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Fundamentals and Mechanism of Action
Fundamentals and Mechanism of Action
• The Magnetic Field: Uses a powerful magnet (up to 60,000 times Earth's magnetic field) to transiently align hydrogen atoms in the body.
• The Magnetic Field: Uses a powerful magnet (up to 60,000 times Earth's magnetic field) to transiently align hydrogen atoms in the body.
• Radiofrequency (RF) Pulse: A specific RF pulse deflects these atoms, imparting energy; when the pulse stops, atoms realign (relaxation phase) and emit radio waves.
• Radiofrequency (RF) Pulse: A specific RF pulse deflects these atoms, imparting energy; when the pulse stops, atoms realign (relaxation phase) and emit radio waves.
• Voxel Localization: Gradient magnets arrange the field so each discrete volume element (voxel) has a unique frequency based on its location.
• Voxel Localization: Gradient magnets arrange the field so each discrete volume element (voxel) has a unique frequency based on its location.
• Chemical Mapping: Unlike CT (based on density), MRI is based on chemical composition, providing exquisite detail and contrast of body structures.
• Chemical Mapping: Unlike CT (based on density), MRI is based on chemical composition, providing exquisite detail and contrast of body structures.
• Data Management: Representations of signal strength are stored on the PACS (picture archiving and communication system), while raw scan datasets are usually not kept due to size and proprietary formats.
• Data Management: Representations of signal strength are stored on the PACS (picture archiving and communication system), while raw scan datasets are usually not kept due to size and proprietary formats.
Scanning Procedures and Modalities:
Scanning Procedures and Modalities:
• Acquisition: Modern scanners acquire volumetric (3D) datasets simultaneously.
• Acquisition: Modern scanners acquire volumetric (3D) datasets simultaneously.
• Sequences: Scans involve multiple sequences in different orthogonal planes to assess various molecule types and tissues.
• Sequences: Scans involve multiple sequences in different orthogonal planes to assess various molecule types and tissues.
• Time Requirements: Because signals are small, many repetitions are needed, making sequences last several minutes.
• Time Requirements: Because signals are small, many repetitions are needed, making sequences last several minutes.
• Contrast Enhancement:
• Contrast Enhancement:
◦ Common for brain and soft tissue imaging to provide specific etiologic diagnoses.
◦ Common for brain and soft tissue imaging to provide specific etiologic diagnoses.
◦ Uses specific MRI agents distinct from those used in radiography or CT.
◦ Uses specific MRI agents distinct from those used in radiography or CT.
◦ Safety Note: Generally safe in animals, but should be used with caution in patients with preexisting renal disease.
◦ Safety Note: Generally safe in animals, but should be used with caution in patients with preexisting renal disease.
Clinical Applications:
Clinical Applications:
• Neuroimaging: The mainstay for imaging the brain and nervous system in small animals.
• Neuroimaging: The mainstay for imaging the brain and nervous system in small animals.
• Musculoskeletal:
• Musculoskeletal:
◦ Ideal for joints and muscles, specifically ligaments and cartilage.
◦ Ideal for joints and muscles, specifically ligaments and cartilage.
◦ Equine Specialty: Crucial for identifying lesions in the equine distal limb.
◦ Equine Specialty: Crucial for identifying lesions in the equine distal limb.
• Angiography: Evaluation of blood vessels deep within the head, neck, and legs.
• Angiography: Evaluation of blood vessels deep within the head, neck, and legs.
• Bone Evaluation: While less effective for cortical bone than CT, it is excellent for bone marrow.
• Bone Evaluation: While less effective for cortical bone than CT, it is excellent for bone marrow.
Safety and Artifacts:
Safety and Artifacts:
• Ferromagnetic Sensitivity: Scanners are extremely sensitive to iron and cobalt; even microchips, BB pellets, or surgical staples can degrade or destroy image quality.
• Ferromagnetic Sensitivity: Scanners are extremely sensitive to iron and cobalt; even microchips, BB pellets, or surgical staples can degrade or destroy image quality.
• Pre-Scan Protocol: Animals should have radiographs of the area of interest before MRI to check for metallic foreign bodies.
• Pre-Scan Protocol: Animals should have radiographs of the area of interest before MRI to check for metallic foreign bodies.
• Projectile Risk: Powerful magnetic fields can accelerate ferromagnetic objects (e.g., oxygen bottles) to nearly 100 mph, posing a lethal risk to patients and personnel.
• Projectile Risk: Powerful magnetic fields can accelerate ferromagnetic objects (e.g., oxygen bottles) to nearly 100 mph, posing a lethal risk to patients and personnel.
• Ionizing Radiation: MRI does not use ionizing radiation, making it desirable for veterinary personnel.
• Ionizing Radiation: MRI does not use ionizing radiation, making it desirable for veterinary personnel.
Patient Management:
Patient Management:
• Immobilization: Small animal studies require general anesthesia due to the length of scans and extreme sensitivity to motion.
• Immobilization: Small animal studies require general anesthesia due to the length of scans and extreme sensitivity to motion.
• Equine Systems:
• Equine Systems:
◦ Standing Systems: Use lower magnetic field strength and require only sedation.
◦ Standing Systems: Use lower magnetic field strength and require only sedation.
◦ Recumbent Systems: Offer significantly improved image quality but require general anesthesia.
◦ Recumbent Systems: Offer significantly improved image quality but require general anesthesia.
VI. Facility and Personnel Requirements
VI. Facility and Personnel Requirements
• Shielding: Rooms must have specialized RF shielding in the walls to block extraneous signals.
• Shielding: Rooms must have specialized RF shielding in the walls to block extraneous signals.
• Coolants: Advanced high-field systems require liquid helium as a coolant to maintain superconductivity.
• Coolants: Advanced high-field systems require liquid helium as a coolant to maintain superconductivity.
• Technologists: Scanners should be operated by technologists with specialized training in radiological technology.
• Technologists: Scanners should be operated by technologists with specialized training in radiological technology.
• Specialization: Due to high costs and technical complexity, MRI is typically found only in academic centers or large referral specialty practices.
• Specialization: Due to high costs and technical complexity, MRI is typically found only in academic centers or large referral specialty practices.
Facility and Personnel Requirements
Facility and Personnel Requirements
• Shielding: Rooms must have specialized RF shielding in the walls to block extraneous signals.
• Shielding: Rooms must have specialized RF shielding in the walls to block extraneous signals.
• Coolants: Advanced high-field systems require liquid helium as a coolant to maintain superconductivity.
• Coolants: Advanced high-field systems require liquid helium as a coolant to maintain superconductivity.
• Technologists: Scanners should be operated by technologists with specialized training in radiological technology.
• Technologists: Scanners should be operated by technologists with specialized training in radiological technology.
• Specialization: Due to high costs and technical complexity, MRI is typically found only in academic centers or large referral specialty practices.
• Specialization: Due to high costs and technical complexity, MRI is typically found only in academic centers or large referral specialty practices.
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