!Study Notes on Magnetic Resonance Imaging (MRI)

Balanced Gradient Echo Sequence

  • A modification of the coherent gradient- echo sequence

  • Uses a balanced gradient scheme to correct for phase errors in flowing blood and CSF

  • The area of the gradient under the line equals that above the line, moving spins accumulate a zero-phase change

  • Gradient scheme is the same as flow compensation or gradient moment rephasing

  • Higher flip angles and shorter TRs are used than in coherent gradientecho producing a higher SNR and shorter scan times

  • Uses: 1. Imaging the heart and great vessels 2. Spinal Imaging, especially the cervical spine 3. Internal auditory meatus, as CSF flow is reduced 4. Joint Imaging 5. Abdominal Imaging

  • (slide 7)

MRA and MRV: Examination of Vessels

Types of Angiography

  • Magnetic Resonance Angiography (MRA):

    •  MRAs are used to look at arteries

  • Magnetic Resonance Venography (MRV):

    • • MRVs are used to look at veins

  • Variants of MRA:

    • Flow-dependent MRA - non contrast - its dependent on the flow of the protons

    • Contrast-dependent MRA - uses Gadolinium to provide effective imaging.

Flow-Dependent MRA Techniques

  • Time-of-flight MRA → Flow dependent MRA = means non contrast MRAs

  • Magnetic resonance angiography (MRA) - utilize bright-blood imaging to demonstrate high-signal flow within the vasculature

  • Flow-dependent angiograms were only suitable for small regions of interest, such as the circle of Willis or the carotid bifurcation

  • Gadolinium contrast-enhanced magnetic resonance angiograms (CEMRA) are the technique of choice for large vessels

TOF MRA

  • TOF angiogram - produce an image with high contrast between high-signal vascular flow and saturated, low-signal stationary background anatomy

  • TOF effect must also be taken into consideration as long TEs will cause the RF-excited bolus to leave the slice before the echo is sampled

  • TOF angiograms are acquired using a flow-compensated gradient echo sequence and may be performed as a sequential 2D acquisition or a 3D volumetric acquisition

  • Gradient moment nulling prevents flow artifacts

  • Presaturation bands are used to stop unwanted flow (venous flow in MRAs and arterial flow in MRV

  • In 2D sequential acquisitions, the saturation band follows the position of each slice as it is acquired (sometimes called a traveling SAT band or travel-sat)

  • TOF angiograms are only suitable for imaging blood that flows perpendicular to the slice

2D Sequential TOF MRA - colleted slice by slice

  • The FOV and slice coverage are only limited by the size of the receive coil

  • Slow-flowing spins experience a short transit through a thin slice

  • This means slow flow vessels are less saturated and appear bright

  • Venetian blind artifact may be present

  • Can not image in plane flow

  • Mainly used as localizer for 3D TOF MRAs and MRV

  • Veins are easier to visualize on 2D because of their slow flow

3D Volumetric TOF MRA - colleted as a volume

  • 3D MRA - thinner slices with higher matrix values allow isotropic voxels

  • MIP reconstructed image is of high spatial resolution and that the voxel size is the same from every angle of rotation

  • Small voxels also reduce the amount of intravoxel dephasing and improve SNR

  • In a 3D volume, spins spend longer in transit through a thick slab compared to a thin slice

  • Multiple Overlapping Thin Slab Acquisitions (MOTSA) – multiple 3D slabs are used to cover one area

Phase Contrast MRA - non contrast MRA

  • Phase Contrast MRA – shows blood flow signal based on the velocity of the blood flow

  • Takes advantage of the fact that the magnetic moments of spins moving along a bipolar gradient acquire a phase shift, whereas those of stationary spins do not

  • The degree and direction of the shift can be manipulated by the use of velocity-encoding (VENC) gradients built into the pulse sequence

  • VENC is a parameter set by the technologist

  • The amplitude of the gradient is modified to suit the speed of flow in centimeters per (cm/s)

  • Only the moving spins are visualized on the reconstructed image

  • Subtraction is used, high signal from early subacute hemorrhage and fat is eliminated from the image

Phase Contrast can create two sets of images from one scan:

  • 1. Magnitude images offer the normal anatomical-looking appearance with high-signal vessels against a black background.

  • 2. Phase images, on the other hand, demonstrate the direction of flow as being white (when in the same direction as the VENC) or black (when in the opposite direction to the VENC

  • Setting a correct VENC is necessary to avoid aliasing → Low VENC leads to signal void in the middle of the vesse

Contrast Agent

  • In order to increase contrast between pathology and normal tissue, enhancement agents may be introduced that selectively affect the T1 and T2 relaxation times in tissues

  • • To make it nontoxic, we bind gadolinium to a chelate (which is just another chemical)

Contrast Enhanced MRA

  • 2D/3D TOF and Phase Contrast can have issues because motion artifact and the potential signal loss in vascular structures due to in-plane flow -These problems can be solved by injecting gadolinium

  • Contrast enhanced MRA uses spoiled T1W 3D gradient echo, followed by a bolus injection of gadolinium and dynamic imaging

  • Contrast in the vessel makes the vessel appear much brighter and high velocity signal loss is reduced

  • This means signal loss is caused by pathology, not time of flight effects

MIP

  • Maximum Intensity Projection – computer algorithm applied after the scanning is complete to make bright pixels brighter and dark pixels darker