Felix Werner Wehrli

The dependence of nuclear magnetic resonance (NMR) image contrast on intrinsic and pulse sequence timing parameters.

In Nuclear Magnetic Resonance (NMR) the image pixel value is governed by at least three major intrinsic parameters: the spin density N (H), the spin-lattice relaxation time T1, and the spin-spin relaxation time T2. The extent to which the signal is weighted toward one or several parameters is related to the history of the spin system preceding detection. On the simplifying, though not generally warranted assumption that the spin density does not vary significantly in soft tissues, relative tissue contrast can be predicted quantitatively provided the relaxation times are known. Signal intensities and contrast were computed on the basis of the Bloch equations and experimentally determined relaxation times as a function of pulse timing parameters and the data compared with those in images recorded at 0.5T field strength. Significant deviations from the equal density hypothesis were found for gray and white substance. Notably partial saturation but also spin echo and inversion-recovery images are not in full accordance with predictions made on the basis of relaxation times alone.

Method for visualization of in-plane fluid flow by proton NMR imaging

A method for visualizing in-plane flow utilizing an NMR pulse sequence to produce a plurality of odd and even spin-echo signals occurring respectively at echo delay times, TE, of 2τ, 6τ, 10τ, etc., and 4τ, 8τ, 12τ, etc. In the preferred embodiment, a fictitious spin-echo amplitude is calculated from the odd and even spin-echo signals at an echo delay time TE =0, for example. The calculated values for the odd spin-echo signals are lower than those calculated for the even spin-echo signals due to incomplete rephasing of the odd spin-echo signals in the presence of a read-out magnetic field gradient and flow. Subtraction of the calculated image pixel value of the odd spin-echo signals from the calculated pixel values of the even spin-echo signals results in a difference image which highlights the flowing nuclear spins. The image pixels due to stationary nuclear spins experience exact cancellation.

Method of accurate and rapid NMR imaging of computed T1 and spin density

A method is provided for accurate and rapid NMR imaging of computed T1 and Mo (spin density) NMR images. The imaging data is acquired using a repetition of a sequence made up of RF and magnetic-field-gradient pulses. Each repetition of the sequence includes at least one step of reducing the net longitudinal magnetization in a predetermined region of the sample to zero. The longitudinal magnetization is allowed to at least partially recover prior to exciting nuclear spins in the predetermined region to produce at least one NMR spin-echo signal due primarily to the recovered magnetization. The spin-echo signal is sampled in the presence of a magnetic-field gradient for encoding spatial information.

SAR reduced pulse sequences

Three techniques were considered for reducing the RF (radiofrequency) power deposition in the body while maintaining scan time efficiency: reducing the RF peak amplitude while increasing the pulse width, substituting gradient echoes for spin echoes, and reducing the flip angle of the phase reversal pulse. The use of gradient echoes was found to be the most efficient means to reduce the power delivered to the patient and to obtain rapid data acquisition. The effect upon SAR (specific absorption rate) and SNR (signal-to-noise ratio) was demonstrated on a phantom when the phase reversal pulse was reduced from the standard 180 degrees to 90 degrees. Data in the body indicated a fairly constant SNR down to a refocusing flip angle between 110 degrees and 135 degrees. An initial clinical evaluation was performed at three institutions using the method of reducing the flip angle of the phase reversal pulse. The scan with theta = 120 degrees was rated by readers in a blinded study as having acceptable diagnostic image quality while the 135 degrees scan had comparable image quality to a conventional 90 degrees - 180 degrees pulse sequence. The use of reduced phase reversal pulses was seen as an efficient protocol to obtain T1-weighted images at rapid data rates while reducing the power delivered to the body by about 40%.

Cervical spine MR imaging using multislice gradient echo imaging: Comparison with cardiac gated spin echo

Forty-one patients with suspected cervical spine disorders were studied using multislice gradient echo imaging (GE) technique, with a 1.5-T system. The images were compared to cardiac-gated spin echo (CGSE) images in the diagnosis of suspected cord and spinal disorders. Images were graded for ability to detect cord lesion, cord-CSF contrast, CSF-bone contrast and contrast between CSF and extradural abnormality. The signal-to-noise ratio and contrast-to-noise ratio were used to compare images. There was 44% decrease in contrast between cord lesion and normal cord on GE when compared to CGSE, except for spinal cord hemorrhage. There was a 40% improvement between bone and CSF contrast on GE compared to CGSE. GE images were significantly better qualitatively as well as quantitatively in the detection of extradural lesions. This effect was more marked in axial plane where CGSE images are extremely suboptimal. CGSE images are better than GE for spinal cord lesions, while GE are superior in the diagnosis of degenerative disease in the cervical spine.

The Dependence Of Nuclear Magnetic Resonance (NMR) Image Contrast On Intrinsic And Operator-Selectable Parameters

In nuclear magnetic resonance (NMR) the image pixel value is governed by four intrinsic parameters: the spin density ρ, the spin-lattice relaxation time T1, the spin-spin relaxation time T2 and, for non-stationary spins, the flow velocity v. The extent to which the signal is weighted toward one or several parameters is related to the history of the spin system preceding the detection pulse. In the present work T1 and T2 were determined in vivo for several regions in the CNS from inversion-recovery (T1) And multiple-echo (T2) images, using least-squares fitting procedures. From averaged values of T1 and T2 in grey matter, white matter and CSF, the signal intensity was calculated on the basis of the Bloch equations and plotted as a function of the intrinsic parameters for the three most common imaging pulse sequences. These data are in excellent agreement with images, recorded from normal volunteers on an experimental whole-body imaging system operating at 12.8 MHz (0.3T). The graphical presentation of contrast further will provide the radiologist with a straightforward tool for image interpretation.

Time-Of-Flight NMR Imaging Of Plug And Laminar Flow

Moving spins have a significant effect upon the received MRI signal, which is seen, depending upon the pulse sequence utilized, as a modulation in signal intensity and/or phase relative to that of stationary spins. This can be used in MRI to distinguish between blood vessels and stationary anatomic structures. Monoplanar time-of-flight techniques [1-9] use pulse sequences which modulate the signal intensity from spins in a vessel flowing perpendicularly through a slice. This modulation is the result of the fact that the signal is composed of signal strengths from different spin populations whose relative ratio will depend upon the thickness of the slice, the velocity of fluid perpendicular to the slice, and pulse sequence parameters. If two different pulse sequences are used with identical signals from stationary anatomy, one producing increased signal in the region of flowing spins and the other producing decreased signal, then a subtraction image can be formed depicting only regions of flow [2]. Here we develop expressions for the signal intensity of two pulse sequences which are expected to give optimum contrast for imaging flowing blood free of overlying non-vascular anatomy. This imaging technique would provide a means for imaging blood vessels without the use of ionizing radiation or contrast injection; furthermore, it provides information about the presence of flow in vessels. As such, it may be promising as a method to evaluate vessels which are not accessible to standard angiographic imaging.