ADC measurements with EPI scans - netkey.at

ADC measurements with EPI scans Poster No.:

C-0724

Congress:

ECR 2013

Type:

Scientific Exhibit

Authors:

C. Biagini , C. Biagini , L. Natale , M. Betti ; Sesto Fiorentino (FI)/

1

2

2

2 1

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IT, Sesto Fiorentino/IT Keywords:

Technical aspects, Equipment, Diagnostic procedure, MRDiffusion/Perfusion, MR, Oncology, MR physics, Abdomen, Cancer

DOI:

10.1594/ecr2013/C-0724

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Purpose

Diffusion-weighted imaging (DWI) has become an indispensable tool in clinical routine, both in neuro as in body MR imaging [1-3]. DWI has the ability to extract information about tissue micro-structure from MR signal intensities, giving the Radiologist useful details to assess the presence or absence of abnormalities, and even their nature. Its application ranges from treatment monitoring in different physiological regions [2,4-5] to functional characterization to tissue geometry as in Diffusion Tensor Imaging [2]. A particular interest is devoted to quantitative approaches, i.e., the measurement of the water diffusion coefficient of a particular region, or lesion, or tissue and its direct comparison with some other "reference" value obtained from other studies. Even more intriguing is the possibility to separate different contributions to the diffusion coefficient itself, as for the so-called pseudo-diffusion and true diffusion [1]. In particular, very recently [6], it has been reported the ability of low b-valueDWIof the prostate and rectal wall to identify and characterize heteroplasic lesions. Analogously, different quantitative approaches have been proposed in order to perform reliable and repeatable interpretations of diffusion data [7-9]. Evenin the case of simple mean diffusivity calculations [10], care must be paid both to the measurement and data analysis procedures. All those measurements are certainly affected by fluctuations due to patients variability (collaboration and patho-physiological conditions) [11-15], but should not be affected by the equipment: as a consequence, we will focus our attention on the problem of reliability of the quantitative measurement of the ADC using Spin-Echo Diffusion-Weighted Echo-Planar Imaging (SE-DW-EPI) It is fundamental to understand deeply the mechanisms which generate the signal and, more importantly, the technical and technological aspects of its acquisition. Despite almost all clinical studies assume the measurements reliable and repeatable and the systematic errors practically negligible (we intend for "systematic error" only eliminable errors due, for example, to malfunctioning of the equipment [16]),an initial validation of the diffusion measurements is necessary. The study of the effect of imaging gradients eddy currents on the diffusion gradients [12] or the study of the ADC only as a function of the spatial direction of application of diffusion gradients for a single b-value, [1] are still not enough. As it is well known, the imaging technique used for ADC measurements in vivo, i.e. the socalled Echo Planar Imaging (EPI), represents the quickest imaging technique, allowing the acquisition of a single bidimensional image in less than 50 ms, even faster than the last generation CT scanner commercially available [17]. Such a high imaging rate has a heavy drawback in the fact that EPI is probably the most unstable and delicate of all the MRI pulse sequences, being prone to a number of artifacts, due both to the

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acquisition technique itself and also to field or gradient inhomogeneities [15,17]. One of those artifacts is the huge sensibility to the chemical shift: in EPI, the water and fat protons spatial separation can be as large as half the Field of View (FoV) along the phase encoding direction [17]. Therefore, it is mandatory to suppress the lipid signal in vivo using one of the available techniques; spectrally-selective inversion pulses, both adiabatic (as in SPectrally Adiabatic Inversion Recovery, SPAIR) or not (as in the conventional Fat Saturation, FatSat), non spectrally-selective inversion pulses (Inversion Recovery, IR) or selective excitation of water protons with composite pulses (Water Excitation, WE). It is also possible to apply two pulses at the same time, as IR+WE or IR+FatSat to improve the fat suppression effect. Each of those techniques uses RF pulses with different duration, envelope and intensity. Actually, all of them found their application domain in different anatomic districts: the choice is usually dictated by technical reasons as the size of the FoV or the importance of field inhomogeneities [18]. Our motivation stems from the necessity of reliability of ADC measurements as a function of the fat suppression technique, i.e., to estimate the RF pulse effect on the final measurement. Our interest has been focused not only on the absolute value of the ADC but also on the distortion of the signal intensity curve as a function of the b value, due to the presence of the RF suppression pulse.

Methods and Materials Measurements have been realized on the 3T MRI equipment (Siemens Verio) of the Centro Oncologico Fiorentino (Sesto Fiorentino, Florence, Italy) with a 12-channel Head Matrix coil, using two different phantoms: the first (phantom A) is a Plexiglas cylinder filled with an aqueous solution of Agarose (1.2% weight fraction) with a diffusion coefficient -3

2

D#1.95*10 mm /s at 20 Celsius degrees, realized by the Medical Physics Unit of the Azienda Ospedaliera Universitaria di Careggi (AOUC), in Florence (Italy) [14]; to take into account thermal effects due to RF deposition, a alcohol thermometer was inserted in the cylinder, in close contact with the solution. The other phantom (phantom B) is the -3

2

-3

mm /s at

1900 ml bottle of the standard phantom set of the equipment, with D#2.0*10 mm /s at 20 Celsius degree. In a second phase we repeated another run of measurements on the 1.5T MRI equipment (Philips Intera-Achieva, versione 12.1) of the Hospital "Misericordia e Dolce" in Prato (Italy), with a 6-channel Sense-Head coil using a 3000 ml phantom (phantom C) of the standard phantom set of the equipment, with D#2.0*10 20 Celsius degree.

2

Each phantom has been kept inside the magnet room at all times, in order to minimize its movement and to ensure thermal equilibrium with the environment. Having verified that the temperature of the solution in the phantom A did not show any variation before,

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during and after the measurement session, it has been considered sufficient to record the magnet room temperature in the next experiments, avoiding the use of the internal thermometer. Measurements have been realized at the beginning and/or at the end of an examination session, in order to average over the stress of the acquisition and reconstruction electronics; each scan has been repeated twice to estimate reliably the SNR, even in the presence of phased-array coils and parallel imaging techniques [19-23]; the order of the execution of the pulse sequences has been inverted in the second acquisition to average on the possible heating of the phantom solution (even if the Specific Absorption Rate (SAR) of SE- DW-EPI scans is close to zero) and of the acquisition electronics; care has been paid in the phantom positioning in order to repeat the sequences in the same geometry during different sessions; after positioning at the isocenter, the phantom has been left to rest for about twenty minutes before starting the acquisition to eliminate every possible convection motion of the solution; a typical example of measurement sessionis represented by the following temporal list of pulse sequences: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

SE-DW-EPI (no saturation) SE-DW-EPI (SPAIR) SE-DW-EPI (FatSat) SE-DW-EPI (WE) SE-DW-EPI (STIR) SE-DW-EPI (STIR) "bis" SE-DW-EPI (WE) "bis" SE-DW-EPI (FatSat) "bis" SE-DW-EPI (SPAIR) "bis" SE-DW-EPI (no saturation) "bis".

All acquisitions did not show variations during every measurement session, demonstrating the high stability of both MRI equipments. Phantom A 2

Initially, the same DW-EPI sequence with two b-values (0, 1000) s/mm has been acquired five times, with a perfect coincidence of all acquisition parameters but the fat suppression technique: one without RF suppression pulse (in the phantom there are no lipids), two with spectrally selective fat suppression pulse (FatSat, SPAIR), one with a water excitation composite pulse (WE) and the last with a non-spectrally selective fat suppression pulse (IR, TI=220 ms). The common scan parameters were: b=0, 1000; FoV=200 mm; Read Matrix=128; Phase Matrix=128; Phase partial Fourier=7/8; slice thickness/gap=9.0/2.7 mm; number of slices=5; TR=2000 ms; TE=90ms; EPI factor=100; 4 averages; bandwidth=1428Hz/ pixel; iPAT acceleration factor=2; Total scan duration=40". The diffusion gradients have been applied consecutively on the three orthogonal encoding directions (phase encoding

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axis, read encoding axis, slice selection axis) and an isotropic Trace image has been extracted [17,24-25]. The temperature of the phantom solution has been recorded before and after each sequence, without noticing any change due to RF power deposition. However, varying in a controlled way the environmental conditions of the magnet room, we acquired the ADC values in a range between 18.0 and 23.0 Celsius degree: this has allowed us to evaluate the temperature dependence of the ADC, corresponding to a variation of (2.42±0.06)% per degree around 20 Celsius degree in very good agreement with Ref. [14]: Using this result, we renormalized all data to the temperature of 20.0 Celsius degree, taken as the reference temperature in what follows. In a second run, we analyzed the importance of SNR on a direct measurement of ADC: 2

a multi-b sequence has been acquired, with nominal b in the range (0,3000) s/mm , 2

with steps of 300 s/mm ; again, this series has been repeated four times, using the four different fat suppression techniques described above. Common parameters were: b=(0, 300, 600, 900, 1200, 1500, 1800, 2100, 2400, 3000); FoV= 200 mm; Read matrix 128; Phase matrix 64; Phase partial Fourier=7/8; slice thickness/gap=9.0/2.7 mm; number of slices=5; TR=2600 ms; TE=116ms; EPI factor=64; 4 averages; bandwidth=1446Hz/pixel; iPAT acceleration factor=2; Scan total duration=2'04". The diffusion gradients have been applied only along the Read direction, after a careful inspection of its greater stability among the various encoding axis on the 3T MRI equipment (to be published). Phantom B The experiment on this phantom has been motivated by the ADC measurement instability at low b-values, as in Ref. [14]: the ADC calculation depends not only on the signal intensity at high b, but also on its value at low b, as in the following well known relation: -(b-b')D

Sb=Sb' e

, (1)

where Sb is the signal intensity in a voxel o in a Region of Interest (ROI) of the b-weighted image, Sb' the signal intensity on the same voxel or ROI in the b'-weighted image and D the diffusion coefficient or ADC. In Ref. [14], for example, the signal intensity at low b cannot be considered a reliable parameter. As an example, we reported in Fig. 1 a sketch of the behavior of the logarithmic intensity as a function of b, normalized at b=0,in which the deviation from linearity at low b has been magnified for clarity. The correct value of the diffusion measurements must fall in the linear region of the curve in order to avoid the artifact due to the non linearity which in vitro must be interpreted as a gradient diffusion disturbance, resulting in a distortion of the b-values which differ from those chosen by the user [12-14]: in fact, the phantom solution has a single diffusion coefficient D and the logarithmic intensity is a linear function of b; in other words, the signal attenuation has a mono-exponential behavior. Therefore, both b and b' in equation (1) must fall in the linear region.

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2

As a consequence, we have chosen 3 b values (0, 500, 1000) s/mm and manually calculated the ADC comparing all values, (0,500), (0,1000) and (500,1000), as well as with the value automatically returned by the equipment. In this run of measurements, besides to the non saturated scan, we have used all possible fat saturation techniques offered by our equipment, i.e., FatSat, SPAIR, WE, IR, IR+WE and finally IR+FatSat. Common parameters were: b=(0, 500, 1000); FoV 200 mm; Read matrix 100; Phase matrix 100; Phase partial Fourier=7/8; slice thickness/gap 9.0/2.7 mm; number of slices 5; TR=2000 ms; TE=90ms; EPI factor=100; 4 averages; bandwidth=1428Hz/pixel; iPAT acceleration factor=2; Total scan duration=1'04". Phantom C The motivation of this run of measurements has been the necessity to verify, on a different equipment with a different field intensity, the deviations observed during the previous runs on the 3T equipment. For technical reasons, the scan has been modified: being interested mainly in the deviation among manually and automatically calculated ADC values, we restricted 2

ourselves to only two b values (0,1000) s/mm and to three fat saturation techniques (FatSat, SPAIR IR); common parameters were: b=(0, 1000); FoV= 250 mm; Read matrix 104; Phase matrix 77; Half Fourier=no; slice thickness/gap=3.0/0.3 mm; number of slices=40; TR=[5544 (no saturation); 5828 (FatSat); 6378 (IR); 7262 (SPAIR)] ms; TE=87ms; EPI factor=77; 4 averages; bandwidth=2143.6Hz/pixel; SENSE acceleration factor=1; Total scan duration=[2'02" (no saturation); 2'08" (FatSat); 2'20" (IR); 2'39" (SPAIR)]. Obviously, the inversion time in the IR sequence has been taken as 180 ms [17]. Post-Processing All images has been analyzed as follows: on phantoms A and B, a ROI containing at least the 80% of the image has been drawn on the third image of the first sequence; the signal intensity has been recorded; copying the ROI on the same image of the other sequences, we have been able to evaluate the ADC values per each fat suppression technique and for each b value; the same ROI has been copied also on the automated ADC map returned by the equipment and the ADC value has been recorded; on phantom C, the same procedure has been repeated on the image 21. After temperature renormalization, we have taken the average value of the ADC for comparisons.

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Images for this section:

Fig. 1: A schematic sketch of the logarithmic signal intensity, normalized at the intensity at b=0, as a function of b. For high values of b (larger than 250-300 s/mm2), the behavior is almost perfectly linear; this is the correct range in which one can reliably evaluate the ADC. Moving towards lower b-values, the behavior deviates from linearity: till now, this deviation has been associated only with cross-term due to the imaging gradients or to gradients non-linearity. No association has never been proposed with RF pulses. In this range, the value of the ADC is always lower than the correct one, and even a linear fit of all points returns an uncorrect answer.

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Results

First of all, it is worth to stress that all measurements have been extremely reproducible on both equipments and for all fat suppression techniques, giving raise to very small standard deviations, of the order of 0.7%, even after temperature renormalization which could introduce by itself a further instability in the data. Phantom A The results in the first run of measurements on phantom A are reported in Fig. 2. The error bars represent three standard deviations: the ADC values obtained with the FatSat, Spair and WE scans are consistent with each other and with non saturated one, apart for a slight overestimation of the ADC with respect to this scan. The IR scan returns an underestimation of the ADC, and the value is inconsistent with all other saturated scans on the basis on the Student's test. The results of the second run of measurements on the same phantom are summarized in 2

Fig. 3, we reported the ADC values normalized at the value obtained for b=300 s/mm as a function of b: all curves show a "plateau" till a b at which one can observe a "knee"; for higher b, the ADC decreases almost linearly. As the phantom cannot show other diffusion contributions apart the self-diffusive one, the ADC must be a constant as a function of b; therefore, this deviation from constancy is an artifact. It can be related to the exponential fall of the SNR with raising b, which has been reported in Figs. 4 and 5: comparing Figs. 3 and 4, one can observe that the ADC starts to decrease for all the scans when the corresponding SNR falls below a threshold value of about 4.5; for larger b, the ADC continues to decrease monotonically, while the SNR shows an unphysical plateau. Phantom B The results of the measurements on Phantom B are reported in Fig. 6: the series indicated with "automatic" represents the ADC values obtained by the maps calculated by the equipment, while the other three series contain the ADC values calculated from a manual comparison of the signal intensities of the images at b=0 and 500 ("manual 0-500"), b=0 and 1000 ("manual 0-1000") and finally with b=500 and 1000 ("manual 500-1000"). This last value manifests a surprising constancy among the non saturated scan and the FatSat, SPAIR and WE ones. The same thing cannot be observed of the value obtained from the ADC map and also of the other manually calculated values of the same sequences, but the results can all be considered consistent with each other, as can be seen in Table 1.

Relative Deviation in %

w.r.t. ADCauto

w.r.t. ADC500-1000

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No saturation

0,0

-0,3

SPAIR

-0,3

-0,7

FatSat

-0,3

-0,7

Water Exc

-0,2

-0,5

IR

-0,9

-4,0

IR+WE

-1,1

-4,9

IR+SPIR -1,0 -4,5 Table 1: Automatic ADC relative deviation of the non saturated scan with respect to: (first column) the automatic ADC value from the saturated scans; (second column) the ADC500-1000 value from all scans. It is apparent the large deviation between the various measurements for the IR-based scans. It is still to understand the large discrepancy between automatically and manually calculated ADC for those scans. The ADC values obtained using the IR, IR+WE and IR+FatSat scans are worth a deeper insight: the automated results deviate towards slightly lower values with respect to the non saturated scan; on the other hand, the manually calculated ones show a striking discrepancy towards higher values and cannot be considered consistent with the automatic ones. Finally, it is worth to stress the perfect coincidence between the three manually calculated ADC values in the case of the non saturated scan. On the contrary, in all saturated scans, the three values fall in an interval whose range spans from 1% for SPAIR, FatSat and WE scans to 2.4% for the IR-based scans; it is important to notice that the values always follow the same pattern: ADC0-500