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Has the Time Arrived to Image Placental Perfusion?

Department of Radiology,
University of California, San Francisco,
521 Parnassus Ave, Rm C-324C, Box 0628,
San Francisco, CA 94143-0628,
Ben.yeh@radiology.ucsf.edu

SUMMARY

Taillieu et al have shown that it is possible to noninvasively measure the placental blood flow, fractional volume of the maternal vascular placental compartment, and rate of transfer of contrast material between the maternal and fetal circulation in gravid mice through the use of dynamic contrast-enhanced MR imaging. Clearly, much work lies ahead before functional evaluation of the placenta becomes a clinical reality, but contrast-enhanced imaging shows promise for functional evaluation of placental disease.

THE SETTING

The clinical reticence to employ contrast material–enhanced imaging for the placenta and fetus has left unexplored many basic questions that can affect fetal health. Whereas contrast-enhanced functional imaging is becoming the norm for most other organ systems, neonatal evaluation remains steadfastly a morphologic assessment. In this issue of Radiology, Taillieu et al (1) describe the evaluation of normal mouse placental perfusion with dynamic contrast-enhanced magnetic resonance (MR) imaging.

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Dynamic contrast-enhanced MR imaging allows one to quantify the rate at which contrast material crosses from the intravascular compartment into other compartments, such as the interstitial space of various tissues and tumors (2). For 22 gravid mice, Taillieu et al (1) performed dynamic contrast-enhanced MR imaging of the maternal uterus and heart at 1.55-second intervals after injection of either twice the usual clinical dose of gadolinium chelate as a bolus or four times the usual dose as a bolus followed by a slow infusion. Such high gadolinium concentrations were required because standard clinical doses of gadolinium chelate do not produce sufficient placental signal changes to be detectable (3).

Changes in signal intensity over time were measured for the maternal heart blood pool, the placenta, and the fetus. The high doses of gadolinium chelate introduced a nonnegligible T2* effect—that is, signal loss due to intravascular local magnetic field inhomogeneities. Therefore, imaging was performed by using T1-weighted spoiled gradient-echo sequences with two different echo times so the authors could calculate a T1 signal intensity that depended only on the T1 effect and excluded T2* effects (4). The calculated signal intensities were converted to gadolinium concentrations, and the time-concentration curves were then fitted to a standard three-compartment model to estimate various vascular parameters. With this method, Taillieu et al found that the gadolinium transfer rates observed between the maternal and fetal placental vascular compartments were similar between the two injection protocols. Furthermore, the authors showed that the calculated fraction of the placenta that was composed of the maternal vascular component was 36.5%, which is similar to the 35.3% previously found in a morphologic study of human placentas obtained from cesarean sections (5).

THE PRACTICE

Clinical use:
It is hoped that new tools for evaluating the maternal placental interface and placental perfusion may allow improved understanding of, and potentially treatment for, a variety of vascular diseases that may affect this interface. For example, the decision to intervene in cases of placental abruption or placenta accreta or after surgery for twin-twin transfusion syndrome may be influenced by knowledge of placental blood flow and the rate of transfer of substances through the placenta. It is possible that monitoring maternal placental transfer may also improve routine prenatal care by revealing the transfer rate of nutrients, waste, or drugs to and from the fetus. For example, knowledge of how maternal-fetal transfer of substances is influenced by hormonal signals such as those that may be associated with food consumption or stress would certainly be advantageous.

Future opportunities and challenges:
Prior to its clinical use, many milestones for placental and fetal dynamic contrast-enhanced MR imaging remain to be reached, not the least of which is the acceptance of high-dose gadolinium chelates or other intravenous MR imaging contrast materials for use in the unborn patient. Establishment of the safety of one or more MR contrast agents will be needed before routine contrast-enhanced MR imaging is accepted in practice. Likewise, the reproducibility and patient-specific accuracy of dynamic contrast-enhanced MR imaging for the determination of placental perfusion parameters must be evaluated in a clinical population, and disease states in which such evaluation is actually useful must be identified. A broader consideration is that the permeability measured with conventional contrast material may not reflect the permeability of the placenta to other molecules of interest owing to differences in physical characteristics, such as size, charge, or serum protein binding (6). Nevertheless, while much work lies ahead before functional evaluation of the placenta becomes a clinical reality, the study by Taillieu et al (1) is an important early step toward shedding light on this subject.

FOOTNOTES

See also the article by Taillieu et al in this issue.

References

1. Taillieu F, Salomon LJ, Siauve N, et al. Placental perfusion and permeability: simultaneous assessment with dual-echo contrast-enhanced MR imaging in mice. Radiology 2006;241(3):737–745.[Abstract/Free Full Text]
2. Tofts PS, Brix G, Buckley DL, et al. Estimating kinetic parameters from dynamic contrast-enhanced T(1)-weighted MRI of a diffusable tracer: standardized quantities and symbols. J Magn Reson Imaging 1999;10(3):223–232.[CrossRef][Medline]
3. Salomon LJ, Siauve N, Balvay D, et al. Placental perfusion MR imaging with contrast agents in a mouse model. Radiology 2005;235(1):73–80.[Abstract/Free Full Text]
4. Kuperman VY, Alley MT. Differentiation between the effects of T1 and T2* shortening in contrast-enhanced MRI of the breast. J Magn Reson Imaging 1999;9(2):172–176.[CrossRef][Medline]
5. Barker G, Cunliffe N, Bardsley WG, D'Souza SW, Donnai P, Boyd RD. Fetal and maternal blood volumes in shed human placentae: discrepant results comparing morphometry to haemoglobin content. Placenta 1988;9(3):289–296.[CrossRef][Medline]
6. Turetschek K, Preda A, Novikov V, et al. Tumor microvascular changes in antiangiogenic treatment: assessment by magnetic resonance contrast media of different molecular weights. J Magn Reson Imaging 2004;20(1):138–144.[CrossRef][Medline]

This Article Figures Only Full Text (PDF) Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Yeh, B. M. Search for Related Content PubMed PubMed Citation Articles by Yeh, B. M. Related Collections Related Article