Archive for the ‘MRI’ Category

Types of Magnets Used In Magnetic Resonance Imaging

These are the Types of Magnets used in Magnetic Resonance Imaging:

1)   Permanent magnets

Characteristics:

Magnetom Espree-Pink: A New Innovation in MRI

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There is a new innovation in the world of Magnetic Resonance Imaging. It is specially designed for breast scans and has made the entire examination more enduring and comforting due to its compact, open design that has a diameter of only 70 centimeters, while the magnet bore is 10 centimeters wider as compared to most conventional systems.

This new innovation is a product of Siemens and was named Magnetom Espree-Pink. This 1.5-Tesla system is the shortest system available on the market today. With this breakthrough, patients can be positioned either feet-first or head-first and the scanner can also offer sufficient space for adipose patients.

Furthermore, the flexible “Sentinelle Vanguard for Siemens” coil solution makes both imaging and biopsies possible by only using a coil. Also, the coil can be optimally adapted to the different breast sizes of various patients due to its variable coil geometry (VCG). Excellent image quality is attained through an improved signal-to-noise ratio and with the help of eight RF channels.

Source: EH Online

Magnetic Resonance Imaging Notes: Respiratory and Cardiac Gating

Respiratory Gating

Respiratory Gating

- the simplest method of eliminating respiratory motion artifact is to suspend respiration entirely though breath hold
- respiratory gating and respiratory compensation were utilized because not all patients can hold their breath long enough to acquire adequate images

Respiratory Compensation

- also known as exorcist, ROPE and COPE
- performed through reordered phase encoding
- throughout the respiratory cycle, anterior chest wall motion is monitored with pressure transducer that surrounds patient’s chest
- phase encoding steps are then reordered to decrease the intensity of respiratory motion artifacts and changes location of the motion artifacts within the data set
- has potential advantage over respiratory gating because acquisition tiem is not increased
- signal averaging results involved results in marked resolution loss and obscures fine detail
- a complicated real-time technique

Respiratory Gating

- MRI with respiratory gating is a simple and practical approach to reduce respiratory motion effects
- Data are collecting during continuous breathing but are used for image reconstruction only if they are collected within a reference range
- Before, reference for respiratory motion was obtained by placing a belt containing a displacement transducer around the upper abdomen
- Recently, navigation spin echo has been used to monitor diaphragmatic motion
- Acceptance or rejection of data can be made in real-time or retrospectively
- Disadvantage is increased imaging time

Cardiac Gating

- Used to reduce cardiac motion and can be monitored by measuring the ECG signal via placement of MRI-compatible electrodes on either the patient’s chest or back
- It is thought that placement of the electrodes dorsally may reduce artifact caused by lead motion
- If leads crossed out each other or looped, it can cause induction of undesired currents and the possibility of surface burns
- R-R interval is measured and image acquisition is triggered by the R-wave
- Alternative method is peripheral gating or plethysmography, where gating is triggered by the peripheral pulse via a small probe placed at the fingertip

Coils

- Standard gradient body coil and the phased array surface coil are the two coils most commonly used in thoracic imaging
- Phased array coil provide good central and peripheral imaging, maintain field homogeneity and improve SNR above that of the standard body coil
- Smaller flexible surface coils, and sometimes dedicated shoulder coils, are also used for imaging the superior sulcus and brachial plexus

Contrast Agents

- Intravenously administered gadolinium chelates is the most commonly use CM for thoracic MRI
- CM used in MRI are all paramagnetic agents that increase signal and have relaxivity rates
- Paramagnetic agents are administered prior to T1-wieghted image acquisition
- The one exception to this in the thoracic are double dose dynamic gadolinium-enhanced sequences
- Prior to administration of CM, patient’s medical history, specifically drug allergies must be evaluated

Specific uses

1. Aorta and Great Vessels
- MRI has become an important method of assessing dissection, aneurysms, pseudoaneurysms and congenital anomalies
- T1-weighted spin echo imaging with intravoxel dephasing is the most useful sequence for imaging the thoracic aorta and great vessels
- A newer method for MRI of the aorta and great vessels is dynamics double dose gadolinium-enhanced 3D imaging
- Injection is carefully timed to obtain images during peak bolus as determined by the initial test dose

2. Cardiac
- Should be performed in at least two planes
- 3D gradient-recalled echo imaging, a rapid technique, can also be used to image the heart
- Cardiac MRI is also performed to assess congenital disease
- Newer cine-2D GRE sequences can demonstrate flow patterns suggestive of valvular stenosis and regurgitations

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