Magnetic resonance angiography

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Magnetic resonance angiography ( MRA ) is a group of techniques based on magnetic resonance imaging (MRI) to describe blood vessels. Magnetic resonance angiography is used to produce images of arteries (and less often veins) to evaluate them for stenosis (abnormal narrowing), occlusion, aneurysms (dilation of blood vessels, risk of rupture) or other abnormalities. MRA is often used to evaluate the artery of the neck and brain, the thoracic aorta and abdomen, renal arteries, and legs (the latter is often referred to as "runoff").

Video Magnetic resonance angiography

Acquisitions

Various techniques can be used to produce images of blood vessels, both arteries and veins, based on flow effects or on contrast (inherent or pharmacologically produced). The most commonly used MRA method involves the use of intravenous contrast agents, especially those containing gadolinium to shorten blood to about 250 ms, shorter than T > ₁ of all other networks (except fat). Short-TR sequences produce bright images of blood. However, many other techniques for performing MRA exist, and can be classified into two general groups: 'flow-dependent' and 'flow-independent' methods.

Flow-dependent angiography

One group of methods for MRA is based on blood flow. These methods are referred to as flow-dependent MRA. They take advantage of the fact that blood in the vessels flows to differentiate blood vessels from other static tissues. That way, a picture of a blood vessel can be produced. MRA-dependent flow can be divided into different categories: There is a phase contrast MRA (PC-MRA) utilizing phase differences to differentiate blood from static tissues and time-of-flight MRA (TOF MRA) that exploit that spin spinning from more experienced blood few excitation pulses rather than static networks, for example when imaging thin slices.

Time-of-flight (TOF) or inflow angiography, using short echo time and flow compensation to make blood flow brighter than stationary tissue. When the blood stream enters the imaged area, it has seen a limited number of excitation pulses so it is not saturated, giving it a much higher signal than the saturated stationary network. Because this method depends on blood flow, slow-flow areas (such as large aneurysms) or streams in the image area may not be well visualized. It is most commonly used in the head and neck and provides a high resolution detail image.

MRA phase contrast

Phase-contrast (PC-MRA) can be used to encode the velocity of blood in the phase of a magnetic resonance signal. The most common method used to encode velocities is the application of bipolar gradients between excitation pulses and readings. A bipolar gradient is formed by two symmetrical lobes from the same area. By definition, the total area (moment 0) of the bipolar gradient, $Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{          G                Â¯¯ <              me     Â ·                             {\ displaystyle G_ {bip}}  Â} , is null:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â?Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{          G                Â¯¯ <              me     Â ·                                    d          t         =         0               {\ displaystyle \ int G_ {beep} \, dt = 0}  Â} (1)

The bipolar gradient can be applied along any axis or combination of axes depending on the direction along which flow to be measured (eg x). ${\ displaystyle \ Delta \ Phi}$ , the phase accumulated during gradient implementation, is 0 for stationary rotation: their phase is unaffected by the application of bipolar gradients. For spin moving at constant speed, $Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{    Â     v                       <Â> x                            {\ displaystyle v_ {x}}  Â} , along the direction of applied bipolar gradient:

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â\mathrm{?}Â\; Â\; Â\; Â\; Â\; Â\; Â\mathrm{?}Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â=Â\; Â\; Â\; Â\; Â\; Â\; Â\; Â?Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{    Â     v                       <Â> x                         ?                  m              Â 1                            {\ displaystyle \ Delta \ Phi = \ gamma v_ {x} \ Delta m_ {1}}  Â} (2)

Fase yang masih harus dibayar adalah proporsional untuk keduanya ${\ displaystyle v_ {x}}  ÂÂ$ dan momen pertama dari gradien bipolar, ${\ displaystyle \ Delta m_ {1}}  ÂÂ$ , sehingga menyediakan sarana untuk memperkirakan ${\ displaystyle v_ {x}}  ÂÂ$ . ${\ displaystyle \ gamma}  ÂÂ$ adalah frekuensi Larmor dari spin yang dicitrakan. Untuk mengukur ${\ displaystyle \ Delta \ Phi}  ÂÂ$ , dari sinyal MRI dimanipulasi oleh gradien bipolar (berbagai medan magnet) yang disetel ke kecepatan aliran maksimum yang diharapkan. Akuisisi gambar yang kebalikan dari gradien bipolar kemudian diperoleh dan perbedaan dari dua gambar dihitung. Jaringan statis seperti otot atau tulang akan berkurang, namun jaringan yang bergerak seperti darah akan mendapatkan fase yang berbeda karena bergerak secara konstan melalui gradien, sehingga juga memberikan kecepatan alirannya. Karena fase kontras hanya dapat memperoleh aliran dalam satu arah pada satu waktu, 3 akuisisi citra terpisah di ketiga arah harus dihitung untuk memberikan gambaran aliran yang lengkap. Meskipun kelambatan metode ini, kekuatan tekniknya adalah bahwa selain pencitraan aliran darah, pengukuran kuantitatif aliran darah dapat diperoleh.

Angiografi bebas arus

While most of the techniques in MRA depend on contrast agents or blood flow to produce contrast (Contrast Enhancement technique), there are also non-contrast enhanced flow-independent methods. This method, as the name implies, does not depend on the flow, but is instead based on the differences T ₁ , T ₂ and chemical shifts from various voxel networks. One of the main advantages of such a technique is that we can describe the slow flow areas often found in patients with vascular disease more easily. In addition, non-contrast contrast methods do not require the administration of additional contrast agents, which have recently been associated with nephrogenic systemic fibrosis in patients with chronic kidney disease and renal failure.

Improved magnetic resonance angiography using MRI contrast agent injections and is currently the most common method of performing MRA. Contrast media is injected into the blood vessels, and the image is obtained both pre-contrast and during the first pass of the agent through the artery. With the reduction of these two acquisitions in post-processing, an image is drawn which in principle only shows the blood vessels, and not the surrounding tissue. As long as the timing is right, this can produce very high quality images. The alternative is to use a contrast agent that does not, because most agents, leave the vascular system within minutes, but remain in circulation for up to an hour ("blood-proofing agent"). Due to the longer time available for image acquisition, higher resolution imagery is possible. The problem, however, is the fact that both arteries and veins are enhanced at the same time if a higher resolution image is needed.

Reduced magnetic resonance angiography without subtraction: recent developments in MRA technology have made it possible to create enhanced high-contrast MRA images with high quality without the reduction of non-contrast mask images. This approach has been shown to improve diagnostic quality, as it prevents motion artifact reduction as well as an increase in background noise of images, both direct results from image reduction. An important condition for this approach is to have excellent body fat suppression over a large image area, which is possible using the mDIXON acquisition method. Traditional MRA suppresses signals derived from body fat during actual image acquisition, which is a method that is sensitive to small irregularities in magnetic and electromagnetic fields and as a result may indicate insufficient fat depletion in some areas. The mDIXON method can distinguish and accurately separate the image signal created by fat or water. Using 'water imagery' for MRA scans, virtually no visible body fat so there is no reduction mask required for high quality MR venograms.

Magnetic resonance angiography nonsagnance: Because contrast agent injections may be harmful for patients with poor kidney function, other techniques have been developed, which do not require injection. This method is based on the difference of T ₁ , T ₂ and the chemical shift from different network of voxel. An important non-elevating method for flow-free angiography is a balanced steady-state precedent imaging (BSSFP) that naturally generates high signals from arteries and veins.

2D and 3D acquisitions

For image acquisition, two different approaches exist. In general, 2D and 3D images can be obtained. If 3D data is obtained, cross-section at random point of view can be calculated. Three-dimensional data can also be generated by combining 2D data from various slices, but this approach produces lower quality images at a different angle than the original data acquisition. In addition, 3D data can not only be used to create cross-sectional images, but also the projection can be calculated from the data. The acquisition of three-dimensional data may also be helpful when dealing with complex vascular geometries in which blood flows in all spatial directions (unfortunately, this case also requires three different encoding streams, one in each spatial direction). Both PC-MRA and TOF-MRA have their advantages and disadvantages. PC-MRA has less difficulty with slow flow than TOF-MRA and also allows quantitative measurement of flow. PC-MRA exhibits low sensitivity when imaging pulsation and non-uniform flow. In general, slow blood flow is a major challenge in flow-dependent MRA. This causes the difference between the blood signal and the static network signal to be small. This also applies to PC-MRAs where the phase difference between blood and static tissue is reduced compared to the faster flow and to the TOF-MRA in which blood magnetization transverses and thus the blood signal is reduced. Contrast agents can be used to improve blood signals - this is very important for very small vessels and blood vessels with very small flow velocities that usually show weak signals. Unfortunately, the use of gadolinium-based contrast media can be harmful if the patient suffers from poor kidney function. To avoid these complications as well as to eliminate the cost of contrast media, non-enhanced methods have been investigated recently.

Techniques not developed in development

The free flowing NEMRA method is not based on flow, but exploits the differences in T ₁ , T ₂ and chemistry shifts to distinguish blood from static tissue.

Gated subtraction fast spin-echo: An imaging technique that reduces the two rapid swivel echoes obtained in systole and diastole. Arteriography is achieved by reducing systolic data, in which the arteries appear dark, from the diastolic density, where the arteries appear bright. Requires the use of electrocardiographic gating. The trade names for this technique include Fresh Blood Imaging (Toshiba), Original SPACE (Siemens) and DeltaFlow (GE).

Dynamic MR 4D angiography (4D-MRA): The first image, prior to the increase, serves as a reduction mask to extract the vascular tree in the next image. Allows the operator to divide the arterial and venous phases of the bloodstream by visualizing the dynamics. Less time spent researching this method so far than other MRA methods.

BOLD venography or weighted imaging vulnerability (SWI): This method exploits the differences in vulnerability between networks and uses phase images to detect these differences. The magnitude and phase data are combined (digitally, by the image processing program) to produce images with enhanced contrast that are highly sensitive to venous blood, haemorrhage and iron storage. Blood vein imaging with SWI is a blood-oxygen-level (BOLD) dependent technique which is why (and sometimes still) is referred to as BOLD venography. Because of the sensitivity of SWI venous blood is usually used in traumatic brain injury (TBI) and for high resolution brain venography.

A similar procedure for flow effects based on MRA can be used for the vein of the image. For example, magnetic resonance venography (MRV) is achieved by pulling the plane inferiorly when the signal is assembled on the plane immediately higher than the excitation plane, and thus imaging the venous blood that has recently moved from the plane. Differences in network signals, can also be used for MRA. This method is based on the different signaling properties of blood compared to other tissues in the body, independent of the effects of MR flow. This works best with balanced pulse sequences like TrueFISP or bTFE. BOLD can also be used in stroke imaging to assess tissue viability.

Maps Magnetic resonance angiography

Artifact

The MRA technique is generally sensitive to turbulent flow, which causes different protons of different magnets to rotate for the loss of the coherent phase (intra-voxel dephasing phenomenon) and cause loss of signal. This phenomenon can lead to overestimation of arterial stenosis. Other artifacts observed at the MRA include:

MRA-stage contrast: The phase wrapping is caused by an estimate below the maximum blood velocity in the image. Fast moving blood about the maximum set speed for the MRA-phase contrast gets aliases and signals wrap from pi to -pi instead of making flow information unreliable. This can be avoided by using the value of the speed coding (VENC) above the measured maximum speed. This can also be corrected by so-called phase-unwrapping. Maxwell's term : caused by the redirection of the gradient plane in the main field B0. This causes the excess magnetic field to become distorted and provides inaccurate phase information for the flow. Acceleration : accelerating the blood flow is not correctly coded by phase contrast techniques and can cause errors in measuring blood flow. Time-of-flight MRA: Laminar flow saturation : On many boats, blood flow is slower near the vessel wall than near the center of the ship. This causes the blood near the vessel wall to become saturated and can reduce the apparent caliber of the vessel. Venetian blind artifacts : Because the technique of obtaining images in a slab, non-uniform flip angles on the slabs can appear as a horizontal line in the composited image.

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Visualization

Sometimes, the MRA instantly produces thick (thick) slices containing all the interesting vessels. More generally, however, the acquisition results in a stack of slices representing the 3D volume in the body. To display this 3D dataset on 2D devices such as computer monitors, some rendering methods should be used. The most common method is the maximum projection intensity (MIP), in which the computer simulates light through the volume and selects the highest value to display on the screen. The resulting image resembles conventional catheter angiography images. If some such projections are incorporated into the cine loop or QuickTime VR object, the deep impression is enhanced, and the observer can get a good perception of the 3D structure. An alternative to MIP is direct volume rendering in which MR signals are translated to properties such as brightness, opacity and color and then used in optical models.

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Clinical use

MRA has successfully studied many arteries in the body, including the cerebral and other blood vessels in the head and neck, the aorta and its main branches in the chest and abdomen, renal arteries, and arteries in the lower limbs. For coronary arteries, however, MRA has been less successful than CT angiography or invasive catheter angiography. Most often, the underlying disease is atherosclerosis, but medical conditions such as aneurysm or abnormal vascular anatomy can also be diagnosed.

The advantage of MRA compared with invasive catheter angiography is the non-invasive character of the examination (no catheter should be introduced in the body). Another advantage, as compared to CT angiography and catheter angiography, is that the patient is not exposed to ionizing radiation. Also, contrast media used for MRI tend to be less toxic than those used for CT angiography and catheter angiography, with fewer people at risk of allergies. Also much less is needed to be injected into the patient. The biggest disadvantage of this method is the relatively high cost and rather limited spatial resolution. Long scan time can also be a problem, with CT much faster. It is also ruled out in patients who are not safe for MRI (such as having a pacemaker or metal in the eye or a certain surgical clip).

The MRA procedure for visualizing cranial circulation is no different from the position for normal MRI brains. Immobilization in the head coil will be required. MRA is usually part of a total brain MRI examination and adds about 10 minutes to a normal MRI protocol.

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See also

• Computed tomography angiography
• Transcranial doppler sonography

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References

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External links

• Magnetic Resonance Angiography at the National Library of Medicine US Subject of Medical Subject (MeSH)

Source of the article : Wikipedia