Adenosine Stress Perfusion Cardiac MRI: Improving Image Quality Using a 32-Channel Surface Coil

doi:10.4236/ojmi.2011.12004

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Open Journal of Me di cal Imaging, 2011, 1, 21-25

doi:10.4236/ojmi.2011.12004 Published Online December 2011 (http://www.SciRP.org/journal/ojmi)

Adenosine Stress Perfusion Cardiac MRI: Improving

Image Quality Using a 32-Channel Surface Coil

Thomas R. Burchell, Redha Boubertakh, Saidi Mohiddin, Marc E. Miquel,

Mark A. Westwood, Anthony Mathur, L. Ceri Davies

The London Chest Hospital, London, UK

E-mail: [email protected]

Received September 14, 2011; revised November 8, 2011; accepted N ovember 18, 2011

Abstract

Purpose: Adenosine stress CMR is commonly used to assess myocardial ischaemia. Obtaining high quality

images requires maximising signal to noise ratio (SNR) over a large double-oblique field of view (FOV)

whilst minimising artefacts. A 32-channel surface coil may provide a higher SNR over a larger FOV com-

pared to standard coils, possibly leading to improved image quality. Materials and Methods: 50 adenosine

perfusion CMR scans were performed on a Philips Achieva CV 1.5T, with either a 5 or 32-channel coil (25

patients each) using standardised acquisition protocols. 3 short axis slices were acquired per cardiac cycle

and the resulting cine images were scored by two blinded CMR specialists on a quality scale of 1 to 5. Phan-

tom studies were performed using similar acquisition parameters and the SNR was calculated and compared

across a range of acceleration factors. Results: The mean patient age was 62 ± 11 years and 50% of patients

were male. The image quality scores were higher using the 32-channel coil (mean 3.8 ± 0.7 vs 3.2 ± 0.9 p =

0.002). The average phantom SNR was greater for the 32-element coil across the range of acceleration fac-

tors measured (103 vs 86 p = <0.001). Conclusions: The 32-channel coil produces significantly higher qual-

ity images and a higher SNR than the 5-channel coil in routine perfusion CMR.

Keywords: Cardiovascular MRI, Myocardial Perfusion, 32-Channel Coil, Adenosine

1. Introduction

Perfusion Imaging

First pass stress perfusion cardiac magnetic resonance

imaging (Stress Perfusion CMR) has been comprehen-

sively shown to be a safe, reliable and reproducible

method of identifying areas of inducible myocardial is-

chaemia without the use of ionising radiation [1-5]. It has

also been shown to predict cardiovascular morbidity and

mortality in patients with known coronary disease [6].

Acquiring high quality diagnostic images in a routine

clinical setting can be technically challenging due to the

limitations of the current hardware and acquisition pro-

tocols. These limitations can lead to a low in-plan e reso-

lution, resulting in dark rim Gibbs artefact [7], non-uni-

form sensitivity over the FOV, poor SNR and long ac-

quisition times.

Various techniques have been used in an attempt to

overcome these limitations:

A combination of highly accelerated parallel imaging

and temporal encoding (e.g. k-t SENSE) can be used to

increase spatial resolution [8,9] however, using temporal

information requires post processing and is prone to

movement artefacts, which cannot be identified until

after the perfusion scan has been completed. Higher field

strengths to increase contrast to noise ratio (CNR) have

shown promise using 3T but CNR inhomogeneities occur

across the left ventricle, particularly in the inferior wall

[10]. Also 3T magnets are not yet widely available in the

clinical setting.

Increasing the number of surface coils has the poten-

tial advantages of increasing the SNR [11-13] and pro-

vides a better field of view coverage, allowing higher

acceleration factors and hence higher spatial resolution

or faster acquisition times [14].

We assessed the difference in visual quality and SNR

between dedicated cardiac 32-element and standard 5-

element surface coils in routine clinical myocardial per-

fusion imaging using an identical acquisition proto col for

both coils.

T. R. BURCHELL ET AL.

2. Materials and Methods

All scans were performed using a Philips Achieva CV

1.5T MR scanner providing a gradient strength of 33

mT/m and maximum slew rate of 180 mT/m/ms (Philips

Medical Systems, Best, The Netherlands). The scanner

was equipped with 32 independent receiver channels. The

scans were performed with either a standard 5-element

(Philips) or 32-element phased-array surface coil (In vivo

Corporation, Gainesville, FL, USA) (Figure 1).

The 32-element coil consists of two parts, an anterior

(flexible) and posterior (fixed), each with 16 elements,

which are 9.1 × 10.6 cm in size and are overlapped to pre-

vent mutual inductance. Both arrays are arranged in a 4 ×

4 honeycomb pattern and measure 30.3 × 36.4 cm overall,

being widest in the left-right direction. The standard

5-element phased-array co il also consists of 2 parts, with

a fixed posterior 3-element and flexible anterior 2-

element coil. The anterior circular coils are 200 mm in

diameter and overlap, the posterior array is made of three

rectangular coils measuring 138 × 200 mm (Figure 1).

2.1. Declaration of Helsinki

This study was conducted in accordance with the prince-

ples of the “Declaration of Helsinki” (as amended in

Tokyo, Venice, Johannesburg, and Edinburgh) and within

the local laws and regulations.

2.2. Patient Studies

2.2.1. Patient s

50 consecutive patients (25 with each coil) through the

routine clinical service underwent perfusion CMR scans.

They were instructed to refrain from any caffeine con-

taining products for >1 2 hours prior to the scan. Both the

Figure 1. 5 and 32-element receiver coils. A schematic draw-

ing of the receiver coils. Pane A shows the posterior section

of the 32-element phased array coil, demonstrating the 16

hexagonal overlapping coil elements. The anterior part uses

a further 16 elements in the same configuration (not shown

for clarity). Pane B shows the anterior 2 circular elements

and the posterior 3 rectangular elements of the 5-element

coil.

acquisition protocols and the image manipulation/analyses

were identical for both surface coils).

2.2.2. Stress P rotocol

Intravenous aden osine was infused at 140 µg/Kg/min for

3 minutes with continuous heart rate and blood pressure

recording every minute. Following this, an intravenous

bolus of 0.05 mmol/kg of Gadoteric acid (Dotarem, Guer-

bet, France) was administered via an antecubital fossa

vein on the contralateral arm to the adenosine, using a

power injector (Spectris Solaris EP, Medrad, Indianola,

PA, USA) at a rate of 4 ml/sec with an immediate 0.9%

saline flush of 25 ml at a rate of 4 ml/sec.

3 short axis slices, each of 10 mm thickness, were ac-

quired per cardiac cycle, at the basal, mid papillary and

apical levels of the left ventricle, with the patient free-

breathing throughout the acquisition. We used a single

shot prospectively gated, balanced steady state free pre-

cession sequence (TR 2.6 ms TE 1.3 ms, Flip angle 50˚)

and typical voxel size of 2.8 × 2.9 × 10 mm with a ma-

trix size of 111 - 131 × 256 - 320, with a field of view

(FOV) between 343 mm and 410 mm to minimise alias-

ing artefacts. A saturation pre-pulse was applied prior to

data acquisition. A 90 degrees preparation pulse was

used with a 100 ms recovery time. This saturation pulse

was repeated before the acquisition of every slice within

a cardic cycle. Parallel imaging was used with a sensi-

tiveity encoding (SENSE) factor of 2.1 - 2.5.

2.2.3. Image Ma nipul a ti o n and Analysis

The 3 stress cine images were saved as lossless avi vid-

eos: Codec: Microsoft MPEG-4 V2, Resolution: 1250 ×

925, Colour Depth: 16 million, Frame rate: 13fps.

The videos were then reviewed independently, by two

experienced cardiac MRI specialists (who were blinded

to the choice of coil) in a randomised and anonymised

sequence, using Windows Media Player (Microsoft, Se-

attle, USA). Each video was assessed for image quality,

noise and artefact and given an overall visual quality

score using a 5 point scale, where 1 = Non Diagnostic; 2

= Poor; 3 = Adequate; 4 = Good; 5 = Excellent.

2.3. Phantom Study

Measurements were performed for both coils, using a

rectangular phantom filled with a CuSO4/NaCl solution

(770 mg CuSO4 + 2000 mg NaCl/1000 ml of water) and

with dimensions of 260 mm × 240 mm × 370 mm (L ×

W × H), in a sagittal oblique orientation, using a non

balanced gradient echo sequence, with the same scan

parameters used for human studies with the exception of

a saturation pulse. Reconstruction filters were disabled to

assess the non-smoothed reconstructed images. A range

T. R. BURCHELL ET AL.23

of SENSE factors were acquired: 1.0, 1.5, 2.0, 2.3, 2.5,

2.7, 3.0, 3.5 and 4.0, with the majority between 2 and 3

to mirror clinical practice. Measurements were repeated

100 times for each sequence to allow for a temporal

mean to be calculated. The noise standard deviation was

assessed on magnitude images using the multiple acqui-

sition method [13]. To compensate for the Rayleigh

noise distribution in modulus images, a correction factor

of 0.655 was used to correct the underestimated noise

standard deviation. The mean signal (MS) and the stan-

dard deviation (SD) were measured. From this data, the

signal to noise ratio (SNR) was calculated as previously

described [15]. A central circular region of interest,

measuring 110 mm, covering approximately 20% of the

total phantom area was used for analysis, in a location

similar to the heart size and position (see Figure 2).

2.4. Statistics

The image quality scores for the 5 and 32-channel coils

were compared using the Mann-Whitney test for both

observers individually and also with their scores combined.

Intra-observer variability was assessed by comparing the

different observer scores, using Bland Altman analysis.

Signal to noise values were compared using a Student’s

t-test. All p-values are two tailed. Statistical analysis was

performed using SPSS Version 17.0 (SPSS Inc, Chicago,

IL, USA).

3. Results

3.1. Patient Studies

Patient characteristics are listed in Table 1. The mean

Figure 2. Phantom measurements. Sagittal oblique phan-

tom cross section. The phantom’s central area (white cir-

cular ROI) is used for signal to noise and standard devia-

tion measurements for both 5 and 32 channel coils.

patient age was 62 ± 11 years (range 36 to 82 years), and 25

(50%) were male. There was an improvement in image

quality score using the 32-channel coil compared to the

5-channel coil for observer 1 (mean score 4.1 ± 0.7 vs. 3.5

± 1 p = 0.04), observer 2 (mean score 3.4 ± 0.7 vs. 3.0 ±

0.6 p = 0.02) and with both observers’ scores combined

(mean score 3.8 ± 0.7 vs. 3.2 ± 0.9 p = 0.002), which was

highly statistically significant. The mean difference in

scores between observer 1 and 2 was 0.6 ± 1.7 (Fig ure 3).

3.2. Phantom Study

Th e min imu m, max imu m an d mea n S NR v alue s fo r bo th

coils are listed in Table 2. The average SNR was greater

for the 32-element coil than the 5-element coil across the

entire range of acceleration factors measured from 1 to 4

(103 vs 86 p = <0.001) (Figure 4).

4. Discussion

There has been a recent and rapid increase in the demand

for myocardial perfusion assessment, especially for pla-

nning reperfusion strategies [16,17].

CMR perfusion imaging has been shown to be

non-inferior to SPECT in demonstrating myocardial is-

chaemia [18] and has the added advantages of providing

validated information about both left ventricular function

and viability as well as being free of ionising radiation.

CMR perfusion, however is still constrained by tech-

nical limitations. Each image is acquired over a relatively

short period of the cardiac cycle as a single shot, yielding

a low SNR, resulting in a relatively low resolutio n.

Ideally, a high quality perfusion study depends on a

number of factors, including; high image resolution, ade-

quate cardiac coverage, a high signal to noise ratio and

minimal artefact.

This study has demonstrated that the 32-channel coil

provides superior observed image quality for qualitative

myocardial perfusion scanning. This is due mainly to

increased SNR and the larger FOV coverage afforded by

this design of phased array coil.

The SNR was shown (in a phantom) to be significantly

higher for the 32-channel coil over a wide range of ac-

Table 1. Baseline characteristics of the study population.

Characteristics5-channel 32-channel p-value

Age 61.5 ± 10.4 62.3 ± 11.4 0.80

Gender 11 male (44%) 14 male (56%)0.57

Weight in kg 79.5 ± 13.9 72.2 ± 9.6 0.035

Peak HR

(during stress) 89.7 ± 20.9 92.0 ± 24.3 0.71

T. R. BURCHELL ET AL.

Table 2. Signal to noise ratios.

SNR 5-CH SNR-32-CH

SENSE Mean Max MinMean Max Min

1.0 140.4 217.0 96.0151.8 233.7109.6

1.5 112.4 176.7 75.7128.2 182.887.8

2.0 98.6 160.4 60.8108.1 171.171.7

2.3 87.1 131.1 54.9100.5 150.573.8

2.5 86.5 140.9 53.398.3 145.966.7

2.7 79.0 145.7 49.294.7 140.364.8

3.0 72.4 99.9 45.7 91.7 133.461.0

3.5 51.9 85.4 25.3 76.8 117.239.6

4.0 42.5 72.3 20.7 77.7 120.541.4

Signal to noise ratios for each coil over a range of acceleration factors.

Results are expressed as mean, maximum and minimum values within the

central region of interest for each coil and each acceleration factor. Note:

there is a gr eater number of measu remen ts between acc elerat ion facto rs 2 to

3 to mirror standard clinical practice.

celeration factors, most importantly in the range of

common clinical practice (R = 2 - 3).

This increased SNR should allow the use of higher

acceleration factors whilst minimising the noise degrada-

tion seen with the use of standard coils. Speeding up the

acquisition in this way would facilitate either an increase

in cardiac coverage (more slices) or a higher image

resolution, reducing dark rim artefact [9], which is an

area of potential further investigation.

Routine use of the 32-channel coil could provide im-

ages that are easier and faster to interpret by the physic-

cian and may improve workflow through the MRI de-

Figure 3. Intra-observer variability. Bland Altman plot sh-

owing variance from the mean for the 2 observer’s scores.

The 0.95 confidence intervals (±1.96 standard deviations)

are shown as dashed lines.

Figure 4. SNR maps. Signal to noise for the 5 channel coil

(top row) and 32 channel coil (bottom) for 3 SENSE accel-

eration factors. The phase encoding direction is AP.

partment. By extension of the increase in SNR and image

quality, they may prove to have a higher sensitivity and

specificity, however this is beyond the scope of this

study.

Hardware limitations:

The 32-channel coil howev er is not without limitations:

As it is still a new technology this coil is no t currently in

widespread use. It is considerably larger in all dimen-

sions than the standard coil and weighs approximately

1.2 kg more, leading to an effective reduction in bore

size and an increased sensation of confinement. The

subsequent increased coverage of the coil may however,

reduce the dependence on coil positioning, especially

when imaging in the double oblique plane.

Study Limi ta tions:

The study used unpaired patient groups, as it was an

observation of a real world clinical service. As a cones-

quence, there is a small difference in weights between

the groups. Although significantly in creased body weight

may impact on image quality, the difference in groups

was less than 10%. The phantom data provides a paired

comparison between the 2 coil types. The clinical data

was included as an indicator of the clinical utility of in-

creased image quality.

Image quality is a subjective measurement, being an

assessment of a combination of noise, artefact, resolution

and contrast, although in our observations the variation

between observer scores was within the 95% confidence

interval.

We have not indicated whether the improved image

quality translates into improved identification of perfu-

sion defects, which will require further investigation and

this is clearly a significant weakness. Assessment of test

sensitivity and specificity would require an invasive

coronary angiogram in all patients, which would not be

normally indicated in the presence of a normal functional

study.

T. R. BURCHELL ET AL.

In conclusion, the 32-channel phased array coil pro-

duces both a high er SNR and significantly higher quality

images than the standard 5-channel coil in routine stress

CMR despite using near identical acquisition protocols

for both coi l s.

5. References

[1] N. Al-Saadi, E. Nagel, M. Gross, et al., “Noninvasive

Detection of Myocardial Ischemia from Perfusion Re-

serve Based on Cardiovascular Magnetic Resonance,”

Circulation, Vol. 101, No. 12, 2000, pp. 1379-1383.

[2] T. Giang, D. Nanz, R. Coulden, et al., “Detection of

Coronary Artery Disease by Magnetic Resonance Myo-

cardial Perfusion Imaging with Various Contrast Medium

Doses: First European Multi-Centre Experience,” Euro-

pean Heart Journal, Vol. 25, No. 18, 2004, pp. 1657-

1665. doi:10.1016/j.ehj.2004.06.037

[3] E. Nagel, C. Klein, I. Paetsch, et al., “Magnetic Reso-

nance Perfusion Measurements for the Noninvasive De-

tection of Coronary Artery Disease,” Circulation, Vol.

108, No. 4, 2003, pp. 432-437.

doi:10.1161/01.CIR.0000080915.35024.A9

[4] J. Schwitter, D. Nanz, S. Kneifel, et al., “Assessment of

Myocardial Perfusion in Coronary Artery Disease by

Magnetic Resonance a Comparison with Positron Emis-

sion Tomography and Coronary Angiography,” Circula-

tion, Vol. 103, No. 18, 2001, pp. 2230-2235.

[5] N. Wilke, M. Jerosch-Herold, A. Zenovich and A. Still-

man, “Magnetic Resonance First-Pass Myocardial Perfu-

sion Imaging: Clinical Validation and Future Applica-

tions,” Journal of Magnetic Resonance Imaging, Vol. 10,

No. 5, 1999, pp. 676-685.

doi:10.1002/(SICI)1522-2586(199911)10:5<676::AID-J

MRI10>3.0.CO;2-L

[6] C. Jahnke, E. Nagel, R. Gebker, et al., “Prognostic Value

of Cardiac Magnetic Resonance Stress Tests: Adenosine

Stress Perfusion and Dobutamine Stress Wall Motion

Imaging,” Circulation, Vol. 115, No. 13, 2007, pp. 1769-

1776. doi:10.1161/CIRCULATIONAHA.106.652016

[7] E. Di Bella, D. Parker and A. Sinusas, “On the Dark Rim

Artifact in Dynamic Contrast-Enhanced MRI Myocardial

Perfusion Studies,” Magnetic Resonance in Medicine,

Vol. 54, No. 5, 2005, pp. 1295-1299.

[8] T. Niendorf and D. Sodickson, “Highly Accelerated Car-

diovascular MR Imaging Using Many Channel Technol-

ogy: Concepts and Clinical Applications,” European Ra-

diology, Vol. 18, No. 1, 2008, pp. 87-102.

doi:10.1007/s00330-007-0692-0

[9] S. Plein, S. Kozerke, D. Suerder, et al., “High Spatial

Resolution Myocardial Perfusion Cardiac Magnetic

Resonance for the Detection of Coronary Artery Dis-

ease,” European Heart Journal, Vol. 29, No. 17, 2008,

pp. 2148- 2155.

[10] P. Araoz, J. Glockner, K. McGee, et al., “3 Tesla MR

Imaging Provides Improved Contrast in First-Pass Myo-

cardial Perfusion Imaging over a Range of Gadolinium

Doses,” Journal of Cardiovascular Magnetic Resonance,

Vol. 7, No. 3, 2005, pp. 559-564.

doi:10.1081/JCMR-200060622

[11] C. J. Hardy, H. E. Cline, R. O. Giaquinto, et al., “32-

Element Receiver-Coil Array for Cardiac Imaging,” Mag-

netic Resonance in Medicine, Vol. 55, No. 5, 2006, pp.

1142-1149. doi:10.1002/mrm.20870

[12] Y. Zhu, C. J. Hardy, et al., “Highly Parallel Volumetric

Imaging with a 32-Element RF Coil Array,” Magnetic

Resonance in Medicine, Vol. 52, No. 4, 2004, pp. 869-

877. doi:10.1002/mrm.20209

[13] S. Reeder, B. Wintersperger, O. Dietrich, et al., “Practical

Approaches to the Evaluation of Signal-to-Noise Ratio

Performance with Parallel Imaging: Application with

Cardiac Imaging and a 32-Channel Cardiac Coil,” Mag-

netic Resonance in Medicine, Vol. 54, No. 3, 2005, pp.

748-754. doi:10.1002/mrm.20636

[14] M. Ohliger and D. Sodickson, “An Introduction to Coil

Array Design for Parallel MRI,” NMR in Biomedicine,

Vol. 19, No. 3, 2006, pp. 300-315. doi:10.1002/nbm.1046

[15] M. Fenchel, V. Deshpande, K. Nael, et al., “Cardiac Cine

Imaging at 3 Tesla: Initial Experience With a 32-Element

Body-Array Coil,” Investigative Radiology, Vol. 41, No.

8, 2006, pp. 601-608.

doi:10.1097/01.rli.0000223896.70095.49

[16] L. Shaw, D. Berman, D. Maron, et al., “Optimal Medical

Therapy with or without Percutaneous Coronary Inter-

vention to Reduce Ischemic Burden: Results from the

Clinical Outcomes Utilizing Revascularization and Ag-

gressive Drug Evaluation (COURAGE) Trial Nuclear

substudy,” Circulation, Vol. 117, No. 10, 2008, pp. 1283-

1291. doi:10.1161/CIRCULATIONAHA.107.743963

[17] P. Tonino, B. De Bruyne, N. Pijls, et al., “Fractional

Flow Reserve versus Angiography for Guiding Percuta-

neous Coronary Intervention,” The New England Journal

of Medicine, Vol. 360, No. 3, 2009, pp. 213-224.

doi:10.1056/NEJMoa0807611

[18] J Schwitter, C. Wacker, A. van Rossum, et al., “MR-

IMPACT: Comparison of Perfusion-Cardiac Magnetic

Resonance with Single-Photon Emission Computed To-

mography for the Detection of Coronary Artery Disease

in a Multicentre, Multivendor, Randomized Trial,” Euro-

pean Heart Journal, Vol. 29, No. 4, 2008, pp. 480-489.

doi:10.1093/eurheartj/ehm617