On the impact of slice profile and thickness definition across vendors in 2D bSSFP on SNR and T1-mapping in cardiac MRI
Jouke Smink1, Guillaume Gilbert2, Marc Kouwenhoven1, and Johan S van den Brink3

1MR Clinical Science, Philips, Best, Netherlands, 2MR Clinical Science, Philips, Montréal, QC, Canada, 3MR Clinical Excellence, Philips, Best, Netherlands


The actual slice thickness and slice profile in 2D imaging are often not taken into account when comparing SNR from different platforms. It can also have an impact in quantitative imaging such as T1-mapping. Inspired by an earlier study, we compared two definitions of slice thickness in 2D bSSFP(the workhorse in CMR): full width at 50% (FW50) and full width at 70% of maximum (FW70). The FW70 pulse definition leads to 30% thicker slices, 9-30% more SNR and it is more vulnerable to partial volume effects. These effects needs to be taken into account when comparing scans from different platforms in multi-center trials.


Signal-to-noise ratio is an important parameter in MRI as it relates to the required scan time and affects reliability of derived quantitative parameters such as T1. The signal S, the voxel volume and the square root of total scan time times scan efficiency are all proportional to the SNR of a generic pulse sequence. With S being dependent on sequence timing, flip angle, proton density and relaxation times. A usually ignored effect is the shape of the voxel. The use of short RF pulses can compromised this shape. A recent study on measuring slice profiles across multiple platforms (>120 1.5T systems from 4manufacturers) using the ACR phantom showed an increase in actual slice thickness in excess of 30% in some cases, which suggests an important impact on the measured SNR of these sequences . Accordingly and as an example, we reproduced the slice profiles for the two extreme cases from this study and used the corresponding RF pulses to explore the implications in clinical sequences like 2D bSSFP as this is a workhorse in cardiac MR imaging being used for cine, late enhancement, T1-mapping and perfusion.


The most common definition of slice thickness is “full width at half maximum” in which the defined slice thickness corresponds to 50% of the maximum signal. Alternatively, in filter techniques the bandwidth is defined as 3dB of the maximum signal, corresponding to a slice thickness at 70% of the maximum signal. A previous study suggests that some vendors may define slice thickness according to the second method. Accordingly, we performed tests using a standard RF pulse respecting the first definition (FW50) and implemented a Gaussian pulse with σ=0.45 and time-bandwidth factor of 1.6 to mimic the alternative definition(FW70). We used a 3T Philips Ingenia system with standard dStream Head Coil for phantom experiments and anterior/posterior body coil combination for volunteer scanning in this study. To determine slice thickness we used the vendor supplied 5 liter bottle (T1/T2=200/100 ms) and for T1-mapping the Eurospin II phantom . For slice profile and SNR measurements, we used bSSFP with 20 mm slice thickness, 0.5x1.0 mm voxel size, TE/TR=3.0/6.0 ms and 45 degrees flip angle. For T1-mapping we used the consensus recommended 5s(3s)3s MOLLI sequence with bSSFP readout, TE/TR=1.0/2.2 ms and 20 degrees flip angle, in-vivo with 8 mm slice thickness but in phantom with 20 mm slice thickness allowing to use the T1-phantom as macroscopic representation of partial volume effects. As an example of clinical scanning we used a routinely used breath hold cine bSSFP with 8 mm slice thickness, TE/TR=1.48/2.9 ms and 45degrees flip angle. To assess the slice thickness and slice profile the readout gradient was aligned with the slice selection axis, using the scanner's research mode. SNR was determined according to the NEMA standard by acquiring a second dynamic scan with all gradients and RF pulses switched off.


Measuring slice profiles in bSSFP confirmed correct implementation of both slice thickness definitions (fig 1). The FW70 pulse definition matched a 30% larger slice thickness according to the FW50 definition and the area under the curve was 25% larger. The phantom SNR measurement showed a 29% increase in SNR for the FW70 pulse definition (fig 2) while in the clinical cine scan the increase was 8% for blood and 19% for myocardium (fig 3). T1-mapping without partial volume resulted in almost identical T1-values for both pulses. However, in case of intentional partial volume, the T1-values differed up to 21% between both pulses (fig 4).


For sharp slice profiles, the difference between FW50 and FW70 definition is only marginal. But for Gaussian pulses, we demonstrated significant differences which are not as manifest as in-plane resolution. Poor slice profiles are often avoided because of interference with other slices in interleaved slice scans. However bSSFP requires short TR and thus slices to be acquired non-interleaved and thus can be a motivation to accept a poor slice profile. Our results show no differences in T1 in homogeneous tissue, but for focal lesions, partial volume effects between the lesion and healthy myocardium could affect the apparent size and contrast of the lesion.


Our study confirms that an alternative slice thickness definition leads to an artificially increased SNR and affects partial volume effects in quantitative mapping techniques. The exact definition of slice thickness in 2D imaging is therefore important and deserves more attention when comparing scans from different platforms. Taking the differences in slice profile definition between vendors into account will lead to truly homogeneous protocols for multi-center trials.


No acknowledgement found.


1. Bernstein MA, King KF, Zhou ZJ. Handbook of MRI Pulse Sequences

2. Reeder SB, Herzka DA, McVeigh ER. Signal-to-Noise Ratio behavior of Steady-State Free Precession. MRM 52: 123-130 (2004)

3. NessAiver M. Measuring slice profiles across the industry with the ACR Phantom. ISMRM 2017: 4339

4. http://www.diagnosticsonar.com: Eurospin II Test System

5. Kellman P, Hansen MS. T1-mapping in the heart: accuracy and precision. JCMR 2014 16:26. http://www.nema.org: NEMA MS 1-2008 (R2014)


Slice profiles measured in a 5L phantom (T1/T2=200/100 ms) by aligning the readout gradient with the slice selection gradient. The voxel size of 0.5 x 1.0 mm ensured at least 50 sample points to cover the slice profile. The FW50 and FW70 pulse definition have a width of 20 mm at 50% respectively 70% of the maximum signal. Note that the FW70 pulse definition corresponds to a slice thickness of 26 mm at full width half maximum. The area under the curves was65093 and 81183 for FW50 and FW70 respectively, an increase of 25% for FW70 compared to FW50.

SNR measured in a 5L phantom from the average signal in a circular ROI and the standard deviation in a large square ROI in a noise image obtained by taking the same scan parameters but switching off all gradients and RF pulses and looking at the real data. All filters and gradient linearity corrections were switched off. The SNR[FW50] = 166 and the SNR[FW70] = 215 which means an increase of 29% for the FW70. This is in agreement with the higher effective slice thickness of the FW70 pulse.

SNR measured in a breath hold cine bSSFP scan: SNR[FW50,blood] = 72, SNR[FW70,blood] = 77, SNR[FW50,myocardium] = 19, SNR[FW70,myocardium] =23. The FW70 pulse definition leads to an artificial SNR increase of 8% - 19%, dependent on the relaxation times of the tissue.

T1-maps were acquired five times for both RF pulses. In the orientation orthogonal to the bottles, T1's were almost identical. However, when measured parallel to the bottles, the slice contains the bottles as well as its surrounding (T1=549 ms) leading to longer values for bottles with shorter T1 or shorter values for bottles with a longer T1. This effect is even stronger for the FW70 pulse which is thicker compared to the FW50 pulse, e.g. bottle 3 with T1=294 ms is 300 ms with theFW50 pulse and 362 ms with the FW70 pulse (21% longer).

T1-maps were acquired in two healthy male volunteers (73 and 45 years old). For the second volunteer two maps were acquired to study reproducibility. T1's in blood and myocardium were well reproducible (difference <3 ms for myocardium and <46ms for blood). We observed minor differences in T1-values between the FW50 and FW70 pulses since T1 remains mostly homogeneous in the myocardium and blood. Partial volume effects could play a more important role in the case of a focal lesions, where partial volume effects between the lesion and healthy myocardium could impact the apparent size and contrast of the lesion.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)