Imaging
Table 1: Peripheral MRA – Major Characteristics of TOF, nce-MRA/FBI and ce-MRA
TOF FoV Spatial resolution Flow artefacts nce-MRA/FBI ce-MRA
1 station 3–4 stations Low
Present 3–4 stations
1mm3 after Down to 0.6mm3 interpolation
isotropic
Possibly second None test sequence needed
Complex vessel course Limitations Limitations Amount of physician interaction needed Acquisition time Robustness
High
High Low
Very high High Intermediate
No limitations Intermediate to low Low High
FBI = fresh blood imaging; FoV = fields of view; MRA = magnetic resonance imaging; nce = non-contrast-enhanced; TOF = time of flight.
is used. A true voxel size of 0.6mm3 is feasible in lower limbs, especially when parallel imaging such as SENSE (Philips), GRAPPA (Siemens) or ASSET (GE) is being used, while time-resolved imaging of contrast kinetics (TRICKS) or time-resolved echo-shared angiography (TREAT) for the distal run-off vessels may help to identify target vessels for surgery in cases of asymmetrical flow. The limitation is the acquisition time for abdominal and lower leg. In order to minimise potential risks of NSF for patients with severely impaired renal function, exact adherence to the manufacturer’s instructions is imperative.
Irrespective of the imaging method used, in acute ischaemia the main challenge of the diagnostic work-up is the depiction of occlusions and collateral vessels because of slow flow and/or small rudimentary vessel segments due to limited spatial resolution. The main challenges in critical limb ischaemia are reliable stenosis grading, the depiction of collateral and patent distal vessels and the depiction and quantification of stenosis length.16
In clinical routine, digital subtraction angiography (DSA) is widely used and is frequently employed as the reference standard in clinical studies to evaluate new imaging procedures. Especially for peripheral angiography, DSA is regarded as the gold standard. However, for critical limb ischaemia, Kreitner et al.17
have shown that, despite a
lower spatial resolution compared with DSA, ce-MRA is valuable in the diagnostic work-up of the most challenging subgroup of PAOD patients; that is, those with chronic ischaemia. These authors find “…significant superiority of MR angiography to DSA for the detection of patent pedal vessels…”.17
The main advantage in this context is the
ability of ce-MRA to detect blood flow as slow as 2cm/second, whereas DSA shows limitations because of its much lower contrast sensitivity. Clinical studies comparing ce-MRA with conventional catheter angiography show convincing overall sensitivities between 91 and 94% for the detection of vascular disease, and specificities between 90 and 93%.18
Large studies demonstrate that ce-MRA of the
whole body has the potential to change the treatment of patients due to additional detected pathologies.19,20
The major characteristics of TOF, nce-MRA/FBI and ce-MRA for peripheral MRA are summarised in Table 1, which is based on our experience (author personal communication).
Overall, the main limitations of peripheral nce-MRA are the need for complex techniques and the relatively long acquisition times required
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for high resolution. Separation of arteries and veins with nce-MRA may be difficult depending on the technique used, and often makes high technical demands on the equipment and the operator. For nce- MRA/FBI sequences, an ECG trigger has to be prepared and the flow- gradient strength has to be optimised. The latter depends both on the body region and the grade of proximal stenosis.
The use of nce-MRA has been established in only one body region. For imaging of the intracranial arteries, nce techniques are preferred because ce-MRA is difficult owing to the small time window between the arterial and venous phases. On the other hand, typical disadvantages of nce-MRA resulting from patient movement are reduced or absent, so that longer nce sequences can be carried out.
Awareness of the potential risk of NSF in some patients has given momentum to the promotion of nce-MRA techniques, mainly by Toshiba Medical USA and to a lesser extent by Siemens, Philips and GE.21
Nevertheless, despite all improvements, nce-MRA still suffers from fundamental disadvantages. These are:
•
more examination time is needed, resulting in reduced patient throughput;
• more artefacts are obtained, which reduces diagnostic confidence; and
•
more technical difficulties arise, reducing reproducibility and robustness.
Therefore, ce-MRA remains the method of choice for most indications, except for certain applications such as imaging of the intracranial arteries. ce-MRA adds diagnostic value with less effort because of its greater robustness, high spatial resolution, which allows the imaging of patent distal vessels, reliable stenosis grading, fewer technical failures, fewer non-diagnostic images and a broader range of applications. ce-MRA is superior from the technical and clinical perspectives. Therefore, we investigated whether ce-MRA is also competitive in terms of costs of investigation.
Evaluation of Investigation Costs Methods
The costs were assessed on the basis of published literature and an earlier cost study for ce-MRA in the lower extremities, in which costs of a ce-MRA with manual table-feed technique and a standard DSA investigation were compared from the hospital’s perspective. The cost study referred to (‘Study 1’) was conducted by Bayer Schering Pharma AG in German hospitals with a sample of 11 observations for each procedure.22
The main emphasis of Study 1 was the observation of time per work step to run the MRA, the association of these times with costs and the determination of fixed costs for personnel and for equipment use. Investigation costs for ce-MRA were found to be ~20% lower than those of standard DSA (€142.38 and €176.63, respectively), excluding the costs of consumables such as contrast agents and catheters.
For nce-MRA/FBI, no direct or published cost data were available for use in the study in question. Therefore, the results for ce-MRA were adapted to nce-MRA/FBI by re-calculations based on data from recent publications,5–9
as explained below.
Investigation costs were the direct costs of the examination plus overheads, the latter comprising costs indirectly linked to the MRA
EUROPEAN CARDIOLOGY
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