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Phase Contrast Flow Accuracy

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					                    CLINICAL VALUE       CARDIOLOGY – REPORTCARD




     Phase Contrast Flow Accuracy
     By S. Gay Luebchow, RT (R)(MR) Advanced Cardiac Applications, GE Healthcare and
     Cindy R. Comeau, BS, RT (N)(MR), Advanced Cardiovascular Imaging, New York, NY


     MRI offers unique advantages as an accurate, reproducible,
     and non-invasive modality for the quantification of flow.
     At Advanced Cardiovascular Imaging (New York, NY) cardiac
     gated phase contrast sequences are acquired on a GE Signa®
     1.5T and the data is analyzed utilizing ReportCARD™ 3.0
     (GE Healthcare), resulting in flow direction, volume, and
     velocity information.
     However, as is typical with MRI, flow results are often
     compromised due to inhomogeneities found in the magnetic
     field environment, called short term eddy currents. Present      Figure 1                        Figure 2
     in all magnet field environments, these phase offset errors
     are a common challenge. Even relatively small velocity offsets
     can have a large effect on flow results; therefore, it is
     imperative to correctly compensate for the inherent phase-
     offset errors to provide accurate flow measurements for
     clinical diagnosis. These errors can potentially overestimate
     or underestimate flow.
     Using a Venc of 200 cm/s for example:

       Venc 200 cm/s                                                  Figure 3a                       Figure 3b
             1% Velocity Error = 2 cm/s
                                                                      To compensate, it is necessary to place an additional
       For a 10 cm2 vessel:
                                                                      background region of interest (ROI) in an area on the image.
              Flow Error = 2*10 = 20 ml/beat
                                                                      It is recommended to place the background ROI in stationary
       For a patient with an 80 ml stroke volume,                     tissue that is located as close to the original ROI as possible.
       this represents a 25% error in flow.                           This is easily performed when analyzing cerebrospinal fluid
                                                                      (CSF) flow data, as the surrounding brain tissue is stationary
                   400                                                (Figure 1).
                   300
                                                                      Flow analysis in areas of the heart and chest are more
      Flow (m/s)




                   200                                                challenging since there is no stationary tissue located near
                                                                      the vessels to be analyzed. The most commonly measured
                   100
                                                                      flow areas in the chest are within the ascending aorta and
                    0                                                 the main pulmonary artery, both of which are located
                         0   250         500         750   1000       centrally in the chest (Figure 2). Using stationary tissue
                                      Time (msec)                     in either the spine or shoulder as a zero baseline is not
                                                                      recommended. ROIs placed in these regions have differing
                                                                      phase offsets compared to the vessel that is being analyzed.


     ReportCARD 3.0 automatically adjusts final flow results
     eliminating phase-offset errors caused by the eddy currents.

16    A GE Healthcare MR publication • Spring 2007
                                                          CARDIOLOGY – REP ORTCARD CLINICAL VALUE




                                                                                                        S. Gay Luebchow, RT

                                                                                                        S. Gay Luebchow, RT (R) (MR), is
                                                                                                        an Advanced MR Cardiovascular
                                                                                                        Applications Specialist for
                                                                                                        GE Healthcare, dedicated to
Figure 4                                                                                                training 1.5T and 3.0T users on
                                                                                                        cardiovascular MRI. Previously
                                                                                                        Gay was an MR Field Applications
By adjusting the window/level in a phase contrast image from a stationary phantom
                                                                                                        specialist for 4 years based in
acquisition (Figures 3a and 3b), the black pixels (red arrow) show the phase error due                  the SE sector of the USA. She has
to eddy currents. Since there is no real flow in the field of view, the measured flow is                extensive cardiac MR experience
the phase-offset error result. Phase offset error seen on a phantom may be missed                       with years of clinical experience
                                                                                                        as an MRI technologist at Wake
if the background correction ROI is located in the area of shoulder or muscle tissue.                   Forest University Baptist Medical
                                                                                                        Center, North Carolina. Gay has
Phantom images are acquired to provide zero flow baseline measurements.
                                                                                                        assisted in the development of
ReportCARD 3.0 automatically adjusts final flow results by this factor, eliminating                     cardiovascular training programs,
phase-offset errors caused by the eddy currents.                                                        and has globally trained
                                                                                                        physicians, applications specialist
ReportCARD 3.0 quantifies flow volume (milliliter per beat) and velocity (centimeter                    and technologists in the area
per second) of blood or CSF. To obtain these values, a ROI is placed surrounding                        of cardiovascular MRI.
the vessel of interest and clicking “Execute” to generate a flow curve. Selecting the
“Phantom Series” option from the flow tab will automatically propagate this ROI
to all phases of the cardiac cycle on the phantom acquisition. The phase-offset
error is automatically calculated and applied to the original flow curve (Figure 4).
ReportCARD checks for the same scan plane position and velocity encoding (VENC)
prior to applying this correction. Further details on the workflow for performing the
phantom acquisition can be found in the ReportCARD 3.0 operator manual.
ReportCARD 3.0 flow analysis also generates Pixel-Wise Velocity Curves, Peak Absolute
Velocity, and Peak Velocity Envelope information. Other valuable software features
include Flow Summary and Velocity Tables all of which can be printed in the report.
           Flow Summary Table                         Pixel-Wise Velocity Curve                         Cindy R. Comeau, BS, RT

                                                                                                        Cindy R. Comeau BS, RT (N)(MR), is
                                                                                                        the Chief Technologist at Advanced
                                                                                                        Cardiovascular Imaging, New York,
                                                                                                        NY. With extensive clinical cardio-
                                                                                                        vascular MRI experience she has
                                                                                                        published training material, book
                                                                                                        chapters, authored publications,
Figure 5                                                                                                abstracts and has given numerous
                                                                                                        presentations at educational
                                                                                                        meetings for both MRI technologists
Conclusion                                                                                              and physicians. Cindy is an active
                                                                                                        member of the Society for Magnetic
By automatically calculating the phase offset error from a phantom acquisition,                         Resonance Technologists (SMRT)
ReportCARD flow analysis improves the accuracy of reporting flow in clinical cardiac                    and currently serves as the 2006-
MRI patient exams.                                                                                      2007 SMRT President.




                                                                                           A GE Healthcare MR publication • Spring 2007       17

				
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