MRA of the Carotid Arteries 27

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H. J. Michaely, MD

Department of Clinical Radiology, University Hospitals – Gross hadern, Ludwig Maximilian University of Munich, Marchioni nistr. 15, 81377 Munich, Germany

K. Nael, MD

Department of Radiological Sciences, David Geffen School of Medicine at UCLA, 10945 LeConte Ave, Suite 3371, Los Angeles, CA 90095, USA

C O N T E N T S

27.1 Introduction 291

27.2 MRA Techniques 292 27.2.1 Phase-Contrast and

Time-of-Flight Techniques 292 27.2.2 High-Spatial-Resolution MRA 293 27.2.3 High-Temporal-Resolution MRA 294

27.3 Measurement of

Carotid Artery Stenosis 296

27.4 Functional Characterization of Carotid Artery Disease 299

27.5 Comparison to Ultrasound and Intravascular Ultrasound 301 27.6 Carotid MRA at 3 Tesla 301

References 303

MRA of the Carotid Arteries 27

Henrik J. Michaely and Kambiz Nael

giving rise to any branches other than the internal ophthalmic artery and exclusively supplies the brain with blood. Pathologies of the external carotid arteries only rarely become symptomatic and are only rarely treated. However, diseases of the ICAs can be life threatening and have therefore been intensively investigated.

The most common disease of the ICA is atherosclerotic plaque formation and stenosis at the bifurcation at the level of the carotid bulbus. These alterations of the ICA have been identiﬁ ed as a main risk factor and were found an etiologic factor for 80%

of all ischemic brain insults in patients. Since stroke related to ischemia from carotid emboli is one of the leading causes of death in western countries with a rising incidence in developing countries, extensive studies have been conducted on which patients benefi t from therapy. Two independent studies, the North American Symptomatic Carotid Endarterectomy Trial (NASCET) (1991) and the European Carotid Surgery Trial (ESCT) (Barer 1998) could prove that patients with a high-grade stenosis of the ICA benefi t from surgery. The studies’ threshold for a high-grade signifi cant stenosis was set at 70% diameter lumen narrowing measured on digital subtraction X-ray angiography (DSA) images as the standard of reference. These fi ndings underline the need for reliable imaging of the ICA. The DSA is still considered as the gold standard.

However, it is expensive (U-King-Im, Hollingworth et al. 2004), bears a non-negligible risk of 1-2% for complications such as embolism, stroke (Willinsky et al. 2003) and even death (Waugh and Sacharias 1992), may lead to impaired renal function (Niendorf et al. 1991; Heyman and Rosen 2003) and exposes the patient to ionizing radiation (Fig. 27.1).

Because of these drawbacks of DSA, magnetic resonance angiography (MRA) and Doppler ultrasound (DUS) have become the diagnostic imaging modali- ties of choice for ICA stenoses (U-King-Im, Trivedi et al. 2004). The application of DSA is nowadays limited to cases with ambivalent ﬁ ndings in MRA and DUS or for therapeutic purposes.

27.1

Introduction

The common carotid arteries arising in most cases from the aortic arch on the left side and the innomi- nate artery on the right side provide the main blood supply to the brain. The common carotid arteries divide at the carotid bulbus into the external carotid artery, which supplies the neck and the soft tissues of the head with blood. In contrast, the internal carotid artery (ICA) runs into the cranium without

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27.2

MRA Techniques

27.2.1

Phase-Contrast and Time-of-Flight Techniques

MRA techniques have undergone a remarkable evolution since the ﬁ rst description of an angiographic magnetic resonance technique roughly 20 years ago (Wedeen et al. 1985). The initial techniques used for carotid MRA were time-of-ﬂ ight (TOF) and phase- contrast (PC) MRA. Both require neither the administration of MR contrast agents nor high-performance gradient systems to detect the vessels. However, their drawbacks are the relatively long acquisition times that are in the order of several minutes and that make them more susceptible to motion artefacts such as breathing artefacts and artefacts from swallowing (Patel et al. 1995; Saloner 1998), which cannot be suppressed throughout a 4-min scan. TOF techniques are in addition susceptible to slow blood

ﬂ ow and tend to show signal loss (Prince et al.

1997) in case of stenotic vessel lesions due to tur- bulent ﬂ ow and subsequent intravoxel dephasing.

While this seemed like a disadvantage of this technique at fi rst, the dephasing effect was then specifi - cally used to detect high-grade stenoses, which lead to spin dephasing (Nederkoorn et al. 2002). How- ever, TOF techniques still perform relatively well in the detection of atherosclerotic vessel disease with sensitivities and specifi cities only 5-13% lower than with contrast-enhanced techniques (Anzalone et al. 2005). For the intracranial MRA, TOF techniques remain the gold standard and seem to profi t strongly from the combination of parallel imaging and 3-T systems (Willinek et al. 2004; Willinek et al. 2003). No reports on the application of parallel imaging for carotid MRA have been published yet.

Similarly, PC techniques – even though not widely used outside the head any more – seem to gain some support again for the depiction of intracranial vessels and in particular of the intracranial veins as shown in Chap. 26.

Fig. 27.1a,b. High-grade left CAS at the bifurcation in 68-year-old patient. The lateral view of the CE-MRA a correlates excel- lently with that of the intra-arterial conventional digital subtraction angiography b. Figure reprinted with permission from:

Michaely HJ, Herrmann KA, Kramer H, Laub G, Reiser MF, Schoenberg SO “The signiﬁ cance of MR angiography for the diagnosis of carotid stenoses” Radiologe, 2004 Oct 44(10): 975-984

b a

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27.2.2

High-Spatial-Resolution MRA

The NASCET trial drastically demonstrated the posi- tive effects of surgical therapy in patients with at least 70% diameter stenosis of the internal carotid artery (NASCET 1991). From these results arises the strong clinical need for an exact depiction and hence exact grading of carotid artery stenoses (CAS). The mean diameter of the internal carotid artery (ICA) is estimated to be 5-6 mm. In case of an at least 70%

CAS, the remaining lumen would be 1.8 mm in the best case. To exactly assess these CAS and to avoid partial volume effects and hence wrong grading of these CAS, the pixel size has to be smaller than the remaining lumen. The literature states that a value of three pixels within the vessel of interest is required for exact grading stenoses (Hoogeveen et al. 1998;

Westenberg et al. 2000). This implies that the opti- mal spatial resolution for depiction of 70% CAS is at the order of 0.6×0.6 mm² in-plane or even better. This knowledge fueled the evolution of MRA techniques to obtain the best possible spatial resolution. This has been greatly fostered by the advent of high-performance MR scanners, which facilitated the application of contrast-enhanced (CE) MRA techniques. CE-MRA is independent of pulsatility and ﬂ ow artefacts and can acquire one entire 3D data set in a single breath- hold, thereby also eliminating motion artefacts from breathing and swallowing (Prince 1994). An initial technique for improved spatial resolution was the elliptical centric reordered k-space (Riederer et al.

2000) readout, which enables longer scan times without venous overlay (Huston et al. 1999). With this technique the k-space is fi lled from the center to the periphery. Therefore, the contrast agent bolus has to reach the vessel area of interest exactly at the start of the sequence. There are two main advantages to this approach. Since the contrast-determining parts of k-space are fi lled at the start of the scan when the contrast agent bolus passes through the arteries, this approach leads to a very weak venous signal and hence almost no venous overlay even with prolonged scan times. Using automatic or semiautomatic fl uoroscopic techniques, the test bolus can be omitted and the scan time decreased (Butz et al. 2004). However, this approach requires more profound operator experience since it is more diffi cult to initiate the scan correctly during real-time visualization of the rapidly passing contrast media bolus.

Parallel imaging represented the next technical evolution leading to a better spatial resolution of CE

MRA. The application of parallel acquisition techniques (Griswold et al. 2002; Pruessmann et al.

1999) to high-resolution MRA of the carotid arteries was used by most groups to decrease the scan time and improve spatial resolution. Since sufﬁ cient signal-to- noise ratio (SNR) or contrast-to-noise ratio (CNR) is available, no major adoptions to non-parallel-imaging techniques have to be done. A spatial resolution of roughly 1.0×1.0×1.0 mm³ can be acquired in 20 s or less on most current 1.5-T scanners with multi-channel technique and a parallel-imaging acceleration factor of R=2. Using parallel imaging a carotid artery scan with slightly decreased – yet diagnostic – spatial resolution can be incorporated into a whole-body protocol (Kramer et al. 2005). The rationale for a slightly decreased spatial resolution is derived from the faster scan times needed to catch the same bolus of contrast agent at the calves. Exemplary sequence parameters for a dedicated high-resolution MRA of the carotid vessels can be found in Table 27.1. However, since only every second line (acceleration factor R=2) or even every third line (acceleration factor R=3) of k-space is acquired, the center of k-space is traversed much faster compared to non-accelerated acquisitions. This may hamper ﬂ uoroscopically triggered elliptical-centric MRA acquisitions since a slightly delayed start of the acquisition can lead to a substantially decreased image contrast and quality. Therefore, Cartesian acquisition seems preferential with parallel imaging since it is less sensitive to slightly mistimed initiation of the scan. On the other hand, a

Table 27.1 Imaging parameters for carotid CE-MRA at 1.5 T (Siemens Magnetom Avanto)

Time-resolved MRA

High-resolution MRA

Repetition time (ms) 2.37 3.69

Echo time (ms) 0.82 1.23

Flip angle 20° 30°

Bandwidth (Hz/pixel) 750 360 Field of view (mm²) 320×200 330×212

Matrix size 256×218 448×349

Voxel size (mm³) 1.5×1.5×3.0 0.9×0.7×0.9 Parallel imaging GRAPPA, R=2 GRAPPA, R=2 Partial Fourier 6/8 (phase-enc.

direction)

Off

Acquisition time 1.6 s/3D data set 0:23 min Slice thickness (mm) 3.0 0.9

Contrast 10 ml at 4 ml/s 20 ml at 1.5 ml/s k-space trajectories Cartesian Cartesian

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recent publication found beneﬁ cial effects of SENSE in combination with elliptic centric k-space sampling in terms of increased venous suppression (Hu et al. 2004).

Most groups used “conventional” phased-array coils for parallel imaging. For cardiac applications, a 32-channel coil that enables acceleration factors of 4-5 is already available (Reeder et al. 2005). When dedicated neu- rovascular coils become available, higher acceleration factors will also be employed for the carotid arteries. As explained later, the inherently higher signal at 3 T rep- resents an ideal compensatory complement of parallel imaging (Michaely et al. 2005a).

To compensate for the loss of SNR and CNR with parallel imaging at 1.5 T already, the application of either 1-molar contrast agents (gadobutrol, Gado- vist, Schering, Berlin, Germany) (Goyen at al. 2001), or agents with higher relaxivity due to weak protein binding like Gd-BOPTA (Mulithance, Bracco, Milan, Italy) (Goyen and Debatin 2003) or strong protein binding like the intravascular MS-325 is recommended (EPIXpharmaceuticals, Boston, MA, and Vasovist, Schering AG, Berlin, Gemany). For the latter, approval by the European Agency for the Eval- uation of Medicinal Products (EMEA) and the FDA is expected soon. The latter also allows imaging of the carotid arteries in the steady state. Theoretically,

these longer scan times should translate into a higher spatial resolution (Fig. 27.2).

In summary, for high-resolution CE-MRA of the carotid arteries parallel imaging successfully lead to an increased performance in terms of imaging speed as well as spatial resolution. It is up to the individual user whether a higher spatial resolution and accelerated acquisition or a combination of both, which is preferred at our institution, is chosen as beneﬁ ting from parallel imaging. Apart from the well-known signal loss, which can be compensated with the appropriate contrast agent, there is no disadvantage associated with the application of parallel acquisitions techniques in CE-MRA of the carotid arteries.

No reports on major aliasing or third arm artefacts are known either. Recent studies even report that the signal loss – though it can be objectively measured – is diagnostically not impairing (Michaely et al.

2005; Summer et al. 2004).

27.2.3

High-Temporal-Resolution MRA

As high-spatial-resolution MRA is always at risk for venous overlay and does not display dynamic ﬂ ow

Fig. 27.2. a Coronal and b sagittal thin-MIP views of a 30-year-old healthy volunteer who underwent carotid MRA at 1.5 T (sequence parameters in Table 27.1) with intravascular contrast agent (Gadofosveset). In these steady-state images 15 min after the initial administration of the contrast agent the carotid arteries can be clearly followed. Due to the intravascular character of the contrast agent there is also strong venous contrast

b a

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information due to its static nature, time-resolved MRA techniques have been presented by various groups (Willig et al. 1998; Klisch et al. 2000; Golay et al. 2001; Fellner et al. 2005; Warmuth et al. 2005;

Lenhart et al. 2002). The overall approach is to trade spatial resolution for temporal resolution and hereby to acquire several 3D data sets in the same time as a single data set of the high-spatial resolution MRA.

Initially, multiphasic 2D time-resolved techniques have been used that can be used as a test-bolus sequence additionally providing information about vascular hemodynamics (Wikstrom et al. 2003).

Time-resolved 3D data sets combine the information about blood fl ow hemodynamics with that of vessel morphology (Fig. 27.3). Initially, multi-phasic 3D gradient-echo sequences were used that yielded a 3D dataset every 10 s (Lenhart et al. 2002; Naganawa et al. 2001) with, however, markedly reduced spatial resolution. Therefore, it is not surprising that the overall sensitivity and specifi city of these sequences for the detection of CAS is signifi cantly lower than those of high-resolution MRA (Lenhart et al. 2002).

The introduction of parallel imaging led to a consid- erable decrease in scan time to 4 to 6 s per 3D data

Fig. 27.3. a Coronal MIP view of a right-sided, short, high-grade CAS at the bulbus, which can also be clearly demonstrated in the sagittal view b. In the time-resolved MRA c using TREAT with a temporal resolution of 2 s/3D frame a markedly delayed ﬂ ow in the affected right ICA can be appreciated. Figure reprinted with permission from: Michaely HJ, Herrmann KA, Kramer H, Laub G, Reiser MF, Schoenberg SO “The signiﬁ cance of MR angiography for the diagnosis of carotid stenoses”

Radiologe, 2004 Oct 44(10): 975-984 c

b a

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set allowing the visualization of more than one frame free of venous overlay (Golay et al. 2001). A different approach, the so-called view-sharing technique, was also reported to allow time-resolved MRA with a high temporal resolution; this technique has been called TRICKS (time-resolved imaging of contrast kinetics) (Naganawa et al. 2001; Korosec et al. 1996;

Mistretta et al. 1998). The combination of these two techniques, view-sharing and parallel imaging, is sometimes referred to as TREAT (time-resolved echo-shared angiography technique) and enables high-spatial-resolution and high-temporal-resolu-

tion data acquisition in a single exam, cf. Chaps. 7 and 11 (Saloner 1998). By this means, spatial resolutions in the order of acquired 1.5×1.5×3.0 mm³ voxel size can be obtained with a temporal resolution of 1.6 s/3D frame (Staikov et al. 2000). This technique will be valuable in the detection of delayed ﬂ ow in case of CAS or reversed ﬂ ow as in the case of subclavian steal syndrome (Figs. 27.4, 27.5). Further progress of this technique can be achieved at 3 T as laid out in the following paragraph.

27.3

Measurement of Carotid Artery Stenosis

Atherosclerotic changes and hence carotid artery stenoses are the most common pathologic changes of the carotid arteries. From the NASCET and ESCT results, it becomes obvious that an exact determination of the degree of CAS is pivotal for further management of the patient as a higher degree of stenosis may warrant an operative therapy. Typically, CAS was measured as the reduction of the diameter as done on a lateral DSA projection view. The NASCET measures carotid artery diameter at the site of stenosis and relates this diameter to the distal non-stenotic vessels segment (NASCET 1991), while the ECST measures stenosis of the remaining lumen at the carotid bifurcation in rela-

Fig. 27.4. a Coronal MIP view of a distal CAS that leads to a signifi cantly delayed fl ow b in the affected vessel and that is there- fore deemed hemodynamically signifi cant. Distal internal carotid artery stenoses in the petrous part of the ICA can be easily demonstrated in the MRA while CTA and DUS often are not able to depict these lesions due to the bony surroundings of the ICA. Figure reprinted with permission from: Michaely HJ, Herrmann KA, Kramer H, Laub G, Reiser MF, Schoenberg SO

“The signiﬁ cance of MR angiography for the diagnosis of carotid stenoses” Radiologe, 2004 Oct 44(10): 975-984 a

b

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c

b a

Fig. 27.5. a Long-segment, high-grade pseudoocclusion (small arrows) of the ICA seen in the DSA projection. Note the arte- facts caused by the patient’s teeth. b The remaining lumen can be easily seen (small arrows) in the 3D reconstruction of the high-resolution MRA. c In the time-resolved TREAT-MRA the stenotic vessel can again be clearly depicted with delayed inﬂ ow.

Figure reprinted with permission from: Michaely HJ, Herrmann KA, Kramer H, Laub G, Reiser MF, Schoenberg SO “The signiﬁ cance of MR angiography for the diagnosis of carotid stenoses” Radiologe, 2004 Oct 44(10): 975-984

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Fig. 27.6. a Coronal MIP view of a me- dium-grade left-sided CAS in a 52-year- old patient. To exactly assess the vessel lumen downstream b and at the level of c the stenosis, reconstructions perpen- dicular to the vessel axis as displayed on the MIP view are obtained c,e. On these cross-sectional perpendicular views the true lumen (arrows) downstream c and at the level of the stenosis e can be exactly calculated to ﬁ nally obtain the degree of area stenosis. Figure reprinted with permission from: Michaely HJ, Herrmann KA, Kramer H, Laub G, Reiser MF, Schoenberg SO “The sig- niﬁ cance of MR angiography for the diagnosis of carotid stenoses” Radiologe, 2004 Oct 44(10): 975-984

c b

a

e

d

tion to the initial, non-stenotic lumen (Staikov et al.

2000). A recent study comparing the results of CAS measured with CE-MRA according to the NASCET and the ECST method in comparison the conventional angiography as the gold standard found a signiﬁ cant higher sensitivity with the use of the NASCET criteria (U-King Im et al. 2004). It seems therefore appropriate to use the NASCET criteria when measuring the degree of stenosis on CE-MRA data. In a meta-analysis, the sensitivity and speciﬁ city of the MRA for the grading of relevant CAS were found to be 95% and 90%

(Nederkoorn et al. 2003), which was higher than the corresponding results for DUS or computed tomogra- phy angiography (CTA). However, the included studies used maximum intensity projections of the MRA that do not beneﬁ t from the three-dimensional character of the MRA. This may be one explanation for the often- cited fact that MRA tends to overestimate the degree

of stenosis. It seems more accurate to use the area of the stenosis instead of the diameter of the stenosis, a technique that proved extremely useful and precise for the renal arteries already (Michaely et al. 2005b).

Measuring the stenosis in the cross-sectional view also takes the excentric nature of atherosclerotic changes better into account than looking at the diameter of the stenosis only (Fig. 27.6). Simply using the source images or DSA-like projections (Nederkoorn et al.

2002) is already advantageous in terms of diagnostic accuracy (Anderson et al. 1994). In cases of compli- cated vascular anatomy, curved reformats are another possible solution for exact vessel diameter assessment (Gonschior et al. 2001).

Fibromuscular dysplasia (FMD) occurring in the supraaortic vessels in 30% of all patients with FMD poses a further diagnostic challenge to MRA (Slovut and Olin 2004). Particularly in patients

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with FMD, the higher spatial resolution associated with the application of parallel imaging will prove highly advantageous since the typical string-of-beads changes of the vessel wall are difﬁ cult to discern on standard resolution MRA images. However, there are no published data on the performance of CE-MRA with parallel acquisition techniques available at this point (Fig. 27.7-27.9).

27.4

Functional Characterization of Carotid Artery Disease

By adding just a few sequences the MRA of the carotid vessels can be enhanced by functional data. Primarily, MR phase-contrast ﬂ ow measurements yield valuable extra information about the hemodynamic impact of

a CAS (van Everdingen et al. 1997). In a study on 86 patients, the mean fl ow in the ICA was 206 ml/min with a maximal fl ow velocity of 37 cm/s, which was on average reduced to 143 ml/min and 23 cm/s in case of CAS. Even the entire fl ow pattern of all supraaortic vessels changes and can be used as an additional functional parameter in the comprehensive assessment of CAS (Cosottini et al. 2005). Applying parallel imaging alone or in combination with other acceleration algorithms such as k-t-BLAST (Tsao et al. 2003) to PC-MR fl ow measurements, acquisition time can be decreased by a factor of up to 3.5 (Beerbaum et al.

2003). Another – yet still experimental way – to char- acterize the fl ow in a stenotic ICA is the application of MRA sequences with a high temporal resolution. This approach does not allow measuring the fl ow volumes and velocities; however, it may allow detecting delayed fl ow in the stenosed vessel or even collateral vessels.

Adding T1-weighted spin-echo sequences post contrast, particularly in combination with custom-

Fig. 27.7. a Sagittal MIP view of the ICA of 43-year-old female patient with FMD: The characteristic string of beads appearance of the ICA can be well appreciated. b The volume-rendered reconstruc- tion of the MRA data of the same patient shows the above-mentioned alterations (open arrow) but also an affection of the contralateral side (solid arrow). Figure reprinted with permission from:

Michaely HJ, Herrmann KA, Kramer H, Laub G, Reiser MF, Schoenberg SO “The signiﬁ cance of MR angiography for the diagnosis of carotid stenoses” Radiologe, 2004 Oct 44(10): 975-984

b a

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Fig. 27.8. a 30-mm thick coronal MIP view of the MRA in a 19-year-old patient after motor vehicle accident. On the left side the ICA cannot be depicted any more correlating to a traumatic occlusion of the vessel (open arrows) b Equally, in the sagittal thick-MIP view the ICA cannot be depicted and reveals a sudden stop of enhancement at the bulbus (sequence parameters in Table 27.1)

b a

Fig. 27.9. a 30-mm-thick coronal MIP view of a 43-year-old male patient with the incidental ﬁ nding of an ICA aneurysm. Due to the tortuous course of the ICA as seen also on the contralateral side in the coronal view the allocation of the aneurysm to the ICA or ECA is demanding. b In the sagittal view a branch coming off the ECA can be seen (arrow), thereby allowing to allocate the aneurysm to the ICA (sequence parameters in Table 27.1).

a b

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ized coils, enables plaque or vessel wall imaging. This ﬁ eld of ongoing research has not reached clinical routine so far, mainly because the local, customized coils are not readily available yet. However, improved coil technology will foster the characterization of plaques by means of MRI in the near future (Hofmann et al.

2003; Yu et al. 2003; U-King-Im 2004). With the cur- rently available hardware solely the plaque surface can be safely assessed. A correlation between the irregularity of the plaque surface with neurological symptoms was proven (Troyer et al. 2002).

27.5

Comparison to Ultrasound and Intravascular Ultrasound

Apart from MRA, ultrasound and particularly DUS have evolved into the main tool for the detection and grading of CAS. While there is little doubt that DUS is cheaper and readily available, there is an ongoing debate about the value of DUS. DUS is a powerful tool in the hands of an experienced examiner, yet the reproducibility of the DUS results is often poor compared to MRA (Bucek et al. 2002; Mathiesen et al. 2000; Mikkonen et al. 1996). Studies comparing the diagnostic accuracy of MRA and DUS found MRA to be more accurate, but also suggested using DUS and MRA in combination for an even higher reliability (U-King-Im, Trivedi et al. 2004). This is particularly due to the fact that DUS is more sensitive for long stenoses, the so-called pseudo-occlusions (Nederkoorn et al. 2003; Wardlaw et al. 2001).

New technologies such as intravascular ultrasound (IVUS), which is already widely used in cardiac and abdominal intervention, could improve the diagnostic accuracy and reliability of ultrasound examinations; however, these techniques are too invasive and bear the risk of plaque rupture with the spread of emboli. As of now, there are no published data for the application of IVUS at the carotid arteries available.

27.6

Carotid MRA at 3 Tesla

The introduction of whole-body, 3-T MR imaging systems with the potential to generate higher signal-

to-noise ratio (SNR) opened up new horizons to improve image quality in MR imaging. The theo- retical SNR advantage at 3 T is especially relevant for MR angiographic applications, where a boost in SNR can signiﬁ cantly improve the image quality (Fig. 27.10–27.12).

The main advantage of MR imaging at 3 T is the SNR gain, which scales approximately linearly with the ﬁ eld strength B_o from 1.5 T to 3.0 T imaging (Willinek et al. 2003; Campeau et al. 2001). Also, since the longitudinal relaxation time (T1) of unenhanced blood increases with ﬁ eld strength, sensitivity to injected gadolinium agents for CE-MRA is relatively increased. The higher SNR can be used to reduce acquisition time, improve spatial resolution, or a combination of both (Willinek et al. 2003;

Campeau et al. 2001; Thulborn 1999). For CE-MRA

Fig. 27.10. 3D CE-MRA at 3.0 T in a normal subject. The whole 3D data set from aortic arch to intracranial vasculatures was acquired in a 20-s breath hold with a voxel dimension of 0.8×0.7×0.8 mm³, using parallel imaging with the GRAPPA algorithm and a high acceleration factor of R=4 (sequence parameters in Table 27.2)

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Fig. 27.11a,b. 3D CE-MRA at 3.0 T. Atherosclerosis in a 58-year-old patient seen a on the sagittal oblique thin MIP (10 mm) and b coronal volume rendering with severe stenosis of the left internal carotid artery (sequence parameters in Table 27.2)

Fig. 27.12a,b. Standardized coronal MIP a and coronal as well as oblique VR images b from high-spatial-resolution 3D CE-MRA at 3.0 T, showing an aneurysm at the supraclinoid portion of the left internal carotid artery (arrow) (sequence parameters in Table 27.2)

b a

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with short acquisition times and high spatial resolution, the inter-relation between SNR and the acquisition parameters has to be thoroughly regarded.

Because SNR is proportional to the voxel size and the square root of the imaging time, rapid acquisition of a high-spatial-resolution image causes loss of SNR in two ways. For implementations of fast imaging techniques such as time-resolved MRA with segmented k-space and parallel acquisition techniques, SNR can fall below the lower limit at 1.5 T while being still acceptable at 3.0 T.

However, other factors, particularly related to the increased specifi c absorption rate (SAR) at higher fi elds, need to be considered as well. To avoid SAR limitations, parallel imaging seems to be a suitable way. As the SAR scales with the square of the fi eld strength, the SAR at 3.0 T is four times the SAR at 1.5 T. Application of parallel imaging with an accel- eration factor of R reduces the SAR to 1/R of its original value (at least with respect to long-term averaging of SAR), implying that parallel imaging can signifi cantly reduced the SAR limitations. In practice this simplifi ed approach has to be adapted as the sampling density (and thus the short-term average of SAR during acquisition) and is not reduced with parallel imaging. Another approach to lower the SAR is to apply lower fl ip angles. Due to the longer T1 times at 3 T, even lower fl ip angles than conventionally used still succeed in suffi cient background suppression.

There are several studies evaluating carotid and intracranial 3D TOF-MRA at 3.0 T that have revealed signiﬁ cant improvement in image quality and diagnostic accuracy compared to the results of 3D TOF- MRA at 1.5 T (Willinek et al. 2003; Al-Kwiﬁ et al. 2002; Bernstein et al. 2001; Gibbs et al. 2004).

Published data on the application of 3D CE-MRA at 3.0 T are however very limited. Bernstein et al. have shown that TOF-MRA and CE-MRA of carotid and intracranial circulation at 3.0 T are feasible, and image quality was superior to that of a well-estab- lished 1.5 T protocol. At our institution, we are using a 3-T MR system to acquire CE-MRAs in different parts of body, including the carotids (Table 27.2), and the results are highly satisfactory to the point that the CE-MRA has been integrated into the clinical routine examinations. It seems likely that the introduction of more sensitive multi-element coil systems at 3.0 T will permit further improvement in imaging quality and performance, as it has at 1.5 T (Bodurka et al. 2004; de Zwart et al. 2004; King et al. 2001).

Table 27.2. Imaging parameters for carotid CE-MRA at 3.0 T (Siemens Magnetom TRIO)

Time-resolved MRA

High-resolution MRA

Repetition time (ms) 2.16 3.07

Echo time (ms) 0.95 1.28

Flip angle 16° 19°

Bandwidth (Hz/pixel) 1090 790 Field of view (mm²) 390×340 390×305

Matrix size 384×260 576×432

Pixel size (mm³) 1.3×1.0×3.0 0.7×0.9×0.8 Parallel imaging GRAPPA, R=3 GRAPPA, R=4 Partial Fourier 6/8 (phase-enc.

direction)

6/8 (phase-enc.

direction) Acquisition time 2 s/3D data set,

total 20 s

21 s

Slice thickness (mm) 3.0 0.8

Contrast 6 ml at 3 ml/s 25 ml at 1.2 ml/s k-space trajectories Cartesian Cartesian

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