China
Africa
Explore chapters and articles related to this topic
Introduction
Published in A Stewart Whitley, Jan Dodgeon, Angela Meadows, Jane Cullingworth, Ken Holmes, Marcus Jackson, Graham Hoadley, Randeep Kumar Kulshrestha, Clark’s Procedures in Diagnostic Imaging: A System-Based Approach, 2020
A Stewart Whitley, Jan Dodgeon, Angela Meadows, Jane Cullingworth, Ken Holmes, Marcus Jackson, Graham Hoadley, Randeep Kumar Kulshrestha
If the nuclei are subjected to a second rotating magnetic field at the precessional frequency and at right angles to the applied magnetic field (B0), the nuclei will acquire energy and the net magnetisation can be deflected away from its initial orientation by an angle known as the flip angle (Fig. 1.31b). This phenomenon is known as resonance and the rotating magnetic field is known as a RF excitation pulse. The magnitude of the flip angle depends on the amplitude and duration of the RF pulse. Resonance results not only in a change in direction of the net magnetisation from the longitudinal axis to the transverse axis but also causes the nuclei to precess ‘in phase’ with each other. A receiver coil placed in the transverse plane will have voltage (signal) induced in it. This signal is the MR signal, its magnitude being dependent on the net magnetisation in the transverse plane. Once the RF pulse is removed the nuclei will gradually return to their alignment with the applied external magnetic field (B0). This process is known as relaxation.
Hardware
Published in Luisa Ciobanu, Microscopic Magnetic Resonance Imaging, 2017
The radiofrequency coils (RF) are used to excite the spin system (transmitters) and to detect the MR signal (receivers). The transmitter generates a rotating magnetic field, known as the B1 field, perpendicular to B0. This B1 field rotates the magnetization, M0, initially aligned with B0, about its axis (Fig. 2.1). The pulse of energy used to generate this rotation is called RF pulse. The angle of rotation of the magnetization, α in Fig. 2.1, is called the tip or flip angle and it depends on the length and the amplitude of the pulse. When the RF pulse is turned off, the transverse component of the magnetization precesses about the main magnetic field at a frequency known as the Larmor frequency (ω0) , determined by the nucleus under study and the strength of the main magnetic field (ω0 = γB0, where V is the gyromagnetic ratio.c
Magnetic Resonance Imaging
Published in Suzanne Amador Kane, Boris A. Gelman, Introduction to Physics in Modern Medicine, 2020
Suzanne Amador Kane, Boris A. Gelman
We can now explore how RF waves at the Larmor frequency can be used to flip the orientation of nuclear spins in MRI. Consider what happens to the nucleus when it experiences both the main magnetic field and an oscillating field due to the RF wave. To visualize the nucleus's response, consider Figure 8.12. The RF waves used for MRI excitation have their oscillating magnetic field oriented perpendicular to that of the main scanner magnet; such a situation can be produced by using an appropriately oriented radio transmitter built into the MRI scanner. The oscillation in time of the RF waves results in a continual rotation of the magnetic field direction (Figure 8.12). This second, oscillating field adds to the main magnetic field, but its value is too tiny to significantly alter the resonant frequency. It does, however, produce an additional torque on the dipole, which tends to gradually tip it over to greater angles. If the frequency of the RF wave is significantly different from the resonant frequency, the torque, due to the oscillating field, averages to zero; this is analogous to how rotating a magnet at the wrong frequency fails to disturb a compass significantly. For waves near the resonant frequency, the action of the tipping torque is continually reinforced, causing the nucleus's dipole to gradually tip over as it precesses (Figure 8.12). The angle by which the dipole is tipped (called the spin flip angle) depends on how long the RF wave acts on it; MRI scanners often use pulses that flip the dipole by exactly 90° or 180°. We call the exciting RF signal a pulse because it is typically switched on for only a short period of time. Any spin flip angle can be achieved by applying a sufficiently long pulse of RF.
Dimensions of pain catastrophising and specific structural and functional alterations in patients with chronic pain: Evidence in medication-overuse headache
Published in The World Journal of Biological Psychiatry, 2020
Foteini Christidi, Efstratios Karavasilis, Lars Michels, Franz Riederer, Georgios Velonakis, Evangelos Anagnostou, Panagiotis Ferentinos, Spyridon Kollias, Efstathios Efstathopoulos, Nikolaos Kelekis, Evangelia Kararizou
All participants underwent the same imaging protocol on a 3T Achieva TX Philips manufactured MRI scanner (Philips, Best, the Netherlands) at Radiology and Medical Imaging Research Unit, Second Department of Radiology, Attikon Hospital. The protocol included a 3D high-resolution T1 (3 D-HR-T1)-weighted sequence (repetition time (TR), 9.9 ms; time echo (TE), 3.7 ms; flip angle, 7°; voxel size, 1 × 1 × 1 mm, sagittal orientation), a T2*-weighted gradient echo combined with echo planar imaging for resting state functional magnetic resonance imaging (rs-fMRI) with whole brain coverage (TR, 2500 ms; TE, 30 ms; flip angle, 90°; acquisition voxel size, 3 × 3 × 3 mm3 and sensitivity encoding reduction factor of 2), as well as T2-weighted fluid attenuated inversion recovery (T2-FLAIR) sequence. During the rs-fMRI scan, participants were instructed to lie still with their eyes closed. An experienced neuroradiologist considered major anatomical abnormalities on participants’ T1-weighted and T2-FLAIR images of the whole brain. In addition, all data were checked slice by slice by an experienced MR physicist to identify motion or other type of artefacts.
The impact of phonological versus semantic repetition training on generalisation in chronic stroke aphasia reflects differences in dorsal pathway connectivity
Published in Neuropsychological Rehabilitation, 2018
Rachel Holland, Sasha L. Johns, Anna M. Woollams
Patients’ imaging data were acquired as part of a larger case series study by Butler et al. (2014). All scans were acquired on a 3 T Philips Achieva scanner (Philips Healthcare, Best, The Netherlands) using an 8-element SENSE head coil. High resolution structural magnetic resonance imaging (MRI) scans were acquired using a T1-weighted inversion recovery sequence with 3D acquisition, with the following parameters: TR (repetition time) = 9.0 ms, TE (echo time) = 3.93 ms, flip angle = 8°, 150 contiguous slices, slice thickness = 1 mm, acquired voxel size 1.0 mm × 1.0 mm × 1.0 mm, matrix size 256 × 256, FOV = 256 mm × 256 mm, TI (inversion time) = 1150 ms, SENSE acceleration factor 2.5. Distortion corrected diffusion-weighted images were acquired using a pulsed gradient spin echo echo-planar imaging sequence implemented with TE = 54 ms, Gmax = 62 mT/m, half scan factor = 0.679, 112 × 112 image matrix reconstructed to 128 × 128 using zero filling, reconstructed resolution 1.875 mm × 1.875 mm, slice thickness 2.1 mm, 60 contiguous slices, 43 non-collinear diffusion sensitisation directions at b = 1200 s/mm2 (Δ = 29.8 ms, δ = 13.1 ms), 1 at b = 0, SENSE acceleration factor = 2.5. Artefacts arising from pulsatile brain movements (Jones & Pierpaoli, 2005) were minimised by cardiac gating the diffusion sequence using a peripheral pulse unit placed on the participant’s finger. Acquisition time for the diffusion MRI data was approximately 28 minutes, although this varied slightly based on the participant’s heart rate. For each diffusion gradient direction, phase encoding was performed in right-left and left-right directions, giving two sets of images with the same diffusion gradient directions but opposite polarity k-space traversal, and hence reversed phase and frequency encode direction, allowing correction for geometric distortion (Embleton, Haroon, Morris, Ralph, & Parker, 2010). A co-localised T2 weighted turbo spin echo scan with 0.94 mm × 0.94 mm in-plane resolution and 2.1 mm slice thickness was also obtained for use as a structural reference scan in distortion correction (Embleton et al., 2010).
Spontaneous Improvement of Visual Acuity in a 13-Year-Old Boy with Neuromyelitis Optica Spectrum Disorder
Published in Neuro-Ophthalmology, 2019
Keiko Yamaguchi, Takaaki Hayashi, Akiko Kiriyama, Kie Iida, Shoyo Yoshimine, Yoichiro Masuda, Keigo Shikishima, Mitsuko Ariizumi, Genichiro Takahashi, Tadashi Nakano
A 13-year-old male Japanese patient reported blurred vision and retrobulbar pain in his right eye (oculus dexter, OD) at one month before his first visit to our department at the Katsushika Medical Center, Jikei University Hospital. As Goldmann perimetry performed by his previous doctor revealed the patient had a central scotoma in his OD, he was referred to our department for further evaluation. His family history was negative for any type of visual disturbance. At the time of his first examination, he had headache and retrobulbar pain in the OD. Encephalopathy and myelitis were not observed. His logMAR best-corrected visual acuity (BCVA) was 0.4 in the OD and −0.3 in the left eye (oculus sinister, OS). Intraocular pressure was 13 mmHg (OD) and 11 mmHg (OS). Relative afferent pupillary defect was positive in his OD. The critical flicker frequency (CFF) values were 8 Hz (OD) and 38 Hz (OS). He had no other neurological findings such as paralysis of limbs, back pain, and hearing loss. Slit lamp examination, funduscopy, and fluorescein angiography showed no remarkable findings in either eye (data not shown). Although visual field testing using Goldmann perimetry revealed central scotoma of the I-4e isopter in OD and suprerotemporal desensitization of the I-1e isopter in OS, therefore suggesting junctional scotoma (Figure 1). Using spectral-domain optical coherence tomography (OCT) (Cirrus, Carl Zeiss Meditec AG, Dublin, CA, USA), we accessed the circumpapillary retinal nerve fibre layer (RNFL) thickness and conducted a macular ganglion cell analysis. Since normative data were not available for individuals younger than 20 years of age, the OCT findings were evaluated based on the supposition that the patient was 20 years of age. OCT findings demonstrated that while there was no thinning of the RNFL, there was significant thinning of the macular ganglion cell layer (Figure 2). Gadolinium contrast-enhanced T1-weighted SPACE (sampling perfection with application optimized contrasts using different flip angle evolutions) magnetic resonance imaging (MRI) of the brain revealed that there were no remarkable findings such as multiple sclerosis lesions. However, there was thickening and enhancement from the right posterior optic nerve close to the optic chiasm (Figure 3), which suggested the presence of retrobulbar optic neuritis. To exclude Leber hereditary optic neuropathy (LHON), genetic testing for the mitochondrial DNA was performed. Results were negative for three primary mutations of G3460A, G11778A, and T14484C, which account for 95% of LHON. In addition, we tested for the anti-AQP4 antibody using both the enzyme-linked immunosorbent assay (ELISA) and the cell-based assay (CBA) since it is reported that the ELISA had a slightly lower sensitivity when compared to the CBA.5 Interestingly, ELISA results were negative for the anti-AQP4 antibody while the CBA results were positive. In addition, the anti-myelin oligodendrocyte glycoprotein antibody was negative. Based on the above-mentioned clinical and MRI findings, the current laboratory data for this patient were considered to meet the requirements of the diagnostic criteria for NMOSD. Thus, this patient was diagnosed with unilateral retrobulbar optic neuritis associated with NMOSD.