Marketing Information Letter

July 20th 2006MIL MRI-2006-003

TO: MR Service Engineers

Phase Contrast Flow Accuracy

MRI has the ability to quantify flowing blood and CSF non-invasively using a phase contrast pulse sequence and analysis software which outputs results in mLs/heartbeat.

In order to ensure the most accurate quantifiable results the user must perform a small correction that uses non-moving structures to set a zero baseline. This document attempts to outline this technique for improved flow accuracy.

Fast 2D Phase Contrast (PC) is a pulse sequence used to evaluate blood and cerebrospinal (CSF) flow. This sequence requires cardiac or peripheral gating and is optimally scanned perpendicular to the direction of the flowing blood or CSF. The resulting multi-phase data sets are then networked to the Advantage Workstation for analysis. There is no analysis tool on the MR Scanner which can quantify flowing blood/csf.

The tool most commonly used for flow analysis is ReportCARD 3.0. This software quantifies flow volume (ml/beat) and velocity (cm/sec) of blood/csf. To obtain these values, a region of interest (ROI) is placed surrounding the area of interest. This ROI is then automatically propagated to all phases by choosing the “execute” option. Several flow graphs are produced. These include Flow Curve, Pixel-Wise Velocity Curve, Peak Absolute, and Peak Envelope. Flow Summary and Velocity tables are also provided.

In order to improve the accuracy of the flow measurements a baseline measurement of something that has zero flow is desirable. This mitigates phase errors, which may be introduced by eddy currents and can add to or subtract from the flow measurement.

To compensate for these errors, it is necessary to place an additional background ROI in an area on the image located in stationary tissue and is as close to the original ROI as possible. This is easily performed when analyzing CSF flow data, as the surrounding brain tissue is stationary. When the “execute” option is chosen after both ROIs have been placed, the resulting data is automatically corrected for with ReportCARD 2.0 and 3.0. (See fig 1)

Fig 1. ROI measuring flow and the background ROI to compensate.

Flow analysis in areas of the heart and chest are more challenging since there is no stationary tissue located near the vessels to be analyzed. The most commonly measured flows in the chest are within the ascending aorta and the main pulmonary artery both of which are quite central in the chest.

Using stationary tissue in either the spine or shoulder as a zero baseline is problematic due to the fact that these tissues are too far from the measured vessel and themselves have differing phase shifts because they are not close to the ROI. Figure 2. MR image of chest.

Therefore, it is necessary to scan a phantom. (See fig 3). The user must perform the following steps while scanning:

  • At the end of the study, do NOT end the exam.
  • Bring the tabletop out and have the patient get up.
  • Place a phantom on the coil where the patient was.
  • Do NOT re-landmark.
  • Advance to scan.
  • Connect the PG to a person or ECG leads to a “chicken heart” for gating capabilities.
  • “Copy and Paste” all the PC series previously scanned.
  • Perform each of these series on the phantom.

Fig 3. Phantom used for flow correction.

Network the phantom series to the AW and launch ReportCARD 3.0. Place an ROI on the vessel to be analyzed. Choose “execute” to propagate to all phases. Review these phases and edit if necessary. Then choose the “Phantom Series” option on the ReportCARD 3.0 menu. (See fig 4). This will open and display all PC series scanned, including the phantom series. Choose the corresponding phantom series. The originally placed ROI will then propagate to all the phases of the phantom. The results are automatically corrected in all curves and tables.

Fig 4

See document entitled “Phantom Correction for Phase Contrast” for ReportCARD 2.0 phantom correction procedure.

Glossary:

ROI: Region of Interest

CSF: Cerebro-spinal Fluid

Authors:

Gay Luebchow. Advanced Cardiac Applications

Stuart Clarkson. 1.5T Global MR Product Manager