compression of the veins in the transverse view. If compression is performed in the longitudinal view, the vein may roll away from the transducer possibly creating a false-negative examination.
The lumen of the normal vein is echo free. Increasing the gain will display low level echoes representing venous blood moving towards the heart. When performing Doppler spectral analysis or color Doppler the gate should be placed in the center of the vessel. In case of a non-obstructing or recanalized thrombosis, the Doppler gate should be placed within the remaining vessel lumen for flow detection.

See also Maximum Venous Outflow and Zero Offset.

(TCCS) Transcranial color coded sonography is a combination of B-mode and pulsed wave Doppler. TCCS is used to study morphological and functional assessment of the circle of Willis, intracranial hemodynamics caused by extracranial artery stenosis, collateral flow and the vascular supply of intracranial lesion. Color imaging of the intracranial vessels allows placing the spectral Doppler volume correctly. This modality has encouraged the widespread use.
Contrast enhanced TCCS analysis of cerebral arteriovenous transit time (cTT) is used as a measure of cerebral microcirculation.
The windows that are used for transcranial Doppler examinations include regions where the skull bones are relatively thin or where naturally occurring gaps allow proper penetration of the sound beam.

See also A-Mode, Cranial Bone Thermal Index, Transcranial Doppler and Transcranial Window.

Spectral Doppler refers to the combination of either continuous wave Doppler or pulsed Doppler with a spectral display. Spectral Doppler provides a quantitative analysis of the velocity and direction of blood flow.
The Fourier spectrum analyzer performs a fast Fourier transformation on the Doppler signal. The amplitudes of the resulting spectra are encoded as brightness. In the 2D spectral display, the frequency shift is depicted in the vertical and the time in the horizontal axis. The range of blood velocities in the volume produces a corresponding range of frequency shifts.

See also Acceleration Index and Triplex Exam.