Relaxation And Image Contrasts In Mri Biology Essay

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Magnetic resonance imaging (MRI) is a widely accepted modality for providing anatomical information. MRI methods are available and reliable to provide information about biological function, in addition includes the anatomical information. MRI also provides good contrast between varying soft tissues of the body, and it is very useful in imaging the brain, heart, muscles and also can detect or determine the cancer compared with other medical imaging techniques such as computed tomography (CT) or X-rays. MRI is harmless in case of patient' received radiation dose because it uses no ionizing radiation compared to the CT scan.

The MR signal is made up of contributions from intrinsic parameters and extrinsic parameters. The intrinsic parameters include the proton density and the proton relaxation times, T1 and T2 while the extrinsic machine parameters includes the selected repetition time, TR, the echo time, TE, and the flip angle pulse. An image constructed due to the protons in different tissues return to their equilibrium state at different rates, and the difference is measured and detected. Manipulation and changing the parameters on the scanner will create contrast between different types of body tissue or between other properties, as in functional MRI (fMRI) and diffusion MRI.

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T1 RELAXATION TIME

The spin lattice relaxation time, T1 which consider as an average lifetime of nuclei in the higher energy state, is dependent on the gyromagnetic ratio of the nucleus and the mobility of the lattice. An increased mobility will increase the vibrational and rotational frequencies, the component of the lattice field to be able to interact with excited nuclei will also become higher. However, at extremely high mobilities, the probability of a component of the lattice field being able to interact with excited nuclei decreases. In addition, different tissues have different T1 values. For example, fluids have long T1s (1500-2000 ms), and water based tissues are in the 400-1200 ms range, while fat based tissues are in the shorter 100-150 ms range.

Even though T1 or the longitudinal relaxation time varies for different tissues, but in higher field strength in specific tissue type determine longer T1. So, the T1 times of different tissues increase to varying degrees. For example, the T1 relaxation time of brain parenchyma is increased by up to 40% when compared to imaging at 1.5 T. Other tissues with significant magnetization transfer also demonstrate increments of 20-40%, whereas for cerebrospinal fluid, the change is negligible. There is also some case where the T1 is even higher, such an increase of up to 73% is reported for the kidneys.

However, the increase in tissue T1 time will usually affect the image SNR by decreasing its level. In addition, the T1 values of different tissues tend to become more uniform at higher field strengths, and the result, the T1 images between tissues will show decrease In contrast. The pulse sequence parameters used at 1.5 T require modification and manipulation when applying to 3 T to help to reduce the intrinsic 3 T image contrast losses. It includes the selection of TR, TE, flip angle and inversion time in order to optimize the image contrast of different tissues in 3T. Imaging the neonatal brain will be affected by the longer T1. The neonatal brain inherently has high water content, especially in the white matter, making grey-white differentiation hard to distinguish. This can be improved by appropriate use of TR, TE and flip angle and the use of sequences such as T1 FLAIR.

T2 RELAXATION TIME

For the T2 relaxation or spin-spin relaxation time, it generally proceeds more rapidly than T1 recovery, and same as T1, different samples and different biological tissues have different T2. In instance, fluids have the longest T2s and water based tissues are in the 40-200 ms range, while fat based tissues are in the 10-100 ms range. Still, amorphous solids have T2s in the range of milliseconds, while the transverse magnetization of crystalline samples decays in around 1/20 ms

For example, with particular values of the echo time (TE) and the repetition time (TR), which are basic parameters of image acquisition, it will develop either the T1-weighted image or T2-weighted image. Based on the property of the T2-weighted image, on a T2-weighted scan, water- and fluid-containing tissues are bright and fat-containing tissues are dark. The reverse is true for T1-weighted images which when the fat-containing tissues will appear bright and the water and fluid-containing tissues appear dark.

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Optimising image contrast is, of course, greatly assisted if there is prior information on spin density and relaxation times, but this can also be a problem because of the limited nature of most in vivo T1 and T2 measurements on biological tissue.

The T2 is fairly constant at different field strengths, decreasing slightly at higher field strength while the spin lattice relaxation time, T1, increases as the field strength increases. Prior measurements of relaxation times at 4.0 T showed increases in T1 relaxation time of 70-90% and decreases in T2 relaxation time of 10-20%. (Gold et al., 2004)

HOW RELAXATION TIMES INFLUENCED THE IMAGE CONTRAST

In MRI, the problem is to choose the optimum choice of the main magnet field strength, or, equivalently, resonance frequency. A 1.5-T magnet continues to be the standard for many clinical MRI applications, but the desire for higher spatial and chemical shift resolution is driving the move to higher fields. However, as a number of authors have pointed out, provided sufficient resolution is available to identify the regions of interest, it is usually image contrast that needs optimising, and for some clinical conditions, this might be greater at lower, rather than higher fields. (Wright et al., 2010)

Regarding about the magnetic field strength, the intrinsic SNR available in an MRI study determines the strength of the main magnetic field, radiofrequency coil being used, even the volume of tissue being imaged. 1.5 T is the most preferable conventional MRI to evaluate the musculoskeletal system.. However, nowadays, systems with higher field strength such as 3.0 T, are now becoming more prevalent in the clinical setting. Imaging at 3.0 T provide twice the intrinsic SNR of imaging at 1.5 T if the coils and the subject are equivalent because the available magnetization varies linearly with field strength. However, the 3.0 T still has a limitation which is when the field-dependent changes in tissue relaxation times and in the chemical shift difference between fat and water and it will results in degradation of the SNR benefit seen at 3.0 T.

At 3.0 T, because the T1 relaxation times have increased, the TR must be longer to maximize the SNR gain. At 3.0 T, TR must also be longer to achieve the same type of contrast on T1-weighted images as achieved with 1.5 T. Similarly, the TE should be slightly shorter to account for decreases in T2 relaxation times. The number of slices and the spatial resolution required may also influence the choice of TR and TE. (Gold et al., 2004)

In addition, 3.0 T MRI with lower-field strength magnet could have significant diagnostic advantages over imaging. Contemporary fast acquisition techniques can give sufficiently high temporal resolution to sample the fast dynamics observed during contrast enhancement of the prostate gland. (Rosenkrantz, 2009). However, as the acquisition time decreases, there may be compromises in SNR. In comparison with the 1.5 T, the higher field strength of 3-T MRI produces an approximately two times higher SNR and this higher field strength allows higher-resolution T2-weighted images and faster dynamic images to be obtained with an SNR comparable to 1.5 T.

A dynamic contrast-enhanced MRI study provides information and condition on blood volume and permeability, which is valuable for understanding behavioral of tumor, diagnosis making and predicting the prognosis of the tumor.

Furthermore, dynamic contrast-enhanced MRI is a well-established method for detecting and quantifying certain pathology, such as tumor angiogenesis and it is independent of T2 relaxation and MR spectroscopic characteristics. The dynamic contrast-enhanced is important, for example in detecting and determine the prostate cancer because due to the conventional MRI methods, it is generally limited in their ability to differentiate prostate cancer from other abnormalities within the peripheral zone. So, the addition of dynamic contrast-enhanced MRI to conventional imaging methods improves the localization of prostate cancer in most cases and at the same time, may improve and enhance diagnostic performance.

Conventional MRI examinations of the prostate gland have long included T2- weighted MR images for diagnosing prostate cancer. However, the reliability of T2- weighted MRI protocols in discriminating prostate cancer from other causes of low signal intensity, such as inflammation, hemorrhage, hyperplasia is low. T2-weighted MRI is very sensitive for prostate cancer but is not specific. There are many false-positives, as the specificity of 37% implies. (Ocak et al., 2007)

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The other example, MRI is excellent in detecting the hemorrhage. T1 and T2 relaxation times are proven to be inversely related to blood osmolality and hemoglobin concentration, but on the other side, it is directly related to the pH of blood. However, there are some factors that influenced the relaxation times of blood. Larmor frequency, temperature, viscosity, the paramagnetic properties of methemoglobin, pH, and hemoglobin concentration are factors that have been shown to influence the relaxation times of blood.

On acute T1 dependent images, hematoma and intraparenchymal hemorrhage have longer T1 relaxation times because it generally tended to have low signal intensity (equal to or less than that of muscle). As the hematoma aged, most hematomas had a bright signal consistent with lower T1 values due to the increased signal intensity.. Hemorrhage, however, had persistently longer T1 values after the acute phase compares to the hematoma. On the T2-dependent images, both acute intraparenchymal hemorrhage and hematoma generally had a signal of high intensity relative to that of muscle and therefore longer T2 relaxation times are developed. As the hematoma aged, the intensity remained strong yet generally became less marked. So, we can summarize that the relaxation times of hematoma are initially long and subsequently decrease with time, while for the intraparenchymal hemorrhage remain long throughout resolution.

In addition, T1 relaxation time is important in determine the lesion contrast enhancement. T1 relaxation time thresholds increase with TE of the fluid-attenuated inversion recovery (FLAIR) MR imaging sequence and the difference from hyperintense to hypointense lesions can be seen. FLAIR MR imaging is important and mostly used in routine brain imaging because of its ability to enhance the visibility of several abnormal lesions. The pulse parameters such as TR, TE, inversion time, T1 and echo-train length have an effect to the FLAIR MR imaging sequence on the visibility of brain lesions. Optimization of the pulse parameter sometimes develop certain T2-hyperintense brain lesions remain inconspicuous on FLAIR MR images. Furthermore, otherT2-hyperintense brain lesions are hypointense on FLAIR MR images. These inconsistencies and difference between FLAIR and T2- weighted MR imaging are mostly because of the result of complex interactions between the relaxation times, T1 and T2 of the lesion and the pulse parameters such as TR, TE, and TI of the FLAIR MR imaging sequence. With T1 relaxation time increase, there was, however, the contrast between the simulated lesions and white matter decrease. Once the T1 relaxation time increased beyond a certain threshold, the simulated lesions transformed from hyperintense to hypointense compared with white matter on all the FLAIR maps.

The upward shift in the T1 relaxation time threshold, lesions with the same T1 and T2 relaxation times could appear hypointense on short-TE FLAIR and hyperintense on long-TE FLAIR MR imaging. Also, if a lesion had a T1 relaxation time that matched that defined by a particular FLAIR MR imaging sequence, the lesion was isointense to white matter (invisible) on the FLAIR MR imaging sequence despite being hyperintense on T2-weighted MR imaging sequences. (Melhem et al, 2001)

T1 relaxation time affects image contrast by manipulating the degree of longitudinal magnetization recovery during the TI and TR-TI intervals. The T1 relaxation time is directly proportional with the degree of longitudinal magnetization recovery and inversely proportional with the signal intensity. As the T1 relaxation time of a lesion increases, the degree of longitudinal magnetization recovery also will decreases and the signal intensity from the lesion become lower. So, in detail, at short TEs, the T1 effect is emphasized and the lesion appears

hypointense to surrounding white matter. Different at long TEs, on the other hand, the T2 effect is emphasized and the lesion develops as hyperintense.

Furthermore, in instance, in one study shows that T1 imaging of the spinal cord is difficult at 3 T. T1 prolongation results in reduced contrast between the grey and white matter and image quality is distracted. In addition, CSF signal intensity at 3 T is greater than at 1.5 T which means there is reduced contrast between the spinal cord and the surrounding CSF. To compensate with this, the gradient sequences such as T1 FLAIR is used to lessen the problems associated with T1 lengthening, with the potential of optimum contrast at 3 T, at normal as well as abnormal tissue interfaces. Studies comparing post-contrast T1 spin echo and gradient images have yielded hazy results with regard to lesion conspicuity and detection, and to overcome this, the different planes of each one is acquired.

Previous studies have demonstrated that there is considerable variation in the T1 and T2 relaxation times of lesions which cannot be accounted for by the age or activity of the lesion. This study provides evidence that the site of the lesion is another factor which may influence the T1 and T2 relaxation times of lesions. (Stevenson et al., 2000)

Therefore, it is necessary to maximize the contrast when developing new diagnostic protocols because many diagnostic applications of clinical MRI depend on the contrast that exists between tissues, such as tumours and the surrounding healthy tissue.

Variety in T2 that is obtained during the contrast uptake requires fast T2 measurement techniques. The spin-echo sequence is one of the most widely used protocols in MR imaging for T2 estimation. To achieve a compromise between signal intensity and total acquisition, the long TR repetition time must be enough but yet, TRs allow recording long echo train which compensates for various imager imperfections, such as artifact motions developed that will affect T2 accuracy. So, in opposition, short TR in spin echo sequence would give the advantage even though it reduce the imaging time, but as a consequence the number of available echoes decreases, introducing inaccuracies in T2 estimation.

In addition, at 3 T the relaxation times are altered, and the T1 of tissues is longer. If the TR is kept constant, there will be a reduction in SNR. Increasing the TR to compensate for the longer T1 will increase the acquisition time. Therefore, the signal to noise gain is always less, (typically 1.7-1.8-fold), and this gain varies for different tissues. (Dagia, 2008). Compare to T1, T2 values tend to decrease with field strength. The higher the field strength will decrease in T2 values. However, tissue contrast on T2 images is not as significantly affected as T1 images. Some recent reports suggest up to 10% decrease in T2 relaxation times at 3 T, compared to 1.5 T to reduce the SNR gain for long TE sequences and the contrast between different tissues can be distinguished well.

Evaluation of the reproducibility of MR imaging measurements is fundamentally different from that of computed tomography (CT) because in CT, the attenuation of a voxel is directly related to its physical density, while the signal intensity on MR imaging is multiparametric and dependent on T1 and T2 relaxation times, hydrogen density, motion, and instrumental parameters such as TE, TR, and the type of pulse sequence.

CONCLUSION

Detection and evaluation of pathology with magnetic resonance imaging not only restricted to the optimal image and evaluation of normal and abnormal anatomy but it also includes on the detection of signal intensity differences between distinct tissue subtypes even before anatomic derangement occurs. Different machine settings will affect the absolute or relative intensity of a region, however, both T1 and T2 relaxation time and the resonating proton density of distinct regions are unaffected. Therefore, quantification of T1 and 12 relaxation times and of spin density can be classified as a fundamental way of characterizing tissue