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StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-.
Mohammed Abed ; Darren P. Sandean .
Last Update: December 11, 2022 .
Proper patient positioning is essential for accurate diagnosis with all imaging modalities. In magnetic resonance imaging (MRI), patient positioning is vital to obtain good quality and an appropriate diagnosis. However, there are numerous approaches for patient positioning concerning anatomical reference or pathology. It is essential to be aware of 'coils,' accessories within the MRI with variable sizes and properties designed to transmit and receive signals from the regions of interest.
How does magnetic resonance work?
A strong magnet produces a powerful magnetic field, which forces the body protons to be aligned with that field. A specific pulse sequence applied to these protons made them spin out of the normal equilibrium state. The pulse sequence is turned off, which results in the protons returning to the original alignment with the field. The re-alignment process occurs in a variable time and releases energy detected by MRI sensors to reflect the various tissues, which is then converted into an image.[1][2]
Contraindications to MRIThe contraindications for MRI include:Metallic Implant
Vascular clips, especially ferromagnetic clips. If the clips are made of titanium or another non-ferromagnetic substance, the scan can be performed.
Foreign bodies: There is a potential risk of foreign material displacing and damaging vital structures. If any suspension regarding foreign material occurs, the exam must be canceled.
Coronary and peripheral artery stents: nowadays, most of these are made of non-ferromagnetic materials, and the studies showed that MRI could be done safely.[3]
Aortic stent-grafts Prosthetic heart valves and annuloplasty rings Cardiac occluder devices Vena cava filters and embolization coils Hemodynamic monitoring and temporary pacing devices Hemodynamic support devices Permanent cardiac pacemakers and implantable cardioverter-defibrillators Retained transvenous pacemaker and defibrillator leads Permanent contraceptive devices Cochlear implantsOther Potential Contraindications
Tattoos and cosmetics Claustrophobia Pregnancy and postpartumSpecial Patient Conditions
Obese patients: obesity carries challenges concerning safety and performance. Among these is the challenge of fitting the patient into the scanner. Nowadays, most vendors design a 70 cm diameter scanner bore that almost carries no difficulties; however, this requires a broad imaging view and negatively impacts the image quality.[4][5] Imaging of obese patients also carries a thermal risk due to their girth and specific absorption rate, which is increased with the radial distance from the isocenter and field strength.[6]
Anxious patients: the enclosing nature of the bore may exaggerate any nervous disorders. Here are a few tips on how to help the anxious patient:
If the coil has a mirror, adjust it so the patient can see the out of the magnet.When feasible, use the prone position to image the patient, as this position allows the patient to see out of the magnet.
Remove the pillow from the patient's head to allow a distance from the face and the magnet. Tell the patient to close the eyes when comfortable to do so.
Tell the patient that they can exit the magnet at any time and may refuse the exam. This makes them feel that they have control.
Pediatric patient: Children often have difficulty tolerating MRI due to long scanning times and noisy exams. The problem is especially evident for children under 6, who found it difficult to stay still and follow the breathing instructions.[7] Deep sedation or general anesthesia is often required to minimize the motion artifacts. However, this approach comes with many technical and clinical risks.[8]
The human body can be divided into 5 main anatomical regions for MR imaging application:
Central nervous system imaging Musculoskeletal imaging Body imaging Cardiovascular imaging Breast imagingWe present different patient positioning with the above classification.
Central Nervous System Imaging
The patient should be centralized to the brain coil with the chin pointed upwards using landmarking, laser marker light, or touch sensors. For patient comfort and safety, the earplugs should be used for hearing protection, immobilization pads and straps should be placed around the head to reduce noise and motion, leg support pads should be placed, and the alarm bell should be tested and given to the patient. The patient's eyes must be closed while using laser marking.
If loop coils exist at the site setting (also called TMJ coils), they should be used as they have a shorter signal penetration length but significantly higher signal-to-noise- ratio. The loop coils shall be placed as close as possible to the orbit. If no loop coils are available, the routine brain can be used.[9]
The patient's supine larynx should be centered on the coil center. After landmarking the center of the brain coil, the patient may be sent for the scan. The measures used in the routine brain are also applicable to ensure patient safety.
Using an 8-channel brain or neurovascular coil, the patient's cheekbones must align with the coil to ensure the best quality.
The patient should lie in a supine position, and the center of the sternum must be aligned with the center of the spine coil.
The patient should lie supine, and the spine coil must be centered on the table about 5 cm. The superior of the iliac bones must be aligned with the coil's center. The coil selection must be wisely selected depending on the region of interest to avoid peripheral signal artifacts.[10]
Musculoskeletal Imaging
The TMJ coils have 2 pieces connected by a holder. With the patient supine, the coil's loop must be placed at the TMJ and as close as possible to the patient's face without causing discomfort. If no TMJ coils are available, routine brain coils can be used.
The shoulder coils are placed on the patient's shoulder with straps. The patient must be supine, and the arm must be parallel to the bed and in external rotation.
With the patient supine position, the coil is wrapped around the elbow of interest. In external rotation, the arm must be parallel to the bed.
If there is a dedicated coil available, it should be placed straight with the patient's wrist on the coil's center and the patient in a supine position. If no dedicated coil is available, the superman position can be used, which is placed in the prone position, and the head is first. The arm is then extended straight with the arm pointing down; however, this position is less comfortable for the patient.[11]
Cardiac coils can be used for hip imaging since they are smaller and have fewer coil elements, hence a better signal than the multi-channel coil. The coil should be placed at the MR table center, aligned to the iliac crest, to reduce the breathing artifact.
The knee is usually imaged using dedicated coils. These transmit/receive coils can transmit and receive RF pulse using the transmitter and receiver elements. The knee coil must be placed at the center of the MR table. The patient's knee is then inserted with a slight bend (about 15 degrees). The patella is aligned to the center of the coil.
The patient's ankle is placed in the center using the ankle coil. With the support pads' aid, the patient's foot can be immobilized and straight during the exam. The other (non-imaged) foot should be placed away from the coil to avoid wrapping or aliasing.
Multi-channel coils can be used to image the long bones bilaterally. The bone's anatomy of interest must be placed in the center of the coil, including the entire region. For femoral imaging, the hip and knee must be included in the coil.
Body Imaging
The patient should be in a supine position. The arms must be raised above the head. The center of the coil should be aligned to the center of the breast mass for females and nipples in males. The inspiratory bellow must be placed on the level of the diaphragm.
The coil must be placed straight to the center of the MR table. With the patient supine, the coil must be placed while the arms are raised above the head. The center of the coil must be at the level of the xiphoid process.
The coil placement is the same as for liver imaging.The coil must be placed squarely at the center of the MR table, with the arms sideways above the coils' level, with the patient in the supine position. The center of the coil must be about 10 cm below the iliac crest level.
Cardiovascular Imaging
The coil must be placed squarely at the center of the MR table, arms raised above the head, with the patient in the supine position. For females, the coil's alignment must be with the center of the breast mass, and for males, with the center of the nipple.
The patient positioning is identical to routine brain imaging.The coil must be placed straight to the center of the MR table. With the patient supine, the coil must be placed while the arms are raised above the head. The center of the coil must be at the center of the sternum.
The coil must be placed straight to the center of the MR table. With the patient supine, the coil must be placed while the arms are raised above the head. The center of the coil must be at the center of the sternum or to the section of the aorta to be imaged.
The coil must be placed straight to the center of the MR table. With the patient supine, the coil must be placed while the arms are raised above the head. The coil center must be aligned between the lower end of the sternum and the belly button.
Breast Imaging
The coil must be placed straight to the center of the MR table. It must be placed with the patient in the supine position, arms extended forward, and folded comfortably. For females, the center of the coil must be at the center of the breast mass, and for males, at the center of the nipples.
The 3 fundamental principles of MRI are:
Accurate and reproducible image quality Good characterization of the disease process Comprehensive information for the area of interestPatient positioning is an essential step in the MRI to achieve these principles. It is important to recognize the value of appropriate coil positioning as part of sequence optimization. Accurate patient position is deemed important in claustrophobic, pediatric, and other age groups.
Patient safety is the most important aspect of an MRI exam. A full patient consent form should be filled out appropriately for an MRI scan. After the form has been filled out, the staff is responsible for clarifying some of the misunderstandings by using direct communication before the exam. These are things to be done while the patient is in the waiting room.
Explain the consent form to the patient and answer his/her questions, if any.Review the form with the patient and ask for additional questions about any stimulator's presence, eg, aneurysmal clips, cardiac valves, pacemakers, or metallic clips.
Pregnant or suspected pregnant patients should have a consultation with the radiologist before performing MRI.
Scan the patient with a metal detector to ensure no loose metallic objects.Other things to be done during the MRI exam:
Lowering the table to allow the patient to lie down. Placing protective pads. Give and instruct the patient on how to use the earplugs. Ask the patient to close the eyes when the laser lights are turned on. Place the safety pads. Give the patient an alarm bell and explain how to use it.Interprofessional team considerations:
The MR radiographer should have an unobstructed view of the door of the MR scanner. A good level of training shall exist for the staff for any possible hazards related to MR safety. MR quench button should be easily accessible, and trained MR staff should know how to use it. MR power cut-off must be accessible, and MR-trained staff should know how to use it.McGowan JC. Basic principles of magnetic resonance imaging. Neuroimaging Clin N Am. 2008 Nov; 18 (4):623-36, x. [PubMed : 19068405 ]
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Disclosure: Mohammed Abed declares no relevant financial relationships with ineligible companies.
Disclosure: Darren Sandean declares no relevant financial relationships with ineligible companies.
