Non-Contrast-Enhanced MRA
Ioannis Koktzoglou1,2

1Radiology, NorthShore University HealthSystem, Evanston, IL, United States, 2Pritzker School of Medicine, University of Chicago, Chicago, IL, United States

Synopsis

This presentation will review established and emerging methods for non-contrast-enhanced magnetic resonance angiography.

Target Audience

Clinicians and scientists interested in learning about the evolving field of non-contrast-enhanced (NE) magnetic resonance angiography (MRA).

Outcomes/Objectives

Learn about established and emerging techniques for non-contrast-enhanced MRA (NEMRA).

Methods

NEMRA, MRA without the use of exogenous contrast agents, has multiple potential uses. It can serve as an alternative to contrast-enhanced MRA (CEMRA) for patients with severe renal insufficiency in whom Gadolinium (Gd)-based contrast agents are contraindicated, eliminate concerns over Gd retention within the body, allow for the saving of contrast agents for other uses (e.g. perfusion imaging), and help to depict arterial anatomy prone to venous contamination during CEMRA.

NEMRA is a long-established yet evolving field that is composed of multiple distinct MRI methods that seek to provide high signal from the vascular pool while suppressing the appearance of non-vascular background tissue. Distinct methods for NEMRA include the “flow-independent” approach of balanced steady-state free precession (bSSFP), as well as the “flow-dependent” methods of time-of-flight (TOF)1-3, inflow inversion-recovery (IFIR)4,5, cardiac-gated subtractive (GS)6-10, arterial spin labeling (ASL)11,12, phase contrast (PC)13, quiescent-interval slice-selective (QISS)14, and velocity-selective (VS)15 imaging.

Flow-Independent NEMRA:

Flow-independent bSSFP-based NEMRA, which capitalizes on the high signal from blood during bSSFP imaging, is widely available and is most often used for evaluating the great vessels and the coronary arteries. T2 preparation and fat suppression are often applied to improve arterial-to-background contrast.16,17

Flow-Dependent NEMRA:

TOF and PC are some of the oldest and most widely available approaches, whereas IFIR, GS, ASL and QISS are available on some modern MR systems, while the VS method resides in the research domain. Brief descriptions of these methods are now provided:

  • Time-of-flight (TOF): TOF NEMRA consists of flow-compensated gradient echo sequence that saturates stationary spins and accentuates the appearance of arterial spins flowing into the imaging slice or slab. Primary uses of TOF reside in the neurovascular arena.1-3
  • Phase Contrast (PC): PC NEMRA is predicated on the application of gradients that impart a phase to the MRI signal, wherein the phase is proportional to the flow velocity.13,18 With the use of cardiac gating and a multi-phase readout, PC can provide time-resolved 3D evaluation of blood flow.19,20
  • Inflow Inversion-Recovery (IFIR): The main application of IFIR NEMRA is in the abdomen for the evaluation of the renal arteries.4,5 IFIR leverages a slab-selective inversion-recovery radiofrequency (RF) pulse over the imaging slab to suppress background and venous signals, an inflow time to allow fully magnetized arterial spins to enter the imaging slab, and a high signal-to-noise readout such as 3D bSSFP.
  • Quiescent Interval Slice-Selective (QISS): QISS NEMRA is cardiac-gated multi-slice technique initially described for the evaluation of the lower extremity arteries.14 QISS applies a saturation RF pulse to suppress background signal within the imaged slice, a quiescent time for inflow of arterial spins into the imaging slice, and a 2D bSSFP readout. Recent works have reported variants of QISS for evaluating other vascular beds.21-24
  • Cardiac-Gated Subtractive (GS): GS NEMRA relies on the differential signal of arterial blood when imaging is performed in systole and diastole.6,7 A common implementation of GS NEMRA consists of cardiac-gated 3D fast spin-echo (FSE) readouts that are acquired in systole when arterial blood appears dark, and in diastole when arterial blood appears bright.8,25,26 Subtraction of the readouts reveals an angiogram. GS NEMRA is primarily applied in the lower extremities, feet, and hands.
  • Arterial Spin Labeling (ASL): ASL is a subtractive approach for NEMRA that consists of two acquisitions that are identical except for differential RF labeling of inflowing vascular spins.11,12 Subtraction of the two acquisitions reveals the blood pool that was labeled differently between the two acquisitions. ASL NEMRA is primarily applied in the neurovascular circulation.27-33 ASL NEMRA coupled with multi-phase readouts can be used for visualization of blood flow.34-36
  • Velocity Selective (VS): VS NEMRA applies tailored magnetization preparations that suppress vascular and/or stationary background spins based on velocity and directionality.15 The approach is typically applied with cardiac gating and a 3D bSSFP readout. Abdominal, lower extremity, and intracranial applications have been reported.15,37,38

Results

Flow-independent bSSFP-based NEMRA is mainly used for imaging the coronary arteries and the great vessels, primarily at 1.5 Tesla. In a large multi-center trial evaluating the coronary arteries, bSSFP-based MRA provided a sensitivity of 88% and a specificity of 72%.39 The method also provides diagnostic image quality for displaying the aorta and pulmonary vasculature.40-43

Main applications of TOF reside in the evaluation of head and neck arteries. 3D TOF is reliable for detecting steno-occlusive disease of the intracranial arteries, and highly sensitive for detecting intracranial aneurysms.44-47 TOF is accurate for the detection of ≥70% stenoses of the internal carotid arteries albeit with poorer sensitivity for moderately severe (50-69%) stenoses.48

The main application of IFIR is for the evaluation of the renal arteries. For the detection of ≥50% renal artery stenosis, four studies performed at 1.5 Tesla have reported a median sensitivity of ≈91% and a median specificity of ≈91%.5,49-51

QISS has primarily been applied for detecting ≥50% stenosis in the lower extremities (infrarenal aorta through ankle). In five studies using CEMRA as the reference standard test, QISS provided a median sensitivity of ≈94 and a median specificity of ≈96%.52-56 In five studies using X-ray digital subtraction angiography as the gold standard, median values for the sensitivity and specificity of QISS have been ≈92% and ≈95%, respectively.57-61

In seven studies of the lower extremities (three of which limited to the calf or foot), GS NEMRA using a FSE readout provided a median sensitivity of ≈87%, and a median specificity of ≈87% for the detection of ≥50% stenosis.26,62-67 However, other studies have reported poor image quality and a high rate of non-diagnostic vessel segments.59,68,69

Discussion and Conclusion

Non-contrast-enhanced MRA can be performed using a variety of protocols and can often serve as a substitute for contrast-enhanced MRA, especially in patients with renal impairment. Protocol reliability, ease of use, and availability are important factors in the utilization of non-contrast-enhanced MRA in clinical practice. Future developments in the field of NEMRA should address these factors as well as incorporate multi-center trials to validate the accuracy and robustness of newer protocols.

Acknowledgements


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Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)