W11 Flashcards
(46 cards)
what directly affects the quality of the MR signal?
- configuration of the RF transmitter
- receiver probes or coils
several types of coil currently used in MR imaging: (4)
- transmit/receive coils:
- surface coils
- phased array coils
- volume coils
transmit coils:
E is transmitted at the resonant frequency of hydrogen = short intense burst of radio frequency = radio-frequency pulse
the main coils that transmit RF in most systems are either 1. a body coil (usually located within the bore of the magnet itself) or
2. a head coil
receive coils
RF coils placed in the transverse plane generate a voltage within them when a moving magnetic field cuts across the loops of wire => this voltage is the MR signal that is sampled to form an image
to induce an MR signal, the transverse magnetization must occur perpendicular to the receiver coils
surface coils:
- improve SNR, b/c generally: the nearer the coil is situated to the structure under examination, the greater the SNR
- small & especially shaped so that they can be easily placed near the anatomy to be imaged with little or no discomfort to the patient
- body coil: transmit RF, surface coil receives MR signal
Surface Coils Applications: (5)
Musculoskeletal Imaging:** Detect fractures, evaluate soft tissue injuries, diagnose arthritis.
**Neuroimaging: Visualize superficial structures near the skull or spinal cord, used in fMRI studies for detecting brain activity.
Breast Imaging:** Obtain high-resolution images of breast tissue.
**Cardiac Imaging: Visualize cardiac anatomy, assess myocardial function, and vascular anatomy.
*Abdominal and Pelvic Imaging: Image abdominal and pelvic structures such as the liver, kidneys, pancreas, and pelvis.
VOLUME COILS:
- surround either the whole body of a specific region => SNR of image obtained this way is usually less than that obtained with surface or phased array-coils
ex: body coil (main coil of the magnet in the magnet bore), head coil
Volume Coils Applications
Whole-Body Imaging: Provide uniform RF excitation and reception for comprehensive imaging of the entire body.
Abdominal Imaging: Visualize organs like the liver, kidneys, pancreas, spleen, and gastrointestinal tract.
Pelvic Imaging: Assess pelvic structures including the bladder, uterus, ovaries, prostate, and pelvic bones.
Thoracic Imaging: Image thoracic structures such as the lungs, heart, mediastinum, and thoracic spine.
Angiography: Visualize larger vessels and vascular territories for arterial and venous imaging.
Large Field of View Studies: Facilitate imaging studies requiring a broad coverage area, such as whole-spine or whole-brain imaging.
PHASED ARRAY COILS
- consist of multiple coils and receivers (usually up to 4) whose individual signals are combined to create one image => improved SNR, increased longitudinal coverage, improved uniformity across a whole volume => advantages of small surface coils are combined with a large FOV for increased anatomy coverage.
Phased Array Coils Applications
Musculoskeletal Imaging: Visualize joints, bones, muscles, and ligaments with improved sensitivity and spatial resolution.
Neuroimaging: Image brain and spinal cord structures with enhanced SNR and coverage, useful for both routine and research MRI studies.
Cardiac Imaging: Assess cardiac anatomy, myocardial function, and vascular structures with improved signal sensitivity.
Abdominal and Pelvic Imaging: Obtain detailed images of abdominal and pelvic organs, aiding in the diagnosis of abdominal pathologies.
Dynamic Imaging: Capture dynamic processes such as blood flow, cardiac motion, and joint movement with high temporal resolution.
pulse sequence -
series of RF pulses, gradients applications and intervening time periods which enable control of the way in which the system applies RF pulses and gradients
image weighting is controlled by
conventional spin echo
selecting the intervening time periods
Pulse sequences are required why?
because without a mechanism of refocusing spins, there is insufficient signal to produce an image
Conventional spin echo (SE or CSE) pulse sequences are used to
to produce T1, T2 or proton density weighted images and are one of the most basic pulse sequences used in MRI.
In spin echo pulse sequence
pulse, rephasing
90° excitation pulse followed by a 180° rephasing pulse followed by an echo
spin echo -
signal in the receiver coil is regenerated after 180° RF pulse => measured
Rephasing the NMV eliminates what?
magnetic field inhomogeneities
typical sequence used to produce a T1 weighted set of images:
a single 180° RF pulse applied after the excitation pulse => a single spin echo
sequence that provides two images per slice location - and what those images are?
dual echo sequence consists of two 180° pulses applied to produce two spin echoes => 2 images per slice location are: T2 weighted and proton density weighted
Fast spin echo (FSE) -
- much faster version of conventional spin echo;
- employs a multiple (2-30) 180° rephasing pulses, each one producing a spin echo => number of 180° RF pulses and resultant echoes is called the ECHO TRAIN LENGTH (ETL) / TURBO FACTOR
- spacing b/w each echo - ECHO SPACING
Short turbo factor =>
short t turbo factor => short effective TE => T1 weighted image and increased scan time
long turbo factor =>
long turbo factor => long effective TE => T2 weighted image & reduced scan time
the bigger the turbo factor -
what happens to blurring?
Image blurring increases with turbo factor, b/c greater number of echoes obtained at different TE from the same image
FSE is usually used for:
brains, spines, joints, extremities, pelvis
