There are many resources available related to Magnetic resonance imaging. The following links direct you to the necessary information sources.
International Society for Magnetic Resonance in Medicine (ISMRM)
http://www.ismrm.org/
ISMRM FAQ
http://www.ismrm.org/ISMRMFAQ.htm
Radiological Society of North America (RSNA)
http://www.rsna.org/
Cardiac MRI
http://www.scmr.org/
MRI Books
http://www.amazon.com/MRI-Practice-Edition-Catherine-Westbrook/dp/1405127872
Tuesday, May 8, 2012
Saturday, April 14, 2012
MRI 1.5T and 3T What is the difference?
The strength of the MR scanner is represented in Tesla (T). Tesla is the SI unit of magnetic flux density. The cgs unit of measurement of magnetic field is gauss (G). One Tesla is equal to 10,000 gauss. It is the measure of the main magnetic field generated by the MR equipment. The main magnetic field of a 1.5T scanner is about 30,000 times stronger than earth's magnetic field. The main magnetic field of 3T is 60,000 times the strength of earth's magnetic field. 1.5T and 3T magnets are common in clinical use. MR signal generated by the scanner increases with the strength of magnetic field . The signal strength is almost double in 3T compared to 1.5T scanner. The signal to noise ratio (SNR) increases with magnetic field strength. The extra signal generated in a 3T scanner can be utilised to reduce the scan time or to increase the spatial resolution of the picture. Other than the physical strength of the magnet there are some differences in the image appearance between 3T and 1.5T.
Chemical shift
Chemical shift is due to the difference in resonance frequencies of water and fat protons in the tissue.Chemical shift effect is prominent in 3T compared to 1.5T. This phenomenon is advantageous in MR spectroscopy imaging due to high spectral resolution. Spectral resolution is the ability to visualise the changes in metabolite peaks. Water and fat peaks are distinct and which will helps in good fat saturation. The increased chemical shift is a disadvantage for muculoskeletal imaging where the cartilage and bone interface show shading effect. Increasing the bandwidth can reduce chemical shift effect.
Susceptibility effect
Magnetic susceptibility is more prominent in 3T compared to 1.5T. It is the ability of a material or tissue to be magnetised in a given magnetic field. The higher susceptibility is beneficial in functional MRI (fMRI). Metal implants show higher susceptibility at 3T fields and susceptibility artifacts are more prominent in 3T. Tissue air interface in the base of skull shows susceptibility artifacts in Diffusion and gradient images.
Dielectric effect
Dielectric artifacts appear due to increased RF inhomogeneity with areas of non uniformity or shading in MRI images. These artifacts are caused by the shortening of RF wavelengths inside the body and exist at all field strengths. Dielectric effects are increased at 3T and the effects can be more with multichannel or phased array coils. Dielectric effect can be reduced by placing a dielectric pad between the patient and coil. Dielectric pad is a high conductivity pad made of a dilute manganese chloride solution. The new generation 3T scanners are equipped with multi drive RF transmission technology to reduce the dielectric effect.
Specific Absorption Rate (SAR)
Specific Absorption Rate (SAR) is the measure of heat energy deposited by Radio frequency field in a given mass of tissue. As per the instruction of FDA* and IEC* the SAR should not exceed 8 watts per kg (W/kg) for any 5 minutes period or 4W/kg for whole body averaged over a period of 15 minutes. The precessional frequency of hydrogen proton is high in 3T and flipping the net magnetization vector requires more RF energy compared to 1.5T. Increasing the field strength from 1.5T to 3T will lead 4 times increase in the SAR value. Most of the current 3T scanners are equipped with real time SAR monitors which helps to keep RF exposure within safety limits.
MR safety
The 3T field is twice the strength of 1.5T field and even small objects can become a deadly projectile and hazardous to patients. The extended fringe field is another area of concern. MR siting requires special attention to contain 5 Gauss exclusion zone within the magnet room or it can interfere with nearby electronic equipments or other imaging modalities. Another area of concern in the MRI environment is the safe usage of medical devices and implants. The implants which is safe in 1.5T environment may not be safe or may be conditional in 3T field. The medical devices such as monitors,ventilators,infusion pump which is safe in 1.5T may cause interference with 3T if the instructions of the manufacturer on the usage of the equipment is not followed.
*FDA - Food and Drug Administration
*IEC- International Electro technical Commission
Please note: This information is provided for the educational purpose only.You should contact the referring physician, Radiologist or MRI technologist for specific information on MRI safety to patients. Reference Manual for Magnetic Resonance Safety,Implants and Devices can be available from Biomedical Research publishing group Los Angels,CA, by Frank G.Shellock Ph.D
Friday, April 10, 2009
MRI for Nurses
What Nurses should know about MRI
Radiology Nurse
The staff nurse in the Radiology department is a very important part of the clinical team. Usually the staff nurse is on rotational posting in various sections of Radiology. These are general X-Rays, Special procedure rooms like IVU, Fluoroscopy, CT scan, Angio suite, MRI etc. In various sections the role and responsibilities varies according to the procedures.
Radiology Nurse in MRI
Magnetic Resonance Imaging(MRI) equipment is a powerful diagnostic Imaging tool. An MRI image is often superior to a plain film x-ray for viewing and understanding soft tissue structure. An MRI image is formed by a large magnet which polarizes hydrogen atoms in the tissues, provides a summation of the spinning energies within the tissues' cells and produces an image representation of this process.
Precautions while sending a patient for MRI form Emergency department,ICU or ward
From an emergency department MRI scan is usually requested for spinal cord injury or acute stroke patients. Patients past medical history may not be known at the time of admission to the emergency department. Patient may be on immobilisers and with monitoring devices and life lines attached and it may not be MR compatible. Patient screening is the primary step before sending for an MRI scan. Make sure there is no cardiac pacemaker and implanted devices on the patient body.If patient had any previous history of surgery then get necessary details of the operation such as date and type of surgery and is there any metallic implants or devices in the patient body. Patient should be transferred to MR compatible monitoring devices and infusion pump before entering into the scan room. The attending Doctors and Nurses should screen by themselves and remove all metallic items from their body before entering into the scan room for patient management.Portable oxygen cylinders should not bring into the MR environment which carries a potential risk of injury to the patient and also damage to the equipment. Oxygen supply should be provided through a wall socket inside the MR room or through a long tubing connected to the oxygen tank placed outside the MR room. If the patient is on ventilator then transfer the patient to MR compatible ventilator before moving into the scan room.
Examples of items that may create a health hazard or image artifacts during an MRI exam include:
Radiology Nurse
The staff nurse in the Radiology department is a very important part of the clinical team. Usually the staff nurse is on rotational posting in various sections of Radiology. These are general X-Rays, Special procedure rooms like IVU, Fluoroscopy, CT scan, Angio suite, MRI etc. In various sections the role and responsibilities varies according to the procedures.
Radiology Nurse in MRI
Magnetic Resonance Imaging(MRI) equipment is a powerful diagnostic Imaging tool. An MRI image is often superior to a plain film x-ray for viewing and understanding soft tissue structure. An MRI image is formed by a large magnet which polarizes hydrogen atoms in the tissues, provides a summation of the spinning energies within the tissues' cells and produces an image representation of this process.
Precautions while sending a patient for MRI form Emergency department,ICU or ward
From an emergency department MRI scan is usually requested for spinal cord injury or acute stroke patients. Patients past medical history may not be known at the time of admission to the emergency department. Patient may be on immobilisers and with monitoring devices and life lines attached and it may not be MR compatible. Patient screening is the primary step before sending for an MRI scan. Make sure there is no cardiac pacemaker and implanted devices on the patient body.If patient had any previous history of surgery then get necessary details of the operation such as date and type of surgery and is there any metallic implants or devices in the patient body. Patient should be transferred to MR compatible monitoring devices and infusion pump before entering into the scan room. The attending Doctors and Nurses should screen by themselves and remove all metallic items from their body before entering into the scan room for patient management.Portable oxygen cylinders should not bring into the MR environment which carries a potential risk of injury to the patient and also damage to the equipment. Oxygen supply should be provided through a wall socket inside the MR room or through a long tubing connected to the oxygen tank placed outside the MR room. If the patient is on ventilator then transfer the patient to MR compatible ventilator before moving into the scan room.
Examples of items that may create a health hazard or image artifacts during an MRI exam include:
- Neurostimulator
- Aneurysm clip
- Cochlear implants
- Cardiac Pacemaker
- Implantable devices - eg. Implantable cardioverter defibrillator (ICD)
- Implanted drug infusion device
- Intravascular filters
- Metal implants such as orthopedic devices
- Metal objects, especially if in or near the eye
- Shrapnel or bullet wounds
- Dentures and bridges
- Any implants that involve materials sensitive to magnetic field
- Medication patch that contains metal(eg.transdermal patch)
Before sending the patient for MRI scan check with the MRI technologist or Radiologist if you have any questions about implanted object or condition that could impact the MRI procedure and patient safety.
Please note: This information is provided for the educational purpose only.You should contact the referring physician, Radiologist or MRI technologist for specific information on MRI safety to patients.
Reference Manual for Magnetic Resonance Safety,Implants and Devices can be available from Biomedical Research publishing group Los Angels,CA, by Frank G.Shellock Ph.D
- Aneurysm clip
- Cochlear implants
- Cardiac Pacemaker
- Implantable devices - eg. Implantable cardioverter defibrillator (ICD)
- Implanted drug infusion device
- Intravascular filters
- Metal implants such as orthopedic devices
- Metal objects, especially if in or near the eye
- Shrapnel or bullet wounds
- Dentures and bridges
- Any implants that involve materials sensitive to magnetic field
- Medication patch that contains metal(eg.transdermal patch)
Before sending the patient for MRI scan check with the MRI technologist or Radiologist if you have any questions about implanted object or condition that could impact the MRI procedure and patient safety.
Please note: This information is provided for the educational purpose only.You should contact the referring physician, Radiologist or MRI technologist for specific information on MRI safety to patients.
Reference Manual for Magnetic Resonance Safety,Implants and Devices can be available from Biomedical Research publishing group Los Angels,CA, by Frank G.Shellock Ph.D
Saturday, March 14, 2009
MRI Fringe Field
Magnetic Resonance Imaging (MRI) Equipment
In an MRI equipment the stray magnetic field outside the bore of the main magnet is called fringe field. These fields are created by magnetic flux lines and the number of flux lines per unit area is called magnetic flux density. The flux density is higher at the magnetic isocenter and the density decreases as we move away from the isocenter of the magnet.
Magnetic Isocenter is the centre of the bore of the magnet in three planes (x,y,z). The magnetic field homogeneity is measured in the isocenter of the magnet. Magnetic field homogeneity is very important for clinical magnetic resonance imaging. Shimming is the process whereby the evenness of the magnetic field is optimized. Shim coils are used to make the magnetic field more homogeneous.
In a superconductive magnet the flux lines run parallel to the bore of the magnet and it is called main magnetic field. Main magnetic field is indicated by B0.
In an MRI equipment the stray magnetic field outside the bore of the main magnet is called fringe field. These fields are created by magnetic flux lines and the number of flux lines per unit area is called magnetic flux density. The flux density is higher at the magnetic isocenter and the density decreases as we move away from the isocenter of the magnet.
Magnetic Isocenter is the centre of the bore of the magnet in three planes (x,y,z). The magnetic field homogeneity is measured in the isocenter of the magnet. Magnetic field homogeneity is very important for clinical magnetic resonance imaging. Shimming is the process whereby the evenness of the magnetic field is optimized. Shim coils are used to make the magnetic field more homogeneous.
In a superconductive magnet the flux lines run parallel to the bore of the magnet and it is called main magnetic field. Main magnetic field is indicated by B0.
The strength of the magnetic field is measured in gauss(G) or tesla (T). Tesla is the SI unit of magnetic flux density. It was defined in 1960 in honor of inventor Nikola Tesla. The cgs unit of measurement of magnetic field is gauss . It is named after the German mathematician and physicist Carl Friedrich Gauss . One Tesla = 10,000 gauss.
Fringe field should be contained within the MRI scanning room. The typical requirement is to contain the 5G(0.5mT) flex line within the walls and ceiling of the scanning room. The stray field outside the scan room can affect patient monitoring devices and medical implants.
Fringe field is contained within the specification by shielding the flex lines.
There are two types of magnet shielding, active and passive shielding. Passive shielding is done using steel plates in the wall and ceiling. This will help the flux lines to contain within the room. An actively shielded magnet uses pairs of coils called bucking coils with equal but opposite current to the main magnetic field coils. Bucking coils counter the main field and the mechanical structure within the cryostat becomes more complex.
Effective magnet shielding in essential to prevent stray field extending outside the scan room. Shielding will also help to reduce the MR siting cost. Moving heavy metal objects may interfere with the main magnetic field if the shielding is not appropriate. Before any MRI installation an experienced engineer should conduct a site survey to find out the shielding requirement .
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