Angiography for Diagnosing Peripheral Arterial Occlusive Disease
published and promoted as potential alternatives to standard ce-MRA.5–9
A more recent review of nce-MRA methods with focus on these newer, commercially available techniques can be obtained in the reference list.10
The major equipment manufacturers have used different technical development strategies for their nce-MRA applications. Toshiba Medical Systems has promoted three nce techniques – nce-MRA/FBI, contrast-free improved angiography (CIA) and time–spatial labelling inversion pulses (time-slip) – and has implemented them in the fourth generation of the Toshiba Vantage system. Recent developments by Philips Medical Systems have comprised technologies termed triggered angiographic non-contrast enhancement (TRANCE) and phase-contrast angiography (PCA- SENSE). TRANCE was introduced with the 11th release of software by Philips, while PCA-SENSE has been available for more than eight years, starting with the ninth release. Siemens has actively promoted new techniques for peripheral MRA (NATIVE truFi), head and neck MRA (NATIVE SPACE) and cerebral perfusion MRA (ASL Perfusion). General Electric (GE) Healthcare also offers a number of more recent pulse sequences on their currently available HDx and HDe systems.
Taken together, the most prominent techniques today for nce-MRA are TOF, PCA, FBI, balanced steady-state free precession (bSSFP), arterial spin-labelling (ASL) and dark-blood imaging. However, of these, TOF and nce-MRA/FBI are most often used for MRA of the peripheral vessels. The following discussion will therefore be restricted to these methods.
The increasing expectation of healthcare payers that providers will deliver cost-effective services heightens the demand for greater procedural efficiency. The aim of this study was to compare the technical and clinical utility of the TOF and nce-MRA/FBI techniques with those of ce-MRA in the specific context of diagnosing peripheral arterial occlusive disease in the lower extremities. In addition, the competitiveness in costs of ce-MRA compared with nce-MRA/FBI was evaluated, the main focus being on costs of the investigation. The three main questions that our analysis was intended to answer were: what are the technical and clinical advantages of the different MRA procedures? How do differences in structure and duration of the procedures influence the costs accruing to the service provider? What are the costs of conducting a diagnosis on a patient with suspected PAOD when ce-MRA is used, compared with nce-MRA/FBI?
Clinical and Technical Utility of TOF, nce-MRA/FBI and ce-MRA – A Mini-review The respective technical and clinical strengths and disadvantages of nce- and ce-MRA techniques differ in nature. The techniques under consideration are those used in an indication for peripheral MRA. These are mainly TOF, nce-MRA/FBI and ce-MRA. The comparison of clinical and technical utility performed here was based on published literature.
Compared with other nce-MRA methods, the advantage of TOF-based techniques is their relatively short acquisition time. However, this acquisition time depends on the size of the FoV, the spatial resolution, the magnetic field strength and the specific sequence used, and also on the possibility of using additional saturation pulses to selectively depict arteries and veins. The radiologist can also achieve suppression
EUROPEAN CARDIOLOGY
of tissue background, which provides good vessel contrast. Usually, operators are familiar with TOF techniques because of their wide use in intracranial MRA. On the other hand, standard TOF images are sensitive to the patient’s movements and to variations in the velocity and direction of blood flow. This may lead to a loss of signal for in-plane vessels, oblique vessels, vessels forming loops, slow-flow regions and turbulent or retrograde flow.11
Saturation of blood imposes
size limitations on the FoV, which in turn leads to inefficient coverage of long vessels and incomplete suppression of fresh thrombi. Most clinically used TOF methods do not provide dynamic information. The strength of TOF is the possibility of arterio-venous separation and single-station unidirectional flow.
nce-MRA/FBI is a more recent technical development. It requires well-trained personnel and long scan times. If used correctly, nce- MRA/FBI provides large anatomical coverage, good spatial resolution even in the peripheral vessels (at least better than TOF) and good separation of arteries and veins.12
Like other spin-echo sequences,
nce-MRA/FBI is less sensitive to susceptibility artefacts than gradient- echo sequences. nce-MRA/FBI can be combined with ASL. On the other hand, extensive scan preparation requirements, including electrocardiogram (ECG) trigger and determination of appropriate spoiler gradients, are time-consuming and make extensive demands on the operator’s time. Therefore, the operator needs to be experienced and specifically trained. This technique is also unsuitable for patients with arrhythmia and, especially, tachyarrhythmia. Ultrafast spin-echo can show low signal or signal voids, leading to impairment of image quality. Furthermore, there is limited robustness
in vessels with varying flow patterns or collateral vessels, and T2 blurring artefacts may also be experienced. nce-MRA/FBI allows multistation MRA and a section thickness of 4mm (interpolated to 2mm). For parallel imaging, 1mm is feasible; however, the signal and arterio-venous separation depend on flow changes during the cardiac cycle; if flow changes are small (e.g. in connection with extended occlusions or reverse blood flow), a reduced signal is to be expected.
Unlike nce techniques, ce-MRA allows large FoVs with an imaging plane along the vessels of interest.13
It reduces or eliminates most of
the artefacts of TOF by avoiding flow-related problems caused by flow direction, flow rate, turbulent flow and blood saturation. The images can be acquired rapidly within the breath-hold capacity of the patient, which is important for abdominal and pelvic MRA in many clinical settings. Along with valuable options of dynamic imaging, ce-MRA has proved to be robust, with high diagnostic output.2–4
In patients with
arteriovenous malformations in particular, time-resolved multiphase ce-MRA of the hand can deliver valuable diagnostic information and supports surgical treatment planning.14
Nevertheless, except for time-resolved ce-MRA techniques, proper timing of the start of the scan relative to the contrast injection is essential in order to ensure high-quality images of the arterial system without venous overlay. Generally, the timing can be determined by fluoroscopic bolus techniques or by administration of a test bolus. For first-pass imaging there is a trade-off between spatial resolution and acquisition time. Easy-to-use protocols such as Smart Prep or Care Bolus are available from respective MRI device manufacturers to ensure optimal and convenient bolus timing.15
The strengths of ce-MRA are its high sensitivity for slow flow and its increased sensitivity and specificity in particular when parallel imaging
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