MR Advance Techniques Vascular Imaging Class III 1
Vascular Imaging There are several methods that can be used to evaluate the cardiovascular systems with the use of MRI. MRI will aloud to evaluate morphology and hemodynamic of the cardiovascular system
Conventional MRI Vascular Techniques MR Imaging techniques for vascular imaging include a number of imaging sequences based on: Spin Echo Fast Spin Echo Inversion Recovery Gradient Echo These pulse sequences are supplemented with a variety of options to enhance vascular imaging. Flow Compensation Pre-saturation bands Contrast enhance
Conventional MRI Vascular Techniques Flowing protons can produce either signal void or signal enhancement. They also produce contrast between vessels and the surrounding tissue.
Conventional MRI Vascular Techniques There two types of contrast in vascular imaging depending on the signal intensity from flowing blood: Black blood imaging Bright blood imaging
Black Blood Imaging Several techniques can be employed to produce images where vessels appear dark. They include: SE FSE IR (Next Class) GRE using pre-saturation pulses
Black Blood Imaging & SE In spin echo pulse sequence fast flowing blood will not received both 90 and 180 RF pulses. Consequently a signal void (dark signal) within the blood vessel will be produce.
Black Blood Imaging & SE The use of pre-saturation band will optimize BBI in SE 90º ½ TE (20 ms) 180º
Black Blood Imaging & FSE These technique follows the same principle of Spin Echo but it uses several rephasing 180 to reduce scan time.
Black Blood Imaging & GRE When fast scan is required (GRE) Black Blood imaging can be optimized with the application of presaturation pulses. Saturation pulses will eliminate phase ghosting and provides intraluminal signal void for excellent distinction between patent and obstructed vessels. Pre-saturation band Pre-saturation band
1 ms 2 ms 3 ms Bright Blood Imaging Bright blood is achieved by using GRE combined with counter current flow and Flow-Comp.
MRA Magnetic Resonance Angiography (MRA) enhances the signal from moving spins in flowing blood and suppressing the signal from stationary spins residing tissues. When stationary tissues are suppressed, the appearance of vasculature is enhanced by increase signal from fresh spins which flow into the imaging volume and received RF excitation for first time (inflow effect).
Magnetic Resonance Angiography MRA There are several techniques that utilize different phenomena to increase signal from flowing spins. Time of Flight MRA (TOF-MRA) Phase Contrast MRA (PC-MRA) Contrast Enhance MRA (CE-MRA) Digital Subtraction MRA (DS-MRA)
Time of Flight MRA (TOF- MRA) TOF-MRA produces vascular contrast by manipulating the longitudinal magnetization of the stationary spins (Saturation). TOF-MRA uses incoherent gradient echo pulse sequences in combination with gradient moment nulling to enhance flow.
TOF-MRA Parameters and imaging options: TR TE Slice position Slice acquisition SAT bands MTC
TOF-MRA The TR is kept well bellow the T1 time of the stationary tissue so that T1 recovery is prevented. This saturates the stationary protons while the inflow effects from fully magnetized flowing fresh spins produces high vascular signal.
TOF MRA The TE should also be kept as low as possible to: Reduce Time of Flight effect and increase flow related enhancement Reduce intra-voxel dephasing and subsequent phase ghosting and signal loss.
TOF-MRA TOF-MRA is most sensitive to flow that is perpendicular to the FOV and the slice. Slices must be positioning as perpendicular as possible to the blood vessels.
TOF-MRA Any flow that is parallel to the slices can be saturated along with the stationary protons if their flow velocities are slow respect to the TR This phenomenon is known as in- plane flow saturation and can result in areas of signal void within the blood vessels.
TOF-MRA TOF-MRA must be acquired counter current flow to the blood vessel of interest to maintain the entry slice phenomenon. 1 100
Spatial Saturation The use of pre-saturation bands in TOF-MRA is important to limit our view to either the arteries or the veins.
Spatial Saturation The saturation band should be placed in a the position so that it will eliminate undesired signal from flowing protons.
Spatial Saturation The saturation band should be placed in a the position so that it will eliminate undesired signal from flowing protons. Pre-saturation band
Spatial Saturation The saturation bands used in TOF-MRI are walking pre-sats.
MTC T1-MTC + + GAD Magnetization Transfer Contrast (MTC) is use to saturate water bound to macromolecules. If this water becomes saturated the overall background signal intensity will drop and the image will look darker (less SNR).
TOF-MRA A disadvantage of TOF-MRA is the high signal intensity in some background tissues, especially those with short T1 relaxation times (fat). Other tissue that may appear bright during TOF-MRA is blood components with short T1 times, such as methemoglobin. They can be a problem in distinguishing sub-acute hemorrhage from flowing blood in TOF-MRA.
TOF-MRA TOF-MRA can be acquired in 2D and 3D. 2D is acquired slice by slice 3D is acquired in a volumetric acquisition
3D TOF-MRA 3D TOF-MRA offers high SNR and thin contiguous slices for good resolution. However spins in vessels with slow flow can be saturated in volume imaging.
3D TOF-MRA Advantages High resolution for small vessels More effective in high velocity flow areas Disadvantages Saturation of in-plane flow Small area of coverage
2D TOF-MRA 2D TOF is optimal in areas of: More effective in slow flow areas (carotids bifurcation, peripheral vascular and venous systems) When a large area of coverage is required 3D 2D
2D TOF-MRA Advantages Large area of coverage More effective in slow flow areas Disadvantages Lower resolution Saturation of in-plane flow (Less than 3D) Venetian blind artifact
TOF-MRA Incoherent GRE (T1WI)(Minimize TOF effects) Flow Comp (Reduce intra-voxel dephasing) Slices perpendicular to the blood Vessels Counter- Current Flow (maximize entry slice) Opposite Pre-Sat (Saturate blood) MTC (Suppress Background Tissues) Very short TR to saturate stationary protons Very short TE to reduce TOF and intra voxel dephasing in flowing protons.
Phase Contrast MRA Phase Contrast MRA (PC-MRA) uses velocity differences, and hence the phase shifts in moving spins to provide image contrast in flowing vessels. The variation in phase originates from physiological conditions such as systolic and diastolic velocity changes. Phase shift can also be generated by a pulse sequence.
PC-MRA Phase shift will be achieved by the phase encoding gradient, the velocity of flow with the use of a bipolar gradient in two different acquisitions. These will create a shift difference between stationary and flowing protons.
PC-MRA The additional gradient used to produce the phase difference between moving spins is called velocity encoding gradient (VENC). Phase contrast MRA is sensitive to flow in any direction within the image volume. X Y Z z x y
PC-MRA PC-MRA is sensitive to flow in three directions (X, Y, Z). The first set of image acquisition is done without the application of the VENC (Magnitude Images). Then another three sets of images are obtained each one has a VENC applied in each direction (X, Y, Z). At the end 4 sets of images are obtained (very long scan time).
VENC The application of the VENC is based on the velocity of the flow of the area of interest, measured in cm/sec. High venc factors of the PC angiogram (more than 40 cm/sec) will selectively image the arteries (PCA - arteriography), whereas a venc factor of 20 cm/sec will perform the veins and sinuses (PCV or MRV - venography). Blood Vessel Aorta Pulmonary Artery Superior Vena Cava Portal Vein Flow Velocity 92 cm/sec 63 cm/sec 10-35 cm/sec 20 cm/sec
PC-MRA Phase contrast provides information about: Flow velocity Vascular anatomy Flow direction
PC-MRA The accumulation of flow induced phase shift is proportional to the velocity of the flow. There is direct correlation between velocity and signal intensity. More velocity more signal.
PC-MRA Advantages Sensitive to a variety of flow velocities Sensitive to flow within FOV Reduce intravoxel dephasing Increase background suppression Magnitude and phase images Disadvantages Long imaging times More sensitive to turbulence
CE-MRA A major problem with TOF MRA is that it is prone to in-plane flow (flowing protons will saturate) and the long scan times in large areas of coverage.
CE-MRA To overcome this, a combination of 3D imaging with IV contrast enhancement (Gadolinium) with rapid dynamic imaging may be used.
CE-MRA CE-MRA is a more invasive technique where a bolus injection of contrast medium is followed by a 3D Incoherent (T1WI) fast gradient echo sequence.
3D-Fast Gradient Echo Very fast pulse sequence have been developed that can acquire a volume (3D) in a single breath hold. These usually employs coherent or incoherent GE. To be able to scan faster: Only one portion of the TE will be read (partial echo) TE will be reduce to the minimum (.7 ms) Only portion of the RF is applied (partial flip angle) Very short TR will be used
Fast Gradient Echo In addition, fast gradient echo acquisition are useful when temporal resolution is required. This is specifically important after the administration of contrast selecting fast gradient echo permits dynamic imaging of the blood vessels. Arterial phase (20 sec) Venous phase (40 sec)
CE-MRA This sequence is general timed to the arterial phase and the repeated several times to acquired images during intermediate and venous phases of the vascular system (Temporal Resolution or Dynamic imaging).
Technical consideration: Injection bolus (hand injection vs power injector) Scan timing (estimating, test injection, or bolus tracking) Image parameters (adequate coverage, SNR, and fat suppression)
Hand Injection
Power Injector
Bolus Detection Testing Bolus Bolus tracking Smart Prep or Fluoro-triggering
1 sec Testing Bolus 2 sec 20 sec after gad injection 3 sec 19 sec 20 sec 25 sec
On this technique the voxel Will send a signal to start Scanning Bolus Tracking
Live visualization of contrast, tech will Start scanning when Contrast arrives ROI
K-space Filling 10 sec 10 sec 10 sec If contrast take 20 sec to arrive to the ROI, and we start scanning 20 sec after injection we will be scanning the central area of the kspace around 30 seconds after contrast injection, resulting in venous contamination.
K-space Filling 10 sec 10 sec 10 sec If contrast take 20 sec to arrive to the ROI, and we start scanning 10 sec after injection we will be scanning the central area of the kspace around 20 seconds after contrast injection, resulting in no venous contamination.
Cylindrical Filling This type of filling is mostly used when image contrast and signal are required in the early phases of the examination. It is very popular in contrast enhance MRA. Scanning and injection can start at the same time, with no risk of contamination
Contrast Enhance MRA Advantages Sometimes it is advantageous to acquired images in the plane that best covers the anatomy. For example, to cover the ascending and descending aortic arch the Sagittal plane is optimal. Coronal plane will be a better option to visualize the abdominal aorta and the renal arteries.
20 SLICES Scan Time 3D=TR x PES x NEX x # Slices 40 SLICES
CE MRA 40 SLICES 20 SLICES
CE & In-plane Flow 3D 2D CE
2D PC MRA CE-MRV
Post Processing Techniques Reconstruction Techniques DS (Digital Subtraction) MIP (Maximum Intensity Projections) minip (Minimum Intensity Projections) MPR (Multi-Planar Reconstruction) Volume Rendering Shaded surface display (SSD) 63
Digital Subtraction Digital subtraction is a technique used to remove signal from stationary protons on the image an enhance the signal from protons in blood stream. These technique can be applied with different purposes (Original + gad) Original Gad - =
Arterial phase (20 sec) Venous phase (40 sec) - =
Digital Subtraction For DS to be effective the subtracted images must have the same: FOV Slice thickness Matrix PES FES
DS-MRA Digital Subtraction MRA is achieved acquiring two sets of images, one during diastole and another during systole, then both images are digitally subtracted. During diastole the velocity of the flow is slower (reduce TOF Phenomenon) and images will display high signal intensity from flowing protons. During systole there is a faster flow velocity, resulting in an increases of the TOF phenomenon and high velocity signal void.
Cardiac Gaiting Cardiac gating monitors cardiac motion by coordinating the excitation pulse with R wave of the cardiac cycle. This achieved by using an electrical signal generated by the cardiac motion to trigger each excitation pulse.
DS-MRA There is indirect correlation between flow velocity and signal intensity. More velocity less signal Systole Less velocity more signal Diastole
Digital Subtraction MRA (DS-MRA) Digital subtraction digitally subtracted the two images to remove the signal from the stationary spins, leaving behind an image of only moving spins based on the phase they were acquired. Systole Diastole Subtraction - =
MIP Images that result from the TOF MRA are called source images. They are reconstructed using a mathematical algorithm call Maximum Intensity Projection (MIP) also known as Maximum intensity pixel (MIP) or Image Viewer Intensifier (IVI) Source Image 3D Image
MIP The maximum intensity projection algorithm is responsible for projecting the brightest pixels, from anatomical stack, to generate an image of projected view of the vessels of interest.
MIP is the process by 2D images (source images) are reconstructed to a 3D image.
1 1 2 2
MIP
RT Side Posterior Circulation LT Side
CE-MRA Post Processing Digital Subtraction MRA reformats/reconstructions Maximum intensity pixel (MIP) (Plain + gad) Plain Gad MIP - =
Minimum Intensity Projections Minimum intensity projections (minip) use the same principle of MIP but opposite, it will make dark pixels even darker on the final image. The minimum intensity projection algorithm is responsible for projecting the darkest pixels, from anatomical stack, darker.
Multi-planar Reconstruction (MPR) Multi-planar reconstruction is a technique used to reconstruct images acquired in one orientation (Axial) and reconstructed in other plane (Coronal) after procedure completion. Good 3D quality is achieved by scanning with very thin slices and isotropic voxels, this combination will allow very good image quality in the reconstructed images.
MPR
Scoliosis
MPR (Multi Plannar Reformation)
3D Volume Reconstruction (Volume Rendering) Volume rendering is a technique used to display a 3D discretely sampled data set. A typical 3D data set is a group of 2D slice images acquired by a CT, MRI scanner. Usually these are acquired in a regular pattern (e.g., one slice every millimeter) and usually have a regular number of image pixels in a regular pattern.
Volume Rendering (VR) Volume rendering allows visualization of structures above and below the surface. Volume Rendering since it shows more than just the surface like in 3D has become the preferred method of display. Volume Rendering shows the entire scanned anatomy in 3-D like view not just the surface like a picture.
Shaded surface display (SSD) Three-dimensional rendering requires operator-specified determination of the range of intensities that will contribute to or be eliminated from the model. The thresholding procedure removes undesirable information from the data set such as noise) by virtue of intensity and allows a greater range of rendering possibilities.
3D Shaded Surface Display (SSD) 3D shaded surface display does not generate images of cross-sections of anatomy (cuts), but rather images of the surface of anatomical structures. The 3D images tend to look like black and white photographs of actual anatomical parts. Because they look so realistic they are used for orthopedics and planning for craniofacial surgery, neurosurgery and radiation therapy.
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