eBook - ePub
Rad Tech's Guide to MRI
Basic Physics, Instrumentation, and Quality Control
William H. Faulkner, Euclid Seeram
This is a test
- English
- ePUB (adapté aux mobiles)
- Disponible sur iOS et Android
eBook - ePub
Rad Tech's Guide to MRI
Basic Physics, Instrumentation, and Quality Control
William H. Faulkner, Euclid Seeram
DĂ©tails du livre
Aperçu du livre
Table des matiĂšres
Citations
Ă propos de ce livre
The second edition of Rad Tech's Guide to MRI provides practicing and training technologists with a succinct overview of magnetic resonance imaging (MRI). Designed for quick reference and examination preparation, this pocket-size guide covers the fundamental principles of electromagnetism, MRI equipment, data acquisition and processing, image quality and artifacts, MR Angiography, Diffusion/Perfusion, and more.
Written by an expert practitioner and educator, this handy reference guide:
- Provides essential MRI knowledge in a single portable, easy-to-read guide
- Covers instrumentation and MRI hardware components, including gradient and radio-frequency subsystems
- Provides techniques to handle flow imaging issues and improve the quality of MRIs
- Explains the essential physics underpinning MRI technology
Rad Tech's Guide to MRI is a must-have resource for student radiographers, especially those preparing for the American Registry of Radiation Technologist (ARRT) exams, as well as practicing radiology technologists looking for a quick reference guide.
Foire aux questions
Comment puis-je résilier mon abonnement ?
Il vous suffit de vous rendre dans la section compte dans paramĂštres et de cliquer sur « RĂ©silier lâabonnement ». Câest aussi simple que cela ! Une fois que vous aurez rĂ©siliĂ© votre abonnement, il restera actif pour le reste de la pĂ©riode pour laquelle vous avez payĂ©. DĂ©couvrez-en plus ici.
Puis-je / comment puis-je télécharger des livres ?
Pour le moment, tous nos livres en format ePub adaptĂ©s aux mobiles peuvent ĂȘtre tĂ©lĂ©chargĂ©s via lâapplication. La plupart de nos PDF sont Ă©galement disponibles en tĂ©lĂ©chargement et les autres seront tĂ©lĂ©chargeables trĂšs prochainement. DĂ©couvrez-en plus ici.
Quelle est la différence entre les formules tarifaires ?
Les deux abonnements vous donnent un accĂšs complet Ă la bibliothĂšque et Ă toutes les fonctionnalitĂ©s de Perlego. Les seules diffĂ©rences sont les tarifs ainsi que la pĂ©riode dâabonnement : avec lâabonnement annuel, vous Ă©conomiserez environ 30 % par rapport Ă 12 mois dâabonnement mensuel.
Quâest-ce que Perlego ?
Nous sommes un service dâabonnement Ă des ouvrages universitaires en ligne, oĂč vous pouvez accĂ©der Ă toute une bibliothĂšque pour un prix infĂ©rieur Ă celui dâun seul livre par mois. Avec plus dâun million de livres sur plus de 1 000 sujets, nous avons ce quâil vous faut ! DĂ©couvrez-en plus ici.
Prenez-vous en charge la synthÚse vocale ?
Recherchez le symbole Ăcouter sur votre prochain livre pour voir si vous pouvez lâĂ©couter. Lâoutil Ăcouter lit le texte Ă haute voix pour vous, en surlignant le passage qui est en cours de lecture. Vous pouvez le mettre sur pause, lâaccĂ©lĂ©rer ou le ralentir. DĂ©couvrez-en plus ici.
Est-ce que Rad Tech's Guide to MRI est un PDF/ePUB en ligne ?
Oui, vous pouvez accĂ©der Ă Rad Tech's Guide to MRI par William H. Faulkner, Euclid Seeram en format PDF et/ou ePUB ainsi quâĂ dâautres livres populaires dans Medicine et Medical Technology & Supplies. Nous disposons de plus dâun million dâouvrages Ă dĂ©couvrir dans notre catalogue.
Informations
1
Hardware Overview
Instrumentation: Magnets
To obtain a magnetic resonance(MR) signal from tissues, a large static magnetic field is required. The primary purpose of the static magnetic field (known as the B0 field) is to magnetize the tissue. The magnet technology utilized is either referred to as: Permanent, Resistive or Superconductive. Regardless of the style or type of magnet used, the B0 field must be stable and homogeneous, particularly in the central area of the magnet (isocenter) which is where the anatomy to be imaged should be placed.
- The vertical field magnet design uses two magnets, one above the patient and one below the patient.
- The frame, which supports the magnets, also serves to âreturnâ the magnetic field.
- Generally, vertical magnets have a reduced fringe field compared with conventional horizontal field magnets.
- The âopen designâ of these systems is often marketed as being less confining to the patient who may be anxious or claustrophobic.
- âOpen MRIâ is marketing terminology and has no basis or meaning in science.
- The radioâfrequency (RF) transmit coil and gradient coils for vertical field magnets (discussed in more detail later) are flat coils located on the âfaceâ of the magnets.
- The receiver or surface coils used with vertical field magnets are solenoid in design.
- For vertical field magnets, field strength and homogeneity can be increased by reducing the gap between the two magnets. The disadvantage to reducing the gap is the obvious reduction in patient area.
Regardless of whether the field is vertical or horizontal, there are three primary types of technology utilized tor MRI system magnets: permanent, resistive, and superconducting.
Permanent Magnets
- MRI systems based on permanent magnet technology use materials which are, as the name implies, permanently magnetized to produce the main external magnetic field (BO).
- Increasing the amount of material used increases the field strength, in addition to size and weight.
- Permanent magnets generally have field strengths of 0.06 to 0.35 Tesla.
- Generally, vertical field permanent magnets have a relatively small fringe field.
- Because of the small fringe field, permanent magnets are often easy to sight, though their weight can be an issue.
- Permanent magnets are sensitive to ambient room temperature.
- Changes in scan room temperature can cause the field strength to vary several gauss per degree.
- Because changes in field strength result in changes in resonant frequency, image quality can vary if the field drifts significantly.
Resistive Magnets
- Resistive magnets are generally used in either a vertical or transverse field system.
- Larger resistive magnetâbased systems can have field strengths up to 0.6 Tesla.
- Whenever electrical current is applied to a wire, a magnetic field is induced around the wire.
- To produce a static field (i.e. not alternating), direct current is required.
- Resistive systems generally also contain an iron core around which the wire is wound.
- Increasing the amount of current or turns of wire increases the field strength and results in heat in the wire.
- Resistive magnets require a constant current to maintain the static field.
- Cooling of the coils is also required as the byâproduct of electrical resistance is heat.
- Resistive magnets can easily be turned off when not in use (permanent and superconductive magnets cannot be turned off).
- The earliest type of magnets used in MRI were resistive.
- Resistive magnets can also be temperatureâsensitive.
Superconductive Magnets
- Superconductive magnets are similar to resistive magnets because they use direct current actively applied to a coil of wire to produce the static magnetic field.
- The main difference is that the coils are immersed in liquid helium (cryogen) to remove the resistance.
- When the temperature of any conductor is reduced, electrical resistance decreases.
- Without the resistance, the electrical current can flow within a closed circuit without external power being applied (i.e. no voltage is needed for current to flow).
- The flow of electrical current without resistance is known as superconductivity.
- Most superconductive magnets are solenoid in design and thus, result in a horizontal magnetic field.
- Recent innovations in magnet design allow for vertical field systems using superconductive magnets.
- Superconductive magnets are capable of achieving higher field strengths compared to permanent and resistive magnet technology.
- Smallâbore horizontal magnets used to image small animals and tissue samples can have field strengths of 10 Tesla or higher.
- Superconductive magnets currently approved for use by the FDA (US) in clinical settings include field strengths from 1.0 Tesla to 7.0 Tesla.
- Higher field strengths produce greater fringe fields.
- To reshape and/or reduce the fringe field for siting purposes, magnetic shielding is employed.
- Passive magnetic shielding uses metal (iron) in the scan room walls.
- Active magnetic shielding uses additional coils as part of the magnet design.
- Helium is not stable as a liquid. The temperature of liquid helium is 4 Kelvin. In order to maintain that temperature, it must be kept in a vacuum. Helium will boil at 4.2 K. If the temperature within the vessel containing the magnet coils and liquid helium rises only slightly, or if the vacuum were to be lost, then the liquid helium will boil and expand at a ratio of approximately 1:750.
- The resultant helium gas will burst through a pressureâsensitive containment system and should vent outside the scan room through a duct system attached to the magnet.
- In the absence of the supercooled environment, the current in the magnet coils will experience resistance, and the static field will be lost.
- This sudden and violent loss of superconductivity is referred to as a quench.
- The major advantage of superconducting technology is high field strength, which results in inherently high signalâtoânoise ratio (SNR).
- The high SNR can be âtradedâ for rapid scan times and increased spatial resolution.
- The major disadvantage of superconducting technology is the high cost associated with acquisition, siting, and maintenance.
B0 Homogeneity
Regardless of the type or style of magnet used for an MR system, the magnetic field must be as homogeneous as possi...