Q1: Answer the following questions (Approximately half a page)
- What is a magnetic field gradient?
- How is the magnetic field gradient used to achieve frequency encoding of the echo?
Q2: Answer the following questions (Approximately half a page)
- Describe the Nyquist theorem
- Calculate the dwell time for a bandwidth of 180,000Hz (+/- 90,000 Hz)
- Calculate the acquisition time of an echo using the dwell time from above for an image matrix of 200×128.
- Calculate the gradient strength (mT/m) required at for this experiment if the FOV = 25cm. (Hint: γ/2π = 42.57 MHz/T)
Q3:Answer the following questions (Approximately 3 pages including pulse sequence diagram)
- Draw a pulse program diagram for a basic spin echo pulse sequence, showing all RF pulses, gradients, acquisition of the echo, TR and TE. Do not copy and paste a picture from the web or a book.
- Describe how images with proton density, T1 and T2 contrast are acquired with spin echo pulse sequences with respect to TE and TR.
- Give two examples of how we can make better use of the TR time period with the spin echo pulse sequence. Briefly explain each.
Q4:Answer the following questions (Approximately 2 pages including pulse sequence diagram)
- Draw a pulse diagram for a gradient echo pulse sequence showing all RF pulses, gradients, acquisition of the echo, TR and TE. Do not copy and paste a picture from the web or a book.
- Describe how proton density (PD), T1, T2* and T1+T2 contrast are acquired with a gradient echo based imaging sequence in terms of a-pulse, TE, TR and any other additional gradients.
The MR image A was taken in by a weighing with T2 weighting. The image of the brain shows cortex, lateral ventricle, and falx cerebri. Usually a T2 weighted image is produced by de-emphasizing T1 contributions. The most significant way of identifying this image as a T2 image as by looking at the fluids which appear at the CSF region, the fluids appear bright a characteristic of a T2 image. Other attributes that associate the image with a T2 image include: the dark bone structure appearing in the skull region of the image, characteristic of a T2 image, the blood vessels or moving blood in the image appears dark also a characteristic of a T2 image (Baur-Melnyk, 2012). Gray matter in this image is gray and white matter, while the white matter appears to be darker that the gray matter a characteristic of a T2 image. At the edges where the skull bone appears dark is the fat containing structures that appear brighter than the skull bone and the bone marrow indicative of the T2 imaging. In summary, the parts that appear bright on a T2 MR image include: increased water, which include edema, tumor, inflammations and the like while the part that appear dark include: parts with low proton density, where there is calcification and fibrous tissue, paramagnetic substances, protein rich fluids and flow voids such as blood vessels (Morris, & Liberman, 2005).
The MR image B is T1 weighted. This can be seen in the dark. This can be determined by the appearance of the CSF that appears to be darker in the image this is because it is mostly formed by water. The lesions could also appear dark on T1 weighting. The image B also shows dark areas in the thick skull bone region, which is a characteristic of T1 imaging (Baur-Melnyk, 2012). White and gray water can also be seen in the diagram where the white matter appears lighter and the gray matter darker. Bright regions of the image are fat containing structures as can be seen on the edges on the brain. Vascular structures in the image should also appear bright. However to clearly see the ventricles in a T1 image contrast must be applied. It is a T1 image the skull appears bright as appears as in image B (Morris, & Liberman, 2005). In summary parts that appear dark in a T1 MR image include: increased water areas such as hemorrhage, infarctions, tumors and the likes while the parts that appear bright include fat, melanin, paramagnetic substances, calcification, laminar necrosis of the celebral infarctions, sub-acute hemorrhage (Baur-Melnyk, 2012).
Sequencing used to produce the weighting
T2 MR Images are produced by the use of long repetition time TR (2000 to 6000 milliseconds) and long echo time TE (100 to 150 milliseconds). The T1 is produced using short repetition times TR (300 to 600ms) so as to exploit the variance in longitudinal relaxation when the process of return to equilibrium is happening, and a short echo time TE (10 to 15ms) to take advantage of T2 dependence in the process of signal attainment. These are the standards for the production of the T1 and T2 MR images (Baur-Melnyk, 2012). A version of T2 production referred to as the fluid-attenuated inversion recovery (FLAIR) would still fall under this repletion time and echo time values.
Suggest possible TE, TR, and TI (if appropriate) values that might produce these
The TR and TE values that can produce these images are:
T1 image can be generated by a repetition time TR of 300 to 600 milliseconds and an echo time TE of 10 to 15 milliseconds. These are the limits that the T1 images fall under which generation of the T1 image lies in. The image B is a low resolution of a T1 image. This is indicated by the low contrast of the parts of the brain (Vlaardingerbroek, & den Boer, 2013). As can be seen from the image the differentiation of the gray and white matter in the image can be difficult as they are not well defined. The more the inversion delay of the person, the more the relaxation and there an effect on the image of producing well defined clearer images since the paramagnetic substances are given more time to settle hence better readings (Morris, & Liberman, 2005).
For the T2 image the repletion time TR would be 2000 to 6000 milliseconds while the echo time would be 100 to 150 milliseconds as per the parameter limits of the MR imaging of a T2 image. As seen before the image A is a T2 image. Another possible scenario in the MR imaging is the use of fluid-attenuated inversion recovery (FLAIR). FLAIR is one of the most sensitive technologies for the detection of white matter lesions in the images (Kraus, Espy, Magnelind, & Volegov, 2014). The images produced by FLAIR have an excellent contrast between the white matter and the gray matter lesions. With the advent of the now more common 7T it is important to use the FLAIR imaging in comparison to others at lower field strengths.
For image B what changes would you expect to see if exactly the same sequence was performed with the same parameters but in 1.5T?
1.5T is a lower magnetic field compared to the 7T used to produce the head MR image in image B. The contrast that is seen in the image is of the regular amount of the image B if a 1.5T field was used to produce the image B the contrast in the image would have been lower and it would have been difficult to identify the different parts of the image. As you move from high intensity to higher field intensities the contrast and clarity of the image is supposed to increase. In this case the image was produced at 7 tesla and now we want to see what would happen if it were to be produced at 1.5 tesla. Due to the low field intensity as seen before the 1.5T will cause a low signal to noise ratio in the image produced. It is known that as the intensity of the field increases the more the image contrast and therefore the clarity of the image increases, making high intensity fields more attractive in the detection of lesions in the parts being inspected (Kraus, Espy, Magnelind, & Volegov, 2014). It is worth noting that the image B was produced by a T1 weighing and a contrast of the structures is necessary so as to identify clearly any lesions and any slow moving blood in the vessels.
Examine the image below. Discuss the origin of the bright (yellow arrows) and dark (red arrows) signal around the kidneys. In this image, which is the read direction? How would you change the imaging parameters to reduce the effect shown?
The kidneys image shows in an axial view. It images shows a view of malrotation of the kidneys. The yellow pointers are indicative of muscle structure which is same as the renal boundary that surrounds the kidneys. The MR image has low contrast in some parts due to the MR sequencing of the image. It has been shown on many occasions that MRI is suitable for the viewing of vascular structures and is particularly suited for the imaging of soft tissues of the abdominal area and the urethral areas of the human body (Kraus, Espy, Magnelind, & Volegov, 2014). The CSF on this image appears to be bright, suggesting that the image was T1 weighting. T1weighted images are suited for the evaluation of anatomic detail of the parts of the body while the T2 weighted images are suited for the provision of sensitive images that can be used in the detection of lesions (Alan, Buckley, & Parker, 2006). It can be concluded that the yellow arrows indicate parts of the other kidney that represent fat, melanin, paramagnetic substances, calcification, laminar or even necrosis if a T1 weighting is used to produce the image. The dark parts would be indicative of the same structures if a T2 weighted image was to be used to produce the image. Other steps have been developed in the MRI imaging arena has as tumor enhancement by the administration of gadolinium-cased contrast (Vlaardingerbroek, & den Boer, 2013). The substance gadolinium is injected so as to improve the quality of the image produced by the MR.
In the image of the kidney image contrast is very apparent since it can be seen that even though it is the same image, the left section shown by the red arrows shows a high contrast (Kraus, Espy, Magnelind, & Volegov, 2014). It is due to the blood vessels that surround the kidney as shown in the image. The image appearance is also indicative of the chemical shift effect around the renal, while the right indicated by the yellow arrows is indicative of low contrast the bright section are the renal surrounding to be of the same contrast with the muscles. The yellow arrows are indicative of the good effect on the image.
Elevation of contrast in this image is necessary so as to get the correct reading from it. These processes, therefore, add the contrast agent gadolinium helps to inoculate the kidneys and make them more visible in the high contrast. It makes the renal outline apparent and therefore can be used in the identification of any abnormalities in the kidney (Alan, Buckley, & Parker, 2006). Abnormalities can manifest in many different forms, including: cysts in the kidney stones that would cause swelling of the kidneys, the positioning of the kidneys can present an abnormality in the structure of the kidney and, therefore, the contrast will help in identifying these abnormalities.
Alan, J., Buckley, D. L., Parker, G. J. M. (2006). Dynamic Contrast-Enhanced Magnetic Resonance Imaging in Oncology. Berlin: Springer Science & Business Media.
Baur-Melnyk, A. (2012). Magnetic Resonance Imaging of the Bone Marrow. Berlin: Springer Science & Business Media.
Kraus, R. Jr., Espy, M., Magnelind, P., & Volegov, P. (2014). Ultra-Low Field Nuclear Magnetic Resonance: A New MRI Regime. OUP USA.
Morris, E., & Liberman, L. (2005). Breast MRI: Diagnosis and Intervention. Berlin: Springer Science & Business Media.
Moy, L., & Mercado, C. L. (2010). Breast MRI, An Issue of Magnetic Resonance Imaging Clinics. London: Elsevier Health Sciences.
Vlaardingerbroek, M. T., and den Boer, J. A. (2013). Magnetic Resonance Imaging: Theory and Practice. Berlin: Springer Science & Business Media.
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