摘要:

The magnetic resonance imaging appearance of blood, as with other body tissues, is affected strongly by magnetic relaxation rates of the water protons. For blood containing only oxyhemoglobin, as for most tissues, the relaxation times are determined by diamagnetic effects related primarily to protein content. However blood containing either deoxyhemoglobin or methemoglobin exhibits additional paramagnetic relaxation effects, which have important consequences for magnetic resonance imaging of hematomas. First, the field inhomogeneity created by the concentration of paramagnetism in the red blood cells lowers the effectiveT2. This effect depends on field strength, and so is more striking at high fields, and is greater if gradient echoes are used. In fact, the observation of a difference inT2with the two different echo methods provides an unequivocal indication of field inhomogeneity such as is produced by erythrocytes. A second paramagnetic relaxation effect is the direct interaction of protons with the electron spin of methemoglobin, which markedly lowers bothT1andT2. This effect is important in the imaging of hematomas that are at least several days old, after significant conversion of hemoglobin to the met form has taken place.

ISSN:0269-3127    

DOI:10.1118/1.595994

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The movement of spins during periods of selective pulses result in a modulation of the signal intensity and phase of the received magnetic resonance imaging (MRI) signal, and is a major cause of signal loss from vessels imaged with slice‐selective pulses. Methods are well developed for compensation of phase perturbations for spins flowing at constant velocity during the time of applied gradients. However, for spins flowing during selective pulses, the magnitude of the amplitude and phase perturbations has not been understood nor to this time has any method of flow compensation been proposed. This is due in part to the difficulty in using the Bloch equations to quantify the amplitude and phase modulation during radiofrequency (rf) excitation since solutions cannot be obtained analytically. In this paper a finite difference method is used to solve Bloch equations for flowing spins during a 90° selective pulse. Compared with stationary spins, the magnetization distribution for flowing spins exhibits a shift of the slice profile in the direction of the flow, an expansion of the profile, phase shifts, and changes in profile shape. The profiles show residual phase errors which become more severe with higher flow velocities, with flow compensation schemes which apply in the case of spins flowing during applied gradients, and in the absence of an rf pulse. The measurement and understanding of the magnetization distribution is important to designing pulse sequences that compensate for flow. Flow compensated pulse sequences are necessary to reduce image flow artifacts and to increase signal of vessels in MR angiographic images.

ISSN:0269-3127    

DOI:10.1118/1.596129

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This study presents experimental results about the effective depth of penetration and about the radiofrequency (rf) power absorption in humans as a function of frequency. The frequency range investigated covers 10 up to 220 MHz. For the main part, the results were derived from bench measurements of the quality factorQ, and of the resonance frequency shift due to the loading of the coil. Different types of head‐, body‐, and surface coils were investigated loaded with volunteers or metallic phantoms. For spin‐echo imaging at 2 T (85 MHz), the local specific absorption rate (SAR) was found to be ≊0.05 W/kg using a π pulse of 1‐ms duration and pulse repetition timeTR=1 s. Measurements of the quality factorQas a function of frequency show that the SAR depends upon the frequencyfaccording to ∼f 2.15. The effective depth of rf penetration as derived drops from 17 cm at 85 MHz to 7 cm at 220 MHz. Head imaging withB1penetrating from practically all sides into the object should be possible up to 220 MHz (5 T) with SAR values staying within the local limit of 2 W/kg as set by the FDA. Whole‐body imaging of large subjects as well as surface coil imaging is depth limited above 100‐MHz frequency. Perturbation methods are applied in order to separate the total rf power deposition in the patient into dielectric and magnetic contributions. The observed effects due to interactions of rf magnetic fields with biological tissue contradict predictions based on homogeneous tissue models. A refined tissue model with regions of high electrical conductivity, subdivided by quasi‐insulating adipose layers, provides a rationale for a better understanding of the underlying processes. At frequencies below 100 MHz, the rf power deposition in patients is apparently more evenly distributed over the exposed body volume than currently assumed.

ISSN:0269-3127    

DOI:10.1118/1.595995

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The trachea and main bronchi of a supine patient in a magnetic resonance (MR) scanner are not contained in a single standard coronal plane, but instead intersect this coronal plane at some angle, usually 20°–35°. We have developed a new MR imaging protocol to determine the oblique imaging plane which best contains the trachea and main bronchi. The resulting oblique images simplify anatomical identification, and allow the user to select additional oblique planes which cut any desired portion of main bronchus in true cross section. Accurate lumen shapes and areas may then be extracted from these cross‐sectional images. The method does not require the patient to be moved or rotated, and does not require hardware modification. We demonstrate the clinical application of the protocol with both a normal volunteer and a patient with an endobronchial tumor. The use of gradient echo pulse sequences together with this protocol to distinguish between vessels and bronchi is presented. We provide phantom verification to demonstrate the quantitative accuracy of the method to provide lumen areas.

ISSN:0269-3127    

DOI:10.1118/1.595996

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Several algorithms have been investigated for reconstructing blood vessels from a limited number of x‐ray subtraction projections, distributed over a limited range of angles. Both computer simulations and anin vivoanimal study were carried out. The best reconstruction performance was achieved using an algorithm that folded in two pieces ofa prioriknowledge of the vascular density distributions: (1) the object is dilute, consisting mainly of a void; and (2) the density distribution in the reconstructions is most likely to be non‐negative. Both the signal‐to‐noise ratio (SNR) and the signal to out‐of‐focus blur were quantitated. Compared to tomosynthetic reconstruction (backprojection), the amount of residual blur from out‐of‐focus planes was significantly reduced with only a small penalty in diminished SNR. The combined effect resulted in significant qualitative image improvement for real arterial distributions as demonstrated in a canine arterial imaging example.

ISSN:0269-3127    

DOI:10.1118/1.595997

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method has been developed that combines dual‐energy subtraction and tomosynthesis for vessel imaging in intravenous angiography. This paper describes the procedure for doing tomosynthesis on a fan‐beam rotational‐motion system and gives the point responses of the imaging system. Phantom studies show that dual‐energy tomosynthesis improves the visualization of desired vessels lying on a selected plane. The results may be feasible for some applications such as clinical diagnosis of coronary artery diseases.

ISSN:0269-3127    

DOI:10.1118/1.595998

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Dual‐energy subtraction imaging allows tissue and bone structures to be separated from each other and attenuating thicknesses measured. Potential applications include chest imaging, bone mineral measurement, angiography, and mammography. However, intrinsic to most x‐ray detectors is the acceptance of scattered radiation as part of the image signal. Added to that is the veiling glare component when an image intensifier is used. Together, they result in erroneous transmission measurement and degrade the accuracy of energy subtraction processing. In this paper, the effects of scattered radiation and veiling glare on energy subtraction images are examined theoretically. A model is derived and used to compute the effects on the thickness signals, image contrast, and image noise as a function of the scatter glare to primary ratios. The ratios were measured on a point‐by‐point basis for a Rando chest phantom. For 96% of the image field studied, the thickness signals may be subject to an error ranging from 0 to −22.5 cm for tissue and 0 to 5.2 cm for bone. The image contrast in the tissue image may be reduced by a factor ranging from 1 to 59. The percentage of the uncanceled bone signals ranges from −52% to 52%. The contrast‐to‐noise ratio may be reduced by a factor ranging from 1 to 18.

ISSN:0269-3127    

DOI:10.1118/1.595975

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The superior spatial resolution obtained with parallel‐beam microradiography over conventional contact microradiography has allowed us to image microstructural features of dental hard tissue not previously reported. Our efforts to extend these techniques to provide a real‐time capability for viewingin situdemineralization and remineralization effects, at and below the 1‐μm level, have resulted in an instrument with several novel and unique features. Using a synchrotron radiation source of x rays and diffraction image magnification, we are now able to change magnification at will (x‐ray zoom lens). In addition, the energy range over which the instrument operates gives one considerable flexibility in optimizing image contrast. The techniques of parallel‐beam microradiography, and diffraction image magnification are applicable to problems in many other areas of science. Using examples within dental research, the uniqueness and versatility of these new techniques are discussed.

ISSN:0269-3127    

DOI:10.1118/1.595976

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have measured the modulation transfer functions [MTF ( f )'s] and the noise power spectra [NPS( f )] of therapy x‐ray detectors irradiated by60Co, 6‐ and 18‐MV radiotherapy beams. Using these quantities, we have calculated the noise‐equivalent quanta [NEQ( f )] and the detective quantum efficiency [DQE( f )] to quantitate the limitations of therapy detectors. The detectors consisted of film or fluorescent screen–film combinations in contact with copper, lead, or tungsten metal plates. The resolution of the detectors was found to be comparable to fluorescent screen–film combinations used in diagnostic radiology, however, the signal‐to‐noise ratio [SNR( f )] of the detectors was limited due to film granularity. We conclude that improved images can be obtained by using alternative detector systems which have less noise or film granularity.

ISSN:0269-3127    

DOI:10.1118/1.596114

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Depth‐ionization measurements have been obtained with an air‐filled Nordic Association of Clinical Physicists (NACP) design parallel‐plate ionization chamber in a water phantom for ten foil‐scattered electron beams from two different machines with nominal energies between 6 and 20 MeV and field sizes from 6×6 to 25×25 cm2. Depths of 50% ionization and practical range have been determined from least‐squares fits to both the raw data and values corrected to parallel‐beam geometry using measured virtual source distances. Depths of 50% dose have also been obtained from fits to divergence‐corrected depth‐dose measurments performed under identical conditions using, ap‐type silicon diode detector. Utilizing accepted conversion factors between mean incident energy (Ē0) and depth of 50% dose for parallel incident beams, and taking advantage of the fact thatp‐type silicon diode detector readings are nearly directly indicative of relative dose, conversion factors betweenĒ0and depth of 50% ionization for divergence‐corrected and raw, uncorrected finite source‐surface distance depth‐ionization data are empirically determined. Those values, obtained using the results of both ETRAN and EGS4 dose calculations as base lines, are compared to values currently recommended for use in clinical dosimetry.

ISSN:0269-3127    

DOI:10.1118/1.596115

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY