Magnetic resonance angiography

Magnetic resonance angiography ( MR angiography, MRA) is an imaging method for the diagnostic imaging of blood vessels ( arteries and veins) using the methods of magnetic resonance imaging ( MRI). For this purpose, different techniques can be used, which (without surgical procedures and injections manage ) are partially or completely non- invasive, based on the administration of MRI contrast agents. In contrast to conventional angiography be included instead of the two-dimensional projection images in the MRA as a rule, three-dimensional data sets, enable an assessment of the vessels from all angles. Another difference from the conventional angiography is that when no MRA catheter must be inserted into the blood vessel system. In many areas, the MRA is a competitive digital subtraction angiography to CT angiography and ultrasound procedures.

Indications

Typical indications for MRA examination, for example, the suspicion of arterial stenoses ( atherosclerosis ), vascular occlusion ( embolism ), venous thrombosis, aneurysms ( aneurysms ), vascular malformations and other vascular diseases, as well as the investigation of the vessel ratios of tumors.

MRA techniques

There are many different techniques to illustrate the magnetic resonance imaging blood vessels. The main techniques are listed below:

Time-of -flight MRA

The time-of -flight MRA (TOF- MRA) utilizes the fact that fresh inflowing blood in the examination volume has a higher magnetization than the stationary tissue whose magnetization is reduced by the applied RF pulses of the MRI pulse sequence ( saturated) is. The blood vessels with fresh inflowing blood are therefore presented hyperintense. For the TOF - MRA usually fast 2D or 3D gradient-echo techniques (FLASH ) can be used; there is no need for a contrast agent.

However, only vessels are shown signal range that contain fresh inflowing blood. This can lead to artifacts when vessels run, for example, over long distances within the examination volume; in this case, the magnetization of the blood is saturated with time. The location of the examination volume can therefore not be chosen arbitrarily, but must be perpendicular to the prevailing direction of blood flow in general.

The name time-of -flight MRA refers to the fact that the magnetization of the blood is carried out outside of the examination volume, and a certain amount of running time is required until the blood reaches the area with higher magnetization shown. German translations such as flight -time or run -time MRA are rarely found.

Phase-contrast MRA

The movement of the flowing blood can be visualized with appropriate investigation techniques as the phase difference in the complex image data. Through these phase differences, the blood flowing from the surrounding stationary tissue is different and can be represented hyperintense therefore. The phase-contrast MRA (PC - MRA of Engl. Phase - contrast MRA, rarely PK- MRA) is usually based on fast gradient echo techniques (FLASH) with additional flusskodierenden gradient pulses and requires no contrast agent.

In contrast to the time-of -flight MRA, the location of the examination volume in the phase-contrast MRA is much more flexible. On the other hand, the exposure times are considerably longer than the time-of -flight MRA.

Contrast-enhanced MRA

By injecting T1 verkürzendem (usually gadoliniumbasiertem ) contrast agent, the blood is shown hyperintense on T1-weighted MRI scans. MRI scans during the passage of the contrast agent through an organ or anatomical region can therefore be used to represent the blood vessels. Contrast-enhanced MRA (CE - MRA of Engl. Contrast -enhanced MRA, sometimes CE-MRA contrast agent MRA ) based on fast usually three-dimensional gradient-echo techniques (FLASH).

Compared to the time-of -flight or phase contrast MRA of recording on contrast-enhanced MRA can be significantly reduced, so that shooting in bated breath are possible as well as dynamic MRA images that can represent the blood flow time-resolved (with time resolutions up down to 1 second per 3D dataset ).

Other techniques

Spin-echo techniques, which are fast-flowing blood signal intensity can be combined with ECG triggering to even portray as the arteries in two consecutive shots hypointense (recording during the rapid systolic blood flow) and once hyperintense (recording during the slow diastolic blood flow). The difference between the two recordings, the arteries appear hyperintense, while other tissue is suppressed.

Other techniques such as steady-state free precession sequences - or Arterial spin labeling methods can be used to represent the blood in the vessels hyperintense.

In contrast to the mostly desired signal rich representation of the vessels there is also the possibility of representing the vessels hypointense ( dark) in the surrounding lighter fabric. This is, for example, the representation of veins ( MR venography ) with T2 * - and used suszeptibilitätsgewichteten pulse sequences.

Post-processing and presentation of the MRA data

For the assessment and diagnostic process, the three-dimensionally recorded data sets must be represented as two-dimensional images (on screen). There are several post-processing: The data can be layered considered according to the original recording orientation, they can be reconstructed ( approximately at right angles to the vessel or the vessel in the image plane ) as multiplanar reconstructions ( MPR) in any plane, it can maximum intensity projections (MIP, Maximum Intensity Projection ) are calculated from different angles, or the data can be displayed as a virtual three-dimensional objects in space.

For all advanced imaging techniques is a recording of isotropic data possible advantageous ie in which the spatial resolution is about the same in all three spatial directions. Typical spatial resolutions are using modern techniques (depending on the size of the vessels of interest and the receiving volume ) between 1.5 × 1.5 × 1.5 mm ³ and 0.5 × 0.5 × 0.5 mm ³.