Improved In-Stent Lumen Visualization using Intravascular MRI and a Balanced Steady-State Free-Precession Sequence

Rationale and Objectives

To investigate the ability of an intravascular magnetic resonance (MR) loopless antenna to reduce the radiofrequency shielding of a vascular stent during signal reception as a way to improve the visualization of the in-stent lumen.

Methods and Materials

Using a balanced steady-state free-precession (bSSFP) sequence and a dedicated vascular phantom, the signal-to-noise ratio (SNR) inside the lumen of a stent is evaluated as a function of the nominal flip angle and compared with the results obtained for a reference vessel without a stent. All experiments are performed using successively an intravascular loopless antenna and surface arrays coils. Using an optimized protocol, in vitro in-stent restenosis visualization and quantification experiments are performed to evaluate the validity of an approach using an intravascular antenna and cross-sectional images to depict a vascular lesion inside a stent.

Results

The use of a loopless antenna effectively eliminates the radiofrequency shielding effect of the stent during signal reception. Furthermore, using a bSSFP sequence with a carefully chosen nominal flip angle, an equally good blood SNR can be obtained inside and outside the stent. Results of in vitro in-stent restenosis quantification measurements using the proposed method illustrate the benefits arising from the use of the intravascular antenna.

Conclusion

In the perspective of MR-guided vascular interventions, the presented results illustrate that the use of an intravascular antenna can significantly facilitate imaging inside a vascular stent. Potential applications include the monitoring of stent deployment as well as visualization and quantification of in-stent restenosis during an intervention.

Vascular stents are widely used during revascularization procedures to improve the short- and long-term success rate of the intervention . However, metallic implants such as vascular stents are also known to produce susceptibility and radiofrequency (RF) shielding artefacts that can considerably hinder the depiction of the stent lumen during magnetic resonance angiography (MRA) and therefore reduce the validity of this approach for a subsequent visualization of the vessel. For instance, a recognized complication of a revascularization procedure using a vascular implant is in-stent restenosis , and the assessment and quantification of this potential problem requires a careful follow-up inspection of the in-stent lumen that can be hindered by the artefacts created by the stent itself. Proposed solutions to increase the luminal depiction inside the stent during MRA include the use of specially designed inductively coupled or artefact-free stents and the employment of an angiography sequence with an increased flip angle .

Along with the development of MRA as a diagnostic tool, several developments toward the execution of endovascular interventional therapeutic procedures under MRA guidance have been made. In comparison to the standard clinical setup using X-rays, magnetic resonance imaging offers a complete elimination of the use of ionizing radiation, a reduction in the use of nephrotoxic contrast agents, functional and anatomical information in three dimensions, and an appreciably improved contrast between soft tissues. However, in the case of a procedure involving the deployment of a stent or a previously implanted stent, these benefits can potentially be locally canceled by the reduction of the lumen depiction caused by the stent.

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Materials and methods

Background MR Physics Theory

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SbSSFP=λr(r→)M0e−TR/T2√(1−e−TR/T1)sin(λt(r→)α)1−(e−TR/T1−e−TR/T2)cos(λt(r→)α)−e−TR/T1e−TR/T2, S

(

→

)

−

(

−

)

sin

(

→

)

−

(

−

)

cos

(

→

)

−

where α is the nominal flip angle applied and M 0 is the equilibrium magnetization. The λr(r→) λ

(

→

) and λt(r→) λ

(

→

) parameters indicate, respectively, the reception and transmission efficiency of the system. For example, the condition λr(r→o) λ

(

→

) = λt(r→o) λ

(

→

) = 1 implies that the effective flip angle at the position r→=r→0 r

→

0 will be equal to the nominal flip angle and that the signal from that point will be received with a 100% relative efficiency. In the case of a voxel located inside the lumen of a vascular stent and imaged using external coils for both transmission and reception, it can be expected that the λr(r→) λ

(

→

) and λt(r→) λ

(

→

) efficiency parameters will be significantly lower than 1, as a result of the RF shielding effect of the stent itself. Effectively, values between 0.38 and 0.57 were reported for an experiment using a fast spoiled gradient echo sequence for nitinol stents placed parallel to the main magnetic of 1.5 T scanner .

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RF Shielding Evaluation and Sequence Optimization

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In-stent Restenosis Visualization

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Results

RF Shielding Evaluation and Sequence Optimization

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Table 1

Relaxation Times and Ratios of In-stent/reference Reception and Transmission Efficiencies for the Intravascular Antenna and the Surface Array Coils

Intravascular Antenna Surface Array Coils Gd-DTPA (mmol/L) T 1 (ms) ∗ T 2 (ms) ∗ λr(stent)Unknown node type: cross_refλr(ref) λ

(

)

Unknown node type: cross_ref

(

) λt(stent)Unknown node type: cross_refλt(ref) λ

(

)

Unknown node type: cross_ref

(

) λr(stent)Unknown node type: cross_refλr(ref) λ

(

)

Unknown node type: cross_ref

(

) λt(stent)Unknown node type: cross_refλt(ref) λ

(

)

Unknown node type: cross_ref

(

) 0 841 (833–849) 138 (133–143) 0.96 (0.92–1.00) 0.65 (0.59–0.71) 0.52 (0.50–0.54) 0.60 (0.56–0.64) 0.36 348 (345–351) 110 (106–114) 1.03 (1.00–1.06) 0.78 (0.73–0.83) 0.53 (0.50–0.56) 0.58 (0.53–0.63) 1.80 110 (107–113) 63 (59–67) 0.96 (0.94–0.98) 0.74 (0.70–0.78) 0.50 (0.48–0.52) 0.63 (0.60–0.66)

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In-stent Restenosis Visualization

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Table 2

Stenosis Quantification Results for the Reference and In-stent Stenoses

Theoretical Value Estimated Value ∗ Intravascular Antenna Estimated Value ∗ Surface Array Coils In-stent stenosis 90% 85% (82%–88%) — Reference stenosis 75% 72% (67%–77%) 73% (67%–79%)

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Discussion

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References

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Improved In-Stent Lumen Visualization using Intravascular MRI and a Balanced Steady-State Free-Precession Sequence

Rationale and Objectives

Methods and Materials

Results

Conclusion

Materials and methods

Background MR Physics Theory

RF Shielding Evaluation and Sequence Optimization

In-stent Restenosis Visualization

Results

RF Shielding Evaluation and Sequence Optimization

In-stent Restenosis Visualization

Discussion

References

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