H. Kramer, MD
Department of Clinical Radiology, University Hospitals – Gross hadern, Ludwig Maximilian University of Munich, Marchioni nistr. 15, 81377 Munich, Germany
C O N T E N T S
40.1 Introduction 451 40.2 Protocol Setup with
Parallel-Imaging Techniques 451 40.2.1 Standard MRI Systems 453 40.2.2 Whole-Body MRI Systems 454 40.3 Design of Screening Studies 454 40.4 Participants 454
40.5 Results 454 40.5.1 Pathologies 454 40.5.2 Examination Time 455
40.5.3 Image Quality: Detection of Pathologies 455 40.5.4 Image Quality:
Magnetic Resonance Angiography 457 40.6 Conclusion: Advantages of
Parallel Imaging 459 References 459
Cardiovascular Screening 40
Harald Kramer
40.1
Introduction
In most industrialized countries cardiovascular dis- ease still ranks number one in morbidity and mor- tality statistics. Until now, Doppler ultrasonography for the greater arterial vessels as well as ECG and ECG stress testing and blood parameters have been the only non-invasive modalities to evaluate these diseases. At the time these diseases get symptomatic, mostly an invasive treatment, such as dilatation and stenting of vessel segments, or even, surgery is neces- sary; therefore, an exam for early detection of initial,
asymptomatic changes seems reasonable (Goyen et al. 2002, 2003; Henschke and Yankelevitz 2000).
In the past few years many trials were started to use magnetic resonance imaging (MRI) as a screening modality in this fi eld of disease, but the standards for this type of exam were too low to fulfi l the require- ments of a fully diagnostic multi-organ evaluation.
Spatial and temporal resolution of MRI was too low and the examination time was too long to establish MRI as a new screening modality.
Recent technical developments in hardware and software, as well as the introduction of parallel im- aging, have changed this situation (Kramer et al.
2004). Magnetic resonance imaging has become the new standard of reference for the evaluation of global cardiac function and perfusion, and magnetic reso- nance angiography (MRA) has replaced most of the solely diagnostic digital subtraction angiography ex- ams. Because of the absence of ionizing radiation and nephrotoxic contrast agents (CA), and because of the increased spatial and temporal resolution, MRI now fulfi ls all conditions of a screening exam.
40.2
Protocol Setup with
Parallel-Imaging Techniques
A whole-body cardiovascular screening exam should include a complete state-of-the-art heart examina- tion including functional imaging of the left ventricle (Wintersperger et al. 2003), perfusion imaging of the left-ventricular myocardium as well as delayed contrast-enhanced (DCE) imaging for the detection of infarcted myocardium. It also should include MRA of the complete arterial vascular tree from the skull base down to the feet (Ruehm et al. 2001). In this protocol also imaging of lungs (Biederer et al. 2002), abdominal organs and brain including intracranial vessels is possible.
Protocol setup depends on which kind of MR sys- tem is used. When using a commonly available state- of-the-art MR system, which means a 1.5-T system with up to 8 receiver channels and table movement up to 150 cm, the complete exam has to be divided into two steps because of repositioning the patient from head fi rst to feet fi rst into the magnet. The fi rst part includes the complete cardiac exam as well as imaging of the lungs and brain and MRA of the su- pra-aortic vessels. The second part consists of im- aging of the abdominal organs and MRA from the diaphragm down to the feet.
When using a dedicated whole-body MR system, which means a 1.5-T system with up to 32 receiver channels and a large range of table movement of up to more than 200 cm, the patient has not to be repo- sitioned (Fig. 40.1). This is only possible due to the implementation of multi-channel MR systems and parallel-imaging techniques. Prior to the fi rst image acquisition, all necessary coils are placed on the pa- tient, which is only possible if enough receiver chan- nels are available. Parallel imaging is used in nearly every step of the exam. This reduces acquisition time while increasing the spatial and/or temporal resolu-
Fig. 40.1a,b. Protocol setup for cardio- vascular whole-body imaging. a Pro- tocol on a standard MR system. Dotted line stands for patient repositioning from head fi rst to feet fi rst to the mag- net. b Protocol on a dedicated whole- body MR system
b a
tion. In particular during the cardiac examination, ad- vantages of parallel acquisition techniques are in the foreground. The thorax has to be covered with mul- ti-element coils to be able to use parallel imaging in anteriorposterior and leftright direction; here, high temporal resolution in combination with short scan times, good signal intensity and spatial resolution are very important. This combination is only possible with parallel imaging, so a complete heart examination with functional and perfusion imaging as well as DCE can be performed within only three breathholds (cf.
Chaps. 35 and 36). Spatial resolution of whole-body MRA can be increased to 1×1×1 mm3 which has been previously only achieved by dedicated MRA exams of a single anatomic area. Quality of lung imaging could be improved due to the increase of signal intensity and decrease of blurring artefacts because of the shorter echo train as described in Chap. 20.
40.2.1
Standard MRI Systems
Standard MRI systems normally have a limited range of table movement. Due to this fact, the patient has to be repositioned during the exam or special table de- vices have to be used. If the exam is divided into two parts, during the fi rst part the patient is positioned head fi rst into the magnet, and the head coil, two body array coils, and the spine array are used. First axial and coronal images of the lungs before administration of
contrast agent (CA) are acquired in a breath-hold tech- nique with a single-shot half-Fourier turbo-spin-echo (HASTE) sequence. This is followed by functional and perfusion imaging of the left ventricular myocardium.
After that, MRA of the carotid arteries is performed with a test-bolus technique. Then T1- and T2-weighted images of the brain are acquired, again followed by post-CA imaging of the lungs with a 3D gradient-echo sequence. Thirteen to 17 min after the last CA injec- tion, delayed contrast-enhanced imaging of the left ventricle is performed. In the second part the patient is repositioned feet fi rst into the magnet and MRA of the arterial vascular tree from the diaphragm down to the feet is performed in four steps. At the end of the complete MR exam T1- and T2-weighted images of the abdominal organs are acquired. For this second part of the examination the peripheral-angio array coil, two body-array coils, and again the spine array as well as the large-fi eld-of-view adaptor, are used (Fig. 40.2a).
If using special table devices, such as the AngioSURF/
BodySURF system (MR-Innovations, Essen Germany), the patient is manually moved between the spine array and a fl exible body-array surface coil, which rests on a holder over the examined anatomic area. This holder follows the contours of the patient to minimize the distance between the coil and the patient. Here the patient has not to be repositioned from “head fi rst” to
“feet fi rst”; on the other hand, there is less fl exibility to use parallel imaging in the leftright direction because of the limitation to just two anterior and two posterior coil elements.
Fig. 40.2a,b. Coil setup for cardiovascular whole-body imaging. a Setup on a standard MRI system: left part for imaging of the upper body part, right part for imaging of the lower body part. b Setup on a dedicated whole-body MR system equipped with a matrix-coil system
b a
40.2.2
Whole-Body MRI Systems
When using a dedicated whole-body MR system with a range of table movement of more than 200 cm, the above-described repositioning of the patient during the exam becomes unnecessary. The sequence of the individual exams on these kinds of MRI systems differs from the common MRI systems. On the MRI system described here a special matrix-coil system is available (Fig. 40.2b). Now T1-weighted imaging of the brain and pre-CA HASTE imaging of the thorax and abdomen is performed in the beginning of the exam, followed by a time-of-fl ight (TOF) angiography of the intracranial vessels. Then functional and perfusion imaging of the heart is performed, followed by a modifi ed MRA pro- tocol. In this protocol, MRA of the supra-aortic vessels is directly followed by MRA of calf and feet with the same CA bolus. This is possible due to the large range of table movement. After the fi rst part of MRA, post-CA, imaging of thorax, abdomen and brain is performed followed by DCE imaging of the left ventricle. The exam is fi nished by the second part of the MRA protocol, imaging of abdominal aorta and thighs and post-CA imaging of the abdominal organs.
40.3
Design of Screening Studies
In our experience of more than 200 participants in a cardiovascular screening protocol using two different MR systems, cardiovascular whole-body MRI is feasi- ble at high quality and can be implemented to clinical routine. On the other hand, it can be further improved.
Due to recent technical developments in hardware and software and the implementation of parallel-im- aging techniques cardiovascular MR screening at a high quality and with a high spatial and temporal resolution is possible within a reasonable time. In our institution the fi rst exams were performed on a routinely used 1.5-T 8-channel MRI system (Mag- netom Sonata, Siemens, Erlangen, Germany). From participant number 43 onward, all individuals were examined on a 1.5-T 32-channel dedicated whole- body MRI system (Magnetom Avanto, Siemens, Er- langen, Germany). To verify diagnostic accuracy all exams were read by two experienced radiologists and pathological fi ndings as well as functional param- eters of the heart were reported. Image quality of
the whole-body MRA was judged by two different radiologists blinded to each other in terms of vessel conspicuity, artefacts and venous overlay on a three- point scale. In-room examination time was reported and compared for both MR systems. Reports of the MRI exam were compared with the fi ndings of the conventional exams in the fi rst 60 individuals, and further comparison is still ongoing.
In the literature some other setups for screening studies are described. In one study, whole-body car- diovascular MRI is included in a prospective trial in cooperation with a public health insurance company (Kramer et al. 2005).
40.4
Participants
Presented data show results from individuals par- ticipating in a company-based health care program.
All of them underwent routine yearly screening for cardiovascular diseases with conventional techniques (e.g. ultrasonography, ECG at rest and stress, echocar- diography, etc.). Mean age was 55r8 years, and all participants were referred from their company medi- cal offi cer. In the above-mentioned other studies participants were referred by their health-insurance company or were self-referring individuals.
40.5 Results
40.5.1 Pathologies
All recently published studies predominantly found manifestations of peripheral vascular disease and ischaemic heart disease, whereas relatively few ma- lignant diseases were found. In the fi rst 180 individu- als of our study we detected 19 cardiac pathologies, namely one primary unknown and one known myo- cardial infarction, 12 cardiac perfusion defects and 5 wall motion abnormalities. The participant with the known infarction suffered from diabetes mel- litus and also revealed a perfusion defect and a wall motion abnormality in the infarcted area (Fig. 40.3).
Most of these cardiac pathologies were not detected
with the conventional modalities such as ECG and echocardiography. The MRA showed 42 pathologies, namely one renal artery stenosis, one jugular glomus tumor, eight ectatic or aneurysmatically dilated aor- tic segments, 12 pathological changes in the carotid arteries including low- and high-grade stenosis and multiple periphery vessel occlusions (Fig. 40.4). The HASTE imaging of the thorax showed 16 pathologi- cal fi ndings, namely four nodules, infi ltrates in four and pathological enlarged lymph nodes in 8 patients (Table 40.1). Nodules were not detected in previously existing chest X-rays. The T1- and T2-weighted imag- ing of the brain showed microangiopathic changes
in multiple individuals, and one meningioma was detected. Abdominal imaging showed multiple cysts and haemangiomas in the liver and kidneys; however, one renal cell carcinoma was detected.
40.5.2
Examination Time
Even though shorter examination times have been re- ported in the literature, mean room time in our study was 102r23 min on the standard MRI system com- pared with 80r7 min on the whole-body MRI system (Fig. 40.5). These scan times seem to be very long, but they are needed to realize a high-quality whole- body examination. The MR exams of <60 min for a complete cardiovascular work-up suffer from reduced image quality. The described scan time reduction re- sults from several reasons. A major advantage of a whole-body MRI system is the absence of the need to reposition the patient. But another important point is the possibility to use parallel-acquisition techniques in combination with a matrix-coil system which make the protocol much more fl exible. There is no need for makeshift techniques such as the positioning of body- array coils next to each other to implement parallel imaging during the heart examination.
40.5.3
Image Quality: Detection of Pathologies
It is always diffi cult to judge the quality of an MRI examination. On one hand, one needs good spatial
Table 40.1. Main pathologies found by whole-body MRI
Cardiac pathologies 19 Others
WMA 5
PD 12
DCE 2
Whole-body MRA 42
RAS 1
JGT 1
CAST 12
AE/AA 8
HASTE lung imaging 16
Nodules 4
WMA wall-motion abnormality, PD perfusion defect, DCE de- layed contrast enhancement of left-ventricular myocardium, RAS renal-artery stenosis, JGT jugular glomus tumor, CAST ca- rotid artery stenosis, AE/AA aortic ectasia/aortic aneurysm Fig. 40.3. a Delayed contrast-enhanced
in an infarcted area of the septum and the anterior wall. b Corresponding
matched perfusion defect a b
Fig. 40.4a–c. Whole-body MRA from head to toe. a A 70%
common carotid artery stenosis. b Infrarenal aortic an- eurysm (diameter 4.5 cm). c Occlusion of anterior tibial artery and retrograde fi lling of the distal tibial artery.
c a
b
and in some cases temporal resolution to get a good exam, and on the other hand, one can have excel- lent resolution but limited image quality in terms of artefacts and low signal-to-noise-ratio (SNR).
Good image quality of MR exams is the combina- tion of good spatial and temporal resolution as well as high SNR. Additionally to that, there must not be any artefacts and, in contrast-enhanced examina- tions, there has to be good vessel conspicuity and no disturbing venous overlay. This kind of image quality has to be rated by readers. The SNR values can be calculated, the readers have to judge occur- ring artefacts and, in the special case of MRA, vessel conspicuity and venous overlay. In our study, the fi rst 60 examinations were read by two experienced radiologists blinded to each other, and inter-reader agreement was calculated by means of kappa val- ues. Detected pathologies were correlated to the primary existing conventional exams. Inter-reader agreement was good to excellent with kappa values (N) of 0.66 in the detection of cardiac pathologies, 0.75 in the detection of pathologies of the brain, the lungs or the abdominal organs and 0.91 in the de- tection of pathological changes of the vascular bed (Table 40.2). Nearly all previously known patholo- gies from the conventional exams were detected, exclusively intimal thickening of the carotid wall without lumen reduction which was not detected by
MRI but by Doppler ultrasonography. On the other hand, multiple additional pathological fi ndings were detected by MRI, e.g. peripheral artery occlusion, pulmonary nodules, wall motion abnormalities and one myocardial infarction (Table 40.3).
40.5.4
Image Quality: Magnetic Resonance Angiography
Of particular interest is the question of whether a whole-body MRA acquisition accelerated by parallel imaging keeps up with the high-quality standards of the present dedicated high-spatial-resolution MRA of a single vascular territory. In our study 90 whole- body MRA data sets were judged by two different radiologists blinded to each other. Forty-fi ve of these data sets were acquired on a standard MR system, and the other 45 on a dedicated whole-body MR sys- tem. Whole-body MRA was divided into 24 vessel segments; each segment was rated on a three-point scale in terms of vessel conspicuity, artefacts and ve- nous overlay. The MRA showed good quality on the standard MR system with good vessel conspicuity in nearly 75% of all vessel segments. Nearly 80% of cases were not affected by artefacts, >85% of all vessel segments had no venous overlay and <1% had major, non-diagnostic venous overlay. Most of the artefacts occurred because of the preliminary suboptimal coil setup. Moderate or poor vessel conspicuity occurred mostly in the calves because of fi eld-of-view limi- tations requiring a four-station angiography of the lower body part with only one CA bolus or because of mistakes in bolus timing. Inter-reader agreement showed good kappa values with 0.67 in terms of ves- sel conspicuity, 0.72 in terms of artefacts and 0.76 in terms of venous overlay. On the whole-body MR sys- tem, MRA image quality could be further increased.
Fig. 40.5a,b. Examination time on both MR systems:
102r23 min on the standard MRI system (a) compared with 80r7 min on the whole-body MRI system (b)
Table 40.2. Inter-reader agreement in terms of detection of pathological changes ranging from good to excellent
Region N Range
Angiography 0.905 0.642–1.000
Heart 0.662 0.542–0.789
Lungs 0.650 0.643–0.656
CNS and abdominal organs 0.787 0.486–1.000
CNS central nervous system a
b
Here >80% of all vessel segments showed good ves- sel conspicuity, >92% were not affected by artefacts and >95% showed no venous overlay. Inter-reader agreement could also be increased to excellent kappa values of 0.75 in terms of vessel conspicuity, 0.7 in terms of artefacts and 0.88 in terms of venous overlay (Table 40.4). Vessel conspicuity could be increased
due to the new MRA protocol with only four steps because of a fi eld of view of 500 mm and the altered chronological order of scanned regions. Artefacts could be reduced because of the special coil setup using the matrix coil system. Spatial resolution could be reduced to 1×1×1 mm3 in the carotid arteries and to <1.6×1×1.5 mm3 in all other regions.
Table 40.3. Comparison between results from conventional exams and whole-body MRI (initial results, fi rst 60 individuals).
(Modifi ed from Wintersperger et al. 2005 Standard exams Patholo-
gical fi ndings
MRI exam Detected in standard exams and in MRI
Additionally detected in MRI
Comments on additional fi ndings in MRI
ECG 0 Cardiac exam
US heart 4 Functional perfusion/
DCE
2/4 7 Wall motion abnormalities (5), DCE (2)
Chest radiograph 1 Lung: HASTE/VIBE 1/1 3 Pulmonary nodules, partially calcifi ed, confi rmed by HRCT (3) Abdomen T1, T2
US liver 0 Liver 0/0 6 Cysts (4), haemangiomas (2)
US gallbladder 5 Gallbladder 5/5 5
US kidneys 3 Kidneys 3/3/ 9 Cysts (9)
US spleen 0 Spleen 0/0 0
US pancreas 0 Pancreas 0/0 1 Cyst (1)
MRA
US carotid arteries 5 Carotid arteries 0/5 2 Abdominal origin (1), low-grade stenosis (1) US abdominal/renal
arteries
1 Abdominal/renal arteries
1/1 1 Low-grade stenosis (1)
Thigh, calf, feet 5 Occlusion of peripheral arteries (5)
Except for 5 cases of intimal thickening, all conventional diagnosed pathologies were detected by MRI without primary knowl- edge. On the other hand, multiple additional pathologies were detected by MRI.
Table 40.4. Image analysis of whole-body MRA in terms of vessel conspicuity, incidence of artefacts as well as venous overlay.
Whole-body MRA was divided into 24 vessel segments and judged on a three-point scale by two blinded radiologists. Because of increased image quality in all analysed categories inter-reader agreement increased as well.
Magnetom Sonata
Vessel conspicuity Percentage Absolute Artefacts Percentage Absolute Venous overlay Percentage Absolute
Good 73.2 1476 None 79.3 1598 None 86.5 1744
Moderate 22.5 454 Minor 18.4 370 Minor 13.0 261
Poor 4.3 86 Major 2.4 48 Major 0.6 11
N 0.671 N 0.716 N 0.756
Magnetom Avanto
Vessel conspicuity Percentage Absolute Artefacts Percentage Absolute Venous overlay Percentage Absolute
Good 80.6 1624 None 92.9 1873 None 95.6 1928
Moderate 15.3 309 Mild 4.4 89 Mild 4.4 88
Poor 4.1 83 Major 2.7 54 Major 0 0
N 0.745 N 0.697 N 0.881
40.6
Conclusion: Advantages of Parallel Imaging
Due to the introduction of parallel imaging as well as recent technical developments, such as matrix- coil systems and dedicated whole-body MRI systems, cardiovascular whole-body MRI screening seems to be feasible. The introduction of parallel imaging was a major breakthrough to overcome the limitations that had restricted MRI in this fi eld of imaging. One of the most important considerations for screening is to perform imaging with the best possible quality.
The introduction of parallel imaging made it possi- ble to combine multiple state-of-the-art exams into one comprehensive protocol within reasonable scan time. Different studies have shown that MR imaging detects the same or even more pathological changes than commonly used conventional techniques when screening for cardiovascular changes. Another very important point in screening is a good to excellent inter-reader agreement, which appears to be the case when using parallel imaging. Parallel acquisi- tion techniques have helped to reduce scan time, to increase spatial and/or temporal resolution. Reduced scan time and increased spatial resolution is very important in MRA. Short scan times are mandatory for whole-body angiography to keep venous overlay to a minimum in the last acquired vessel region. On the other hand, spatial resolution is very important, because for the purpose of cardiovascular screen- ing, asymptomatic early changes should be detected.
High temporal resolution is mandatory for functional cardiac imaging.
References
Biederer J, Busse I, Grimm J et al. (2002) Sensitivity of MRI in detecting alveolar infi ltrates: experimental studies. Rofo 174:1033–1039 [in German]
Goyen M, Quick HH, Debatin JF et al. (2002) Whole-body three- dimensional MR angiography with a rolling table platform:
initial clinical experience. Radiology 224:270–277
Goyen M, Herborn CU, Kroger K, Lauenstein TC, Debatin JF, Ruehm SG (2003) Detection of atherosclerosis: systemic imaging for systemic disease with whole-body three-di- mensional MR angiography. Initial experience. Radiology 227:277–282
Henschke CI, Yankelevitz DF (2000) Screening for lung cancer.
J Thorac Imaging 15:21–27
Kramer H, Schoenberg SO, Nikolaou K et al. (2004) Cardio- vascular whole body MRI with parallel imaging. Radiologe 44:835–843 [in German]
Kramer H, Schoenberg SO, Nikolaou K, Huber A, Struwe A, Winnik E, Wintersperger BJ, Dietrich O, Kiefer B, Reiser MF (2005) Cardiovascular screening with parallel imag- ing techniques and a whole-body MR imager. Radiology 236:300–310
Ruehm SG, Goyen M, Barkhausen J et al. (2001) Rapid magnet- ic resonance angiography for detection of atherosclerosis.
Lancet 357:1086–1091
Wintersperger BJ, Nikolaou K, Dietrich O et al. (2003) Single breath-hold real-time cine MR imaging: improved tem- poral resolution using generalized autocalibrating par- tially parallel acquisition (GRAPPA) algorithm. Eur Radiol 13:1931–1936
