The comparison of MRI imaging In 3.0 T & 1.5T By a.r.shoaie
Dec 25, 2015
The comparison of MRI imagingIn 3.0 T & 1.5T
By a.r.shoaie
The Differences between3.0-T and 1.5-T Imaging
The doubling of the overall signal-to-noise ratio is major difference (SNR) at
The 3.0 T (ie, S/N is directly proportional to B0).
technical points 3.0 T (MR) imaging offers higher
(SNR) and contrast-to-noise ratio (CNR)than 1.5-T MR imaging does, which can be used to improve image resolution.
and shorten imaging time.
What happend An increase in SNR can
be achieve good spatial resolution, good temporal resolution so imaging times decreased
Physical points
At high magnetic field strengths is a function of increased SNR, which allows smaller pixels and thinner sections for a given field of view (FOV) .
T1 Relaxation Times T1 relaxation times for most
tissues are about20%–40% longer at 3.0 T than they are at 1.5 T.
For comparable imaging times, this may result in lower soft-tissue contrast at 3.0 T than at 1.5 T.
Signal intensity–time curves showlonger T1 relaxation times at MR imaging inliver (solid curves) and tumor tissue (dashedcurves) at 3.0 T than at 1.5 T. To generate a levelof T1 contrast between the two tissue types at3.0 T commensurate with that at 1.5 T, longerrepetition times (TR, represented by dotted verticallines) are required.
Relaxation Time at 3.0 T
The problem with T1 prolongation is that it reduces the available signal in a reduction in available imaging sections in a given time.therefore, T1 contrast can be considerably decreased, especially with a multisection 2D GRE sequence.
Increased CNR improves lesion conspicuity, requires less intravenous contrast agents material in Angiography&cholangiography by increasing spatial and temporal resolution.
The higher CNR seen at 3.0T imaging could lead to cost savings by halving the dose of gadolinium typically used at 1.5 T
Gadolinium Effects at 3.0 T
3.0-T MR IMAGING OF
THE ABDOMEN IN COMPARISON
WITH 1.5 T
Increased spatial resolution at
3.0-T MR angiography in two patients.
(a, b) Coronal (a) and sagittal (b) maximum-
intensity projection images obtained in a
47-year-oldwoman with
hypertension show normal single
bilateral renal arteries and normal celiac andsuperior mesenteric
arteries. (c–e) Coronal (c)
and axial (d, e) maximum-intensity
projectionimages
Increased fluid conspicuity due to higher SNR at 3.0 T compared with 1.5 T (a) Coronal 40-mm
thick-slab 3.0-T MR cholangiopancreatographic image shows chronic pancreatitis and pancreatic head
adenocarcinoma. Higher SNR improvesvisualization of hepatic biliary radicals beyond that in
b, a 1.5-T MR cholangiopancreatographic image obtained in another patient with chronic
pancreatitis.
Coronal maximum-intensity projectionimage from 3D MR cholangiopancreatography at 3.0T shows improved SNR and increased conspicuity of
polycystic liver disease in a 45-year-old woman. 3.0 Tallows more robust 3D volumetric acquisitions with
sections as thin as 1 mm within a reasonable acquisition
time of 3 minutes 22 seconds. respiratory triggering.
Increased conspicuity of a hepatocellular carcinoma at 3.0 T compared with 1.5 T .The 3.0-T examination was performed 1
month before the 1.5-T examination. T2-weighted 1.5-T (a) and 3.0-T (b) fast spin-echo MR images show multiple
T2-intense masses (arrows), which are more apparent at 3.0 T than at 1.5 T. In b, note the increased susceptibility
artifact related to a metallic clip (arrowhead), a feature barely perceptible in a.
Multiple HCCs in a 62-year-old man. A large HCC is visible on T2- and T1-weighted images obtained on the 1.5-T system (a-
d). However, the T2-weighted image obtained on the 3.0-T system (e) clearly demonstrates another small lesion (arrow) not appreciated on the T2-weighted image obtained on the
1.5-T system (a). The larger lesion is also better delineated on the 3.0-T images (f-h).
T2-weighted single-shot respiratory-triggered and multishot breath-hold images obtained
with 1.5-T (a, b, c) and 3.0-T (d, e, f) systems are compared,
Metastasis from rectal cancer in a 67-year-old woman. A metastasis (white arrows) from rectal cancer is
visible on both 1.5-T (a) and 3.0-T (b) TSE images, but the margin and internal architecture of the lesion are
better delineated on the 3.0-T image. The renal anatomy (black arrow) is sharply delineated on the 3.0-
T image.
Rectal cancer in a 67-year-old man. Because a 3.0-T system provides greater SNR, a single-shot sequence (d-f) may be used instead of a multishot sequence (a-c) and provide comparable image quality and
spatial resolution in a much shorter imaging time and with less motion artifact. Single-shot sequences performed within 50–60
seconds produce images that are comparable to those obtained with multishot sequences requiring much longer imaging times of 6–8
minutes
Cardiac procedures
Cardiac cases are not suitable with 3.oT
Strength of magnet causes big changing in R-waves ,so triggering is not good.
Therefore 1.5T machine is much better than 3.o T machine.
Abdominal ApplicationsThat Benefit from 3.0 T
The ability of contrast-enhanced evaluation of organs, gadolinium-enhanced.
MR angiography, MR cholangiopancreatography, diffusion-weighted imaging.
Mr spectroscopy
Liver leions For the evaluation of liver lesions,
higher SNR and greater resolution achieved with the 3.0-T system could translate into better detection of malignant lesions on T2-weighted images obtained with adjusted imaging parameters.
Especial application
MR spectroscopy are specific examinations that shows
improved spatial and temporal resolution at 3.oT.
Diffusion-weighted Imaging
Diffusion-weighted sequences have various potential applications in
abdominal MR imaging Diffusion-weighted images show particular promise for monitoring
how tumors respond to therapy.
Other applications in3.0T
Diffusion-weighted imaging has important role in the
prostate, uterus, and rectum and for lymph node mapping
Disadvantages of 3.0 Tfor Abdominal MR Imaging
Because the Larmor frequency increases with field strength, higher-frequency RF pulses are required, resulting in an increase in energy transmission and absorption in tissue. e (SAR) increases by a factor of four in a 3.0-T system compared with that in a 1.5-T system .
Several potential problems remain for abdominal imaging
at 3.0 T.
Limitations on energy deposition in pulse sequence timing and flip angles . Magnetic susceptibility and chemical shift artifacts .
Photograph shows a stretcher that waspulled into a 3.0-T magnet by the static magnetic field
surrounding an MR imaging system. This accidentmight have had catastrophic consequences if a patient
had been undergoing MR imaging at the time. Thepresence of a higher-field-strength magnet requires
placement of the 5-G line at a greater distance from themagnet than is needed for a 1.5-T magnet
Chemical shift effect
Chemical shift artifacts. Single-shot fast spin-echo images in a 41-year-old womanwith normal kidneys at 1.5 T (a) and at 3.0 T (b) show chemical shift artifacts, which are caused by increased spectral separation.Note the increased water-fat misregistration at the renal cortex at 3.0 T (arrow).
Cost and Design Issues
Clinical 3.0-T MR units are more expensive than 1.5 T units are;
they tend to cost as much as double what1.5 T units do.
Technical problems
Three-tesla magnets tend to require at least triple the amount of cryogen needed for for 1.5-T magnets;
for example, they consume liquid helium
at a rate of approximately 0.09–0.15 L/hr, versus 0.03 L/hr for 1.5-T magnets.
Dielectric Effect at 3.0 T
Because the wave velocity of the RF pulse is constant, the wavelength becomes shortened as frequency increases. With this shortening of RF wavelength and induction of eddy currents (small electrical currents caused by rapid on-and-off switching of the gradient), inhomogeneity of the RF field occurs.
Dielectric Effect at 3.0 T
Dielectric effects of (a) single-shot and (b) multishot TSE sequences.
(a) There are signal voids (large arrows) due to the dielectric effect in some areas of the abdomen. This is
most commonly observed with single-shot TSE sequences because they use long echo trains with a
large number of refocusing RF pulses. (b) The dielectric effect is less prominent with multishot TSE sequences
(small arrows).
MAJOR DIFFERENCES IN IMAGING
WITH 3.0T IN COMPARISON BY 1.5T The dedicated receiver coils and
increased gradient performance, 3.0-T magnetic resonance (MR) systems are gaining wider acceptance in clinical practice.
The expected twofold increase in signal-to-noise ratio (SNR) compared with that of 1.5-T MR systems may help improve spatial resolution or increase temporal resolution when used with parallel acquisition techniques.
Table 1. Summary of Sequence Optimization at 3.0 T
SAR at 3.0 T
Because the Larmor frequency increases with field strength, higher-frequency RF pulses are required, resulting in an increase in energy transmission and absorption in tissue.
e (SAR) increases by a factor of four in a 3.0-T system compared with that in a 1.5-T system , causing patients to feel an unpleasant heating sensation.
Imaging of epilepsy. Consecutive coronal FLAIR images of the brain obtained at 3 T .a show an ill-defined area of increased signal
intensity (arrow) in the right occipital lobe, a finding
consistent with focal cortical dysplasia
Fiber tractographic results. (a, b) Left: Three-dimensional reconstruction of corticospinal tract (red) in the same subject at (a) 3.0-T
and (b) 1.5-T
Mr imaging at 3.o T in childern
Potential advantages of 3.0-T imaging in children include acquisition of good-quality images even with a small field of view(FOV).
The shorter overall acquisition time of 3.0-T imaging is useful in childern who may not be able to cooperate for long time.
MR angiography in a 10-year-old girl with
central nervous system
vasculitis. (a–d) Axial (a, c)
andcoronal (b, d)
maximum intensity
projection images from
time-of-fl ight MR
angiography, obtained at 1.5 TSignifi
cant improvement in resolution
and in visualization
of small peripheral vessels is
seen on theimages
obtained at 3 T.
Whole-body MR imaging. Coronalcombined short inversion time
inversion-recovery images,
obtained at 1.5 T (a) and 3 T (b), show the whole body. The 1.5-T
imageshows fairly uniform
signal intensity throughout the
body and less signal loss in the lungs from
susceptibilityin comparison with the 3-T image. However, the
3-T image shows individual muscles more
clearly owingto high SNR and spatial
resolution
Conclusions T here are both advantages and
disadvantages to imaging the abdomen at 3.0 T rather than at 1.5
The increase in SNR and CNR may be used to improve image resolution, shorten imaging time, or both.
Thank you for your attention