MR Angiography Imaging Techniques
This course will identify the MR Angiography Imaging Techniques, as well as discuss optimization of the parameters.
MR Angiography Techniques By the end of this course you will be able to: List the different types of flow effects in MRI Explain Time of Flight MRA Explain Phase Contrast MRI Demonstrate non-contrast MRA techniques Demonstrate contrast MRA techniques Flow Effects in MRI Time of Flight MRA Phase Contrast syngo Native QISS Contrast Enhanced MRA syngo TWIST Laminar Flow - consistent velocities across a vessel. Turbulent Flow - fluctuates at different velocities. Vortex Flow - passes through a stricture or stenosis. The center of the vessel has fast flow where as the flow spirals near the walls of the vessel. 3D single slab ideal for vessels with greater velocities. 3D multi-slab medium replacement velocity. vessel region divided into several 3D slabs. 2D Slices used to image vessels with very low replacement velocities. No contrast material is used. Maximize flow related enhancement. Minimize flow related dephasing. Suppress back ground tissue. Blood flows fully magnetized – generates high signal Any inflowing spins appear bright Both arteries and veins appear bright Saturation bands can be used to suppress unwanted signal Imaging plane is configured perpendicular to the vessel Sub Millimeter Slice Thickness Shorter TR Smaller flip angles Optimal Slice positioning High Resolution New Volume of Blood enters the slice for every TR period TOF is sensitive to flow velocity If flowing blood stays longer in the slice it is exposed to more RF pulse Reduces the blood signal More SNR Strong saturation of background tissue at 3T due to longer T1 relaxation times Better CNR reduces need for MT prep pulses Shorter out-of-phase TE Complex flow or turbulence causes signal loss (Improved by reducing TE). Inefficient coverage of long vessel segments. Short T1 tissues (fat, blood, Contrast enhanced regions) can appear the same as flow, obscuring vessels. Patient motion significantly degrades image quality. Projects brightest pixels from multiple angles. Can use either a 2D or 3D data set. Uses a flow-induced phase shift for selectively displaying flowing blood. Phase shift measured can be used for flow encoding in the image as well as for flow quantification. VENC optimal Max black/white and no aliasing VENC slightly too low Very little aliasing VENC much too high Washed-out black/white VENC much too low Very much aliasing In-plane measurement - the flow-encoding direction can be selected according to the three patient orientations foot to head, anterior to posterior, or Right to left. The selection depends on the slice orientation. Through-plane measurement - the flow is displayed vertically to the vessel. The vessel is shown as a cross-section; the flow direction is either into or out of the slice. Rephased Magnitude Phase Magnitude of flow compenstated echo Flow is bright Background visible Difference in echo magnitudes Flow is bright Background suppressed Difference in phase shifts Forward flow is bright Reverse flow is black Background is midgray syngo NATIVE TrueFISP: Non-ce MRA optimized for the use in thorax (e.g. thoracic aorta) and abdomen (e.g. renal arteries) syngo NATIVE SPACE: Non-ce MRA optimized for the use in peripheral angiography – with Inline Subtraction & MIP Inversion Volume Imaging Volume ← Venous inflow of blood with inverted magnetization -> low signal Respiratory synchronization with Navigator or respiratory cushion Allows high resolution 3D imaging Free Breathing Can be used with no respiratory synchronization where appropriate; e.g., renal transplants Greatly accelerates scan time With ECG triggering Allows optimal timing of scan in relation to blood flow In some cases more efficient as each TI can be guaranteed to include a systolic event Can reduce motion blurring from pulsatile vessels Without ECG triggering Can be used for convenience Needs longer Inversion Times to guarantee a good volume of “non-inverted” blood Abdominal Angiography. Normal Upper abdominal arterial anatomy. Note demonstration of hepatic, splenic, proximal SMA, renals etc. Proximal gastro-duodenal artery is seen but distally is lost due to bowel motion which cannot be compensated. CE-MRA syngo Native TR = 3.5ms TE = 1.2ms Tacq = 13.27sec Slice = 1.2mm Resolution = 227x384 TR = 866.3ms TE = 1.6ms TI = 790ms Tacq = 3min 21 sec Slice = 1mm Resolution = 216x304 Renal Arteries Ensure Triggering Optimal Capture cycle prior to applying scan Good cardiac output – Trigger pulse = 1 Poor cardiac output – Trigger pulse = 2 Preferred method – Respiratory Cushion. Postioning Position slab over renal arteries. Position FoV close to origin of vessel of interest. Don’t include too much of proximal aorta in FoV. Add a second inversion pulse and p and place inferior to first inversion volume . Patients with known heart failure may have to use a longer TI. TSE-based acquisition Enhanced flow sensitivity (example in read direction) with additional spoiler gradients Spin-Echo acquisition shows inherent flow sensitivity > Subtraction of data sampled in diastolic & systolic phase ECG or pulse triggering Segmented data acquisition Fat suppression possible (STIR preparation) Determination of optimal cardiac phases: 2D TD scout scan Triggered PC flow measurement 2D CINE scan ECG Triggering optimal Run localizer Position FoV Cine_TD scout Capture cycle Load Cine _TD scout into Mean Curve Magnify images (1 leg at a time) Draw ROI over artery in each leg Start Evaluation Select > Scaling/Sorting icon X-Axis = Trigger Time Determine TD Maximum Flow from Mean Curve Graph Move vertical bar to highest peak of intensity curve Trigger time > subtract 30 to 50 ms Open > NATIVE SPACE sequence Position anatomy of interest Physio > Signal 1 Tab Captured Cycle TD Peak Flow > Type in the number from Mean Curve Evaluation Typical spatial resolution 0.9 x 0.9 x 1.5 mm Scan Times around 3 minutes per station Heart Rate dependent Peripheral non-contrast MRA If there are different blood transit times in each leg > Run 2 NATIVE measurements Select > Physio > Signal 1 Tab > increase Trigger pulses to 2 Enhance Flow Sensitivity - Physio tab > signal one tab > flow sensitivity field Default - normal method of using flow-spoiling gradients as implemented with the syngo SPACE sequence. Weak - no flow spoiling gradient. Medium - higher flow sensitivity than off, due to sophisticated spoiler gradients. Strong - highest flow sensitivity 2D ECG gated single-shot balanced steady-state free precession acquisition of one axial slice per heartbeat. Image contrast is generated using in-plane saturation to suppress background tissue. Tracking saturation pulse to suppress venous signal prior to quiescent inflow period QISS 3T CE MRA 3T T1 shortening of blood Fast protocols to get images from arterial phase only Abdominal – Thorax Carotids Peripherals Minimally invasive No saturation effects No flow induced dephasing Slice orientation independent of flow direction Short acquisition times Timing is equally important at 1.5T and 3T Transit Time - the time required for the bolus to move from the point of injection to the vascular structure. Test Bolus - determines the physiological transit time of the contrast agent Data acquisition window - determines the measurement duration between arterial and venous enhancement. Care Bolus Test Bolus syngo TWIST Angio Dot Engine Flexibility: Time-resolved MRA in all body regions without the need of coil or patient repositioning with full 50 cm FoV. Accuracy: Time-resolved MRA with high temporal and spatial resolution: up to one 3D dataset per second and submillimeter resolution (in-plane). Speed: iPAT For dynamic imaging of vascular pathologies such as: Arterial-venous malformations (AVM) Carotid fistula Peripheral obstructive artery disease (POAD) Intra-cerebral or thoracic shunts Tumor vasculature Sub-divide k-space into 2 regions and scan them in the alternating scheme A BT A BT A BT ... Scan complete k-space only once in the beginning (= the "mask") Administer contrast agent Region A is the center of k-space, determining the image contrast The size of Region A can be flexibly chosen, between 10% and 100% of total k-space Region B is the periphery of k-space, determining the spatial resolution The sampling density SDB of Region B can be flexibly set, between 0% and 50% Minimally Invasive Low contrast dose Scalable temporal resolution No loss of signal Short scan times Reduce TR Rectangular FoV Partial Fourier Spatial Resolution Reduction Parallel Acquisition Technique Clinical need: No timing issue when doing peripheral run-offs Low dose ce-MRA No timing issue (left/right) High spatial resolution Excellent image quality Dr. J. Barkhausen, University Clinic of Essen, Germany; MAGNETOM Avanto Several well-known strategies exist to reduce the scan time per 3D volume: Short TR Rectangular FoV Partial Fourier Reduction in spatial resolution In-plane slice Parallel imaging In-plane (phase-encoding direction) Both (phase-encoding and slice-encoding directions) With these strategies it is possible to get scan times ranging from one to several seconds, depending on selected parameters. Test Bolus Visual display of arterial/venous timing window Auto Voice Commands Integrated into the scanning workflow System plays voice commands automatically when programmed Ensures optimal timing of scanning, breathing and contrast media Add pauses between automatic breath-hold commands if necessary Indirect form of saturation. Background signal from specific solid tissue reduced - signal from blood is unchanged. Protons bound to macro molecules with very high molecular weight have a broader resonance spectrum than free protons. With a preparation pulse slightly offset to the resonance frequency, it is possible to saturate bound protons without immediate effect on free protons. The saturation of bound protons is transferred to adjacent free protons. The signal of the free protons is reduced. Without MTC With MTC TONE excitation Signal of blood subjected to constant as well as varied excitation angles. Generates a relative uniform signal distribution within the blood vessels of a 3D slab. RF Pulses are switched with a tilted slice profile. Tilt of the slice profile generates flip angles varying from slice to slice. Unsaturated flow with inflow at one side of the slab - smaller flip angles. Partially saturated flow through the slab - larger flip angles. Flip angle increase across the 3D slab optimized for flow velocity and slab thickness. Thicker 3D slabs and slower flow - larger flip angle variations. Thiner 3D slabs and faster flow - smaller flip angle variations. Longer TR Thicker Slice 3-5mm Slice thickness Larger flip angle Straight sections of vessels Used in slow flow vessels 2D ToF works well at 3T in areas of directional flow Problematic areas: Breathing and swallowing In-plane flow saturation Signal losses at air-tissue interfaces Complex flow or turbulence causes signal loss (Improved by reducing TE) Short T1 tissues (fat, blood, contrast enhanced regions) can appear the same as flow, obscuring vessels Patient motion significantly degrades image quality