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