Signal-to-noise ratio in MRI.
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Electricity↗
Biomedical subjects
T W Redpath
Publications and source records attributed to T W Redpath.
Temporomandibular joint MRI: a 2-D gradient-echo technique.
Magnetic resonance imaging (MRI) of the temporomandibular joint (TMJ) has been most commonly performed using spin-echo sequences. Gradient-echo sequences have previously been investigated in the context of 3-D and 'dynamic' (pseudo-kinematic) imaging. We have used gradient-echo to improve image quality in static studies with rapid acquisition, high spatial resolution (512 x 512 matrix) and excellent contrast resolution. Using pragmatic methods and a non-randomized study group we have demonstrated a definite advantage to image quality, and thus diagnostic confidence, from the use of a gradient-echo high spatial resolution sequence incorporating split acquisition open mouth views.
Adolescent↗
A bandwidth indicator for magnetic resonance imagers.
Signal-to-noise ratios in magnetic resonance imaging are crucial in determining image quality, and dependent on a number of factors, one being the signal bandwidth per pixel. Not all manufacturers clearly state the bandwidth per pixel used for all sequences. A small battery-powered portable device is described which produces bright sharp lines on the magnetic resonance image at 10 kHz intervals in the frequency encoding direction. The bandwidth per pixel can then easily be calculated using electronic distance callipers, provided the image matrix and field of view are known. The device is expected to be especially of value when acceptance testing on poorly documented imaging systems.
Biophysical Phenomena↗
MRI developments in perspective.
Following Paul Lauterbur's seminal 1973 paper in Nature, considerable work was needed to overcome a number of physical, engineering and technical problems before the new technique of magnetic resonance imaging (MRI) could be applied clinically. Much of that pioneering work was done in the UK. Since the first head and whole-body images were obtained in the late 1970s, MRI has become a widely used clinical imaging modality capable of yielding tomographic images of excellent spatial resolution and tissue contrast. This review outlines the historical development of MRI in the context both of the technical problems which had to be overcome, and of the clinical uses of MRI. Current areas of research, such as the use of MRI to map brain function, the measurement of physiological parameters such as tissue perfusion, and the use of open-access real-time MRI to guide interventional procedures, are briefly discussed.
Equipment Design↗
The influence of stimulated echoes on contrast in fast spin-echo imaging.
Tissue contrast obtained using fast spin-echo (FSE) and conventional spin-echo (SE) sequences is not identical and a number of mechanisms are thought to contribute to these contrast differences. The effect of stimulated echoes has previously been apparently ruled out as a contributing mechanism. Signal-to-noise ratios of single-slice matched FSE and conventional SE sequences were compared in aqueous solutions of CuSO4, Cr2(SO4)3 and MnSO4 with various T1 and T2 values. Enhancement of the FSE signal was observed in short T2 solutions and the effect was greater in samples where the T1 to T2 ratio was high. Reducing the refocusing pulse flip angle to increase the contribution from stimulated echoes also resulted in slightly increased enhancement. Experimental results were verified by computer simulations. Our results show that stimulated echoes do contribute to the contrast differences between FSE and conventional SE images and may be significant in the imaging of brain hemorrhage.
Computer Simulation↗
Choice of contrast enhancement index for dynamic magnetic resonance mammography.
Indices are often used in dynamic MRI of the breast to quantitate signal enhancement within suspicious lesions. Two indices are commonly used: one calculates the difference in pre- and postcontrast signal intensity, normalised to a base-line signal intensity such as that of fat (which does not enhance) whilst the other calculates the ratio of pre- to postcontrast signal intensity. The results of a computational simulation are presented which demonstrate the superiority of the normalised signal difference index, based on the criterion that the best index is that which is least influenced by initial tissue T1. This hypothesis was tested by comparing the two indices in a group of patients with clinical or mammographic suspicion of recurrent breast carcinoma. Of 37 patient examinations using Gadolinium enhanced MRI of the breast, 11 patients showed 13 lesions with some degree of enhancement, which were subsequently diagnosed histologically as either benign or malignant. The normalised signal difference index showed no overlap between the benign and malignant groups, whereas some overlap was observed with the signal ratio index. The clinical findings are therefore consistent with the results of the computational simulation.
Adult↗
A variable threshold edge-detector for improved quantitation of gated tomographic imaging of the left ventricular blood pool.
The accurate measurement of left ventricular volume from tomographic MUGA studies is difficult due to the limited resolving power of the gamma camera, which causes errors in the detection of the correct ventricular boundaries. Therefore, the use of fixed threshold or second-derivative edge-detectors results in overestimates at small volumes. A variable threshold edge-detection technique was developed to overcome this. Computer-simulated short-axis slices through the heart over a range of left ventricular dimensions were convolved by the Point Spread Response Function of the system to model the acquired image. The maximum pixel value and the threshold value required to detect the true ventricular edge from each simulation were then combined into a look-up table for the calculation of the required threshold value. As the dimension of the ventricle decreased, the threshold value chosen to detect the ventricular edge increased. Left ventricular volumes and ejection fraction measurements were calculated for seven patients using cine-MRI as the gold-standard technique for validation of the proposed method. The single photon emission tomographic studies were analysed using both the standard second-derivative edge-detection software and the proposed variable threshold technique. The variable threshold technique was shown to increase significantly the accuracy of ventricular volume measurements and ejection fraction calculations. The average error in the measurement of volumes was reduced from 41.4 +/- 45.1% to 18.5 +/- 14.6% and the accuracy of ejection fraction measurement was increased from 29.7 +/- 4.6% to 11.3 +/- 6.9%.
Algorithms↗
Measurement of spin-lattice relaxation times with FLASH for dynamic MRI of the breast.
Using a variable flip angle gradient refocused imaging technique, dynamic quantitative T1 relaxation maps acquired before and after the administration of a gadolinium contrast bolus enable the concentration-time curve of the paramagnetic agent in breast tissue to be calculated. This imaging technique has the aim of improving the diagnostic accuracy of MR mammography. Measurements with phantoms of calibrated T1 values have been carried out to investigate the accuracy of the method, with particular reference to errors caused by incomplete spoiling of residual transverse magnetization and inaccurate radio frequency (RF) flip-angle settings. A clinical example is presented. The method has potential use for any patient study which necessitates rapid quantitation of changing in vivo T1 values as a result of contrast agent injection.
Breast Neoplasms↗
Application of the principle of optical phase-contrast microscopy to velocity phase-encoded MRI of blood flow in the aorta.
A new method of presenting magnetic resonance phase information is described and an example of its application given in the context of velocity phase-encoded MRI of blood flow in the aorta. The method takes as its starting point Zernike's technique of phase contrast microscopy. It exploits the parallel between the transform plane in Fourier optics and kappa-space in MRI. In the example described two datasets are acquired, one with and the other without velocity encoding, as in conventional phase-encoded velocity imaging. A dataset is formed which is corrected for unwanted phase variations caused by static field inhomogeneity. The method then effectively combines phase and magnitude information into a single image. The technique is complementary to existing methods of displaying phase information.
Aorta↗
Dynamic magnetic resonance mammography of both breasts following local excision and radiotherapy for breast carcinoma.
The purpose of this study was to evaluate a dynamic gadolinium-diethylene-triaminepentacetate (Gd-DTPA) enhanced magnetic resonance mammography (MRM) protocol, incorporating image subtraction and using a dedicated double breast coil, in patients with treated breast cancer. 36 patients who had undergone breast conserving surgery and radiotherapy, with clinical or mammographic suspicion of locally recurrent breast carcinoma, were examined. 3D rapid volume imaging of both breasts was performed before and repeated three times, at 1, 2 and 3 min, following Gd-DTPA enhancement. Image subtraction allowed identification of enhancing lesions and quantitative analysis of these was performed. Five patients had six lesions demonstrating rapid enhancement in the first minute, all of which were histologically confirmed recurrent or second primary tumours. Slower enhancement was seen in six benign lesions and diffuse enhancement was seen in one patient within 6 months of treatment. The dynamic protocol used allowed sufficient temporal resolution for distinguishing malignant from benign lesions at the site of previous surgery and ensured that both breasts were imaged in their entirety, thus enabling the diagnosis of multifocal and second primary tumours.
Adult↗
Pre-surgical dental implant assessment by magnetic resonance imaging.
The use of magnetic resonance imaging (MRI) for pre-surgical implant assessment of available bone in the maxilla and mandible is described. An acrylic surgical template with suitable MRI reference markers allows potential implant sites, as chosen on the MR images, to be accurately identified at surgery. MRI is a tomographic modality capable of giving accurate information on the three-dimensional relationships of all the relevant anatomic structures. Our results show that MRI is reasonably tolerant of artifacts caused by metal pins and amalgam fillings. Unlike computerized tomography (CT) and other x-radiographic techniques, MRI uses no ionizing radiation, and is capable of angulating and offsetting its scan plane at will. Good bone detail is available because cancellous bone yields a strong signal from the marrow fat, while cortical bone and dental enamel are dark. The excellent anatomic detail provided by thin-slice high-resolution MRI allows for assessment of the suitability of sites to receive an implant in terms of bone quality and thickness, and the relative position of the site to important structures such as the inferior dental nerve and nasal sinuses. The MRI technique used is described in detail. The principles underlying image contrast are outlined where appropriate and examples shown. To date, we have used MRI in conjunction with rotational panoramic x-rays (OPG) to plan 26 implants (21 maxillary, 5 mandibular) in 12 patients.
Alveolar Bone Loss↗
Aortic distensibility and stiffness index measured by magnetic resonance imaging in patients with Marfan's syndrome.
OBJECTIVES: To use magnetic resonance imaging to measure the elastic properties of the aorta of adults with Marfan's syndrome and to compare these results with those obtained by echocardiography. PATIENTS AND METHODS: 12 patients with Marfan's syndrome and 12 controls matched for age. Transverse luminal areas of the ascending and descending aorta were measured using electrocardiographic gated magnetic resonance imaging. Echocardiography was used to measure the diameter of the ascending aorta and aortic arch in patients with Marfan's syndrome. Blood pressure was measured during both scans. RESULTS: In diastole, transverse luminal areas of the ascending and descending aorta were significantly greater in patients with Marfan's syndrome when measured by magnetic resonance imaging and corrected for body surface area; P < 0.02 and P < 0.05 respectively. Patients with Marfan's syndrome had a higher stiffness index (112.77 v 5.78, P < 0.05) and a lower distensibility (0.0066 v 0.0105, P < 0.05) than controls. Results produced by MRI and echocardiography were not significantly different. CONCLUSIONS: Magnetic resonance imaging gives good quality reproducible images of the ascending and descending aorta. In patients with Marfan's syndrome, aortic distensibility and stiffness index measured by magnetic resonance imaging were abnormal (but did not always relate directly to the size of the aorta.
Adult↗
Evaluation of tilted, optimized, non-saturating excitation pulses in 3D magnetic resonance angiography of the abdominal aorta and major branches in volunteers.
Ten normotensive volunteers were imaged with FISP-3D (Fast Imaging Steady Precession) pulse sequences (a steady state gradient recalled echo, SS-GRE, technique) without and with modification by tilted, optimized non-saturating excitation (TONE) customized radio frequency (RF) pulses, for evaluation of the abdominal aorta--in particular the renal arteries. Maximum intensity projection (MIP) angiograms were constructed for each sequence in each volunteer; 15 observers "blinded" to sequence type preferred the images generated from the TONE applied sequence. Analysis showed that this preference was significant.
Aorta, Abdominal↗
Cine magnetic resonance imaging of pulsatile cerebrospinal fluid flow using CSPAMM.
Spatial modulation of magnetization (SPAMM) is a well established imaging technique which superimposes a tagging pattern on conventional magnetic resonance (MR) images, allowing movement to be visualized. A modification to the SPAMM technique, called complementary spatial modulation of magnetization (CSPAMM), which improves the contrast of the tagging pattern is explained. The application of CSPAMM to the visualization of pulsatile cerebrospinal fluid flow (CSF) using an 8 frame cardiac-gated cine sequence is described. Various combinations of binomial pulses, up to the fifth order, were investigated to see which produces the optimum tagging pattern in the CSPAMM images. The flip angles of the imaging RF pulses were studied to see which would give equal maximum CSF signal intensity in all the cine images. The optimized cine CSPAMM technique was compared in vivo with SPAMM and CSF motion was found to be more easily visualized in the CSPAMM images.
Cerebrospinal Fluid↗
Technical note: use of a double inversion recovery pulse sequence to image selectively grey or white brain matter.
The design of a double inversion recovery (DIR) sequence, to image selectively grey or white brain matter, is described. Suitable choice of inversion times allows either cerebrospinal fluid (CSF) and white matter to be suppressed, to image the cortex alone, or CSF and grey matter to be suppressed, to image the white matter. The DIR sequence was found to give clear delineation of the cerebral cortex.
Adult↗
A cine-SPAMM sequence for NMR imaging of pulsatile cerebrospinal fluid flow.
The technical difficulties in designing a SPAMM sequence to image pulsatile cerebrospinal fluid (CSF) flow at different phases of the cardiac cycle are described. The criteria used to select the most appropriate order of binomial SPAMM sequence are outlined. Data collection times required to view both cephalad and caudad flow for all R-R intervals were considered. The flip angles of the RF pulses required to produce images with equal CSF intensity throughout the cardiac cycle were investigated in detail.
Cerebrospinal Fluid↗
Fat suppressed magnetic resonance imaging at 0.5 T using binomial radiofrequency pulses.
Fat suppressed MR imaging can be achieved by selectively saturating lipid protons, just before applying a conventional spin-echo rf pulse sequence. The difference in the Larmor frequency between fat and water protons is only 3.5 ppm, so that the frequency response of the suppressing pulse, or pulses, has to be carefully designed. The choice of suitable binomial and modified trains of hard rf pulses was investigated by computer simulation, phantom testing and T1 weighted spin-echo imaging of a normal volunteer's optic nerve at 0.5 T.
Computer Simulation↗
Noise correlation in multicoil receiver systems.
When two or more rf coils are used to receive NMR signals from the same part of the patient, the overall sensitivity of an image formed from some combination of these signals may be degraded if noise voltages in the coils are correlated. Whereas it is straightforward to calculate correlation caused by noise sources in the coils themselves, there is disagreement about noise correlation when that noise arises from thermally driven currents within the body itself. A general analysis based on the principles of reciprocity, superposition, and energy conservation yields an equation for sample-induced noise correlation in terms of circuit impedances. Furthermore, the analysis yields a very simple expression for the total noise correlation (i.e., noise from patient and coil elements) between the coils. This latter expression is applicable to any two-port receiver coil assembly, including single coils with two modes, such as a birdcage coil, and in general, to any two ports of an n-port receiver system.
Artifacts↗