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MRI (Magnetic Resonance Imaging)

Peter Mansfield got noble prize for NMR (scientific principal behind MRI). Q
Paul Christian Lauterbur was an American chemist who shared the Nobel Prize in Medicine in 2003 with Peter Mansfield for his work which made the development of magnetic resonance imaging (MRI) possible.
  1. It is a noninvasive method of mapping the internal structure of body by producing images by the virtue of gyromagnetic property of protons, with the greatest advantage of not using ionizing radiation for imaging.
  2. It portrays the distribution of hydrogen nuclei and parameters relating to their motion in water and lipids.
  3. Almost all images produced to date have been by the virtue of gyromagnetic property of protons and the nuclear magnetism of hydrogen nucleus (or proton) Q.
  1. How MRI works:
    1. Random movement in the hydrogen atoms in the absence of magnetic field
    2. Alignment of hydrogen atoms in the presence of a strong magnetic field.
    3. Realignment of the hydrogen atoms in the presence of radio-frequency wave/ pulse.
    4. When the radio-frequency is switched off, the atoms return to their original position and release energy in the form of electrical voltage signal
    5. This electrical signal (produced in receiver coil) is digitized and analyzed in a computer, to produce MR images.
      1. When Protons flip towards RF pulse it results in:
        1. T1Spin lattice relaxation
        2. T2 Spin spin relaxation
Extra Edge:

Unlike CT images in which contrast is determined by differences in one parameter (the linear X-ray attenuation coefficient μ), multiple parameters influence MRI Signal including nuclear or proton density, T1 and T2.


T1-weighted images have a short time to repetition (TR) (less than 1000 ms) and a short time to echo (TE) (less than 20 ms). T2-weighted images have a long TR (more than 2000 ms) and a long TE (more than 40 ms). 
  1. The principle pulse sequences are:
    1. Partial saturation (PS) or gradient or field echo
    2. Spin echo
    3. Inversion recovery
  2. Michael Faraday (1791-1867) created a complete cage of metal or metallic meshwork. If a region in space is completely surrounded by a Faraday cage (MRI rooms), ambient electromagnetic waves are effectively screened from enclosed region. Copper or Aluminum foils can transform any room into Faraday Cage. A continuous sheet or wire-mesh of copper or aluminum is used to shield the MRI rooms so as to protect the imager from external electromagnetic radiations, which is known as Faraday cage. Q
  3. The induction coils (transmitter/ receiver coils) used in MRI are Maxwell coils.
    1. Water (CSF) looks white (Hyperintense) on T2 (*Hint to remember World War-2)
    2. Thus, CSF looks hyperintense on T2 weighted image and hypointense on T1 weighted image.
Most of the structures or pathologies  are hypo on T1 and hyper on T2 weighted images
Things bright on T1W  Dark on both T1W & T2W
Fat Air
Hemorrhage Flowing blood (on SE / FSE images)
Proteinaceous substance Cortical bone
Melanin Ligaments, tendons, and other dense fibrous tissues
Paramagnetic agents (gadolinium)  
Substance                                            T1 weighted                    T2 weighted
Water/Vitreous/CSF                               Black                                  extremely white
Fat                                                       White                                  less white
Muscle                                                  intermediate                        Intermediate
Air                                                        Black                                   Black
Fatty bone marrow                                 White                                   Light Grey
Brain: White matter                                Light Grey                            Grey
Brain: Grey matter                                  Grey                                    very light grey


Extra Edge

To differentiate between T1- and T2-weighted images, look for simple fluid. Fluid tends to be hyperintense to virtually everything else on T2-weighted images.  On T1-weighted images, fluid is of low signal.
Good places to look for fluid include the urinary bladder and the cerebral spinal fluid (CSF).


MRI T1W Image : Axial Brain Image :
 MRI t2w Image Axial Brain :
  1. Advantages of MRI: Q
    1. No use of ionizing radiations (radiowaves are used in MRI)
    2. Greater inherent soft tissue contrast (e.g. Tumors of brain including cerebral metastases are usually better demonstrated on MRI than on CT due to greater inherent soft tissue contrast).
    3. Provide direct multiplanar imaging, which helps to define the relationship of the tumor to adjacent structures, and thus helps in planning of surgery ( e.g. facilitates the distinction between intraaxial and extraaxial tumors).
    4. MRI is vastly superior to CT in evaluating posterior fossa tumors as CT is frequently hampered by ‘beam-hardening’ artifact from the base of skull.
    5. Vascular imaging possible without use of intravenous contrast.
  2. Disadvantages of MRI:
    1. Longer time of acquisition                        
    2. Claustrophobia. Q                         
    3. Costly.
Clinical Applications of MRI
  1. Ideal test to detect bone metastasis to only spine .( all other bones - metastasis : bone scan )
  2. Imaging of pancoast’s tumor ( superior sulcus tumor )
  3. Imaging of posterior mediastinal masses
  4. For all brain tumors ( contrast enhanced mri )
  5. Chronic subarachnoid hemorrhage
  6. Investigation of choice in traumatic paraplegia
  7. The most sensitive and specific investigation in renal artery hypertension
  8. Investigation of choice in aortic dissection
  9. In perinatal asphyxia , neurological damage can be predicted by mri
  10. The best investigation for parameningeal rhabdomyosarcoma
  11. Best choice to radiologically evaluate a posterior fossa tumor
  12. Gadolinium enhanced mri is investigation of choice for vestibular schwannoma.
  13. Best diagnosis for dissecting aorta ( aortic dissection )
  14. The most accurate investigation for assessing ventricular function
  15. Investigation of choice for evaluation of suspected perthes disease
  16. In a 40 year old female patient on long term steroid therapy presents with recent onset of severe pain in the right hip. Imaging modality of choice for this patient is mri.
  17. Investigation of choice for a pregnant lady with upper abdominal mass
  18. First investigation of choice for spinal cord tumor
  19. Investigation of choice for multiple sclerosis
    1. The technique of magnetic resonance angiography (MRA) can be achieved without the risks of intravascular injection of contrast and may ultimately replace conventional studies. The technique used in MRI for angiography is called TOF (time of flight)
    2. Heavily T2-weighted sequences which demonstrate fluid-filled structures as areas of very high signal intensity have been developed to show the biliary and pancreatic ducts in magnetic resonance cholangiopan­creatography (MRCP). It seems likely that this technique will take over from diagnostic endoscopic retrograde cholangiopancreatography (ERCP).
    3. The major strength of MRI is in intracranial, spinal and musculoskeletal imaging, where it is superior to any other imaging technique because of its high contrast resolution and multiplanar imaging capability.
    4. Cardiac MRI is firmly established and the value of breast MRI, particularly in multifocal and recurrent cancer, is increasingly recognised.
    5. It is currently the best investigation for staging cervical cancer and for anorectal sepsis.
Magnetic resonance imaging (MRI) has significantly advanced the ability to image musculoskeletal structures. 
Extra Edge
  1. MRI has the advantages of providing multiplanar images with fine anatomic detail and contrast resolution.
  2. Other advantages are the lack of ionizing radiation and adverse effects and the superior ability to visualize bone marrow and soft tissue peri articular structures, which have led to the increased use of this modality.
  3. The advantages of MRI are counterbalanced by high cost and long procedural time, factors that have limited its use in the evaluation of musculoskeletal disorders.
  4. MRI should be used only when it will provide necessary information that cannot be obtained by less expensive and noninvasive means.
  5. Visualization of particular structures can be enhanced by altering the pulse sequence to produce either T1 - or T2-weighted spin echo, gradient echo, or inversion recovery [including short tau inversion recovery (STIR)] images.
  6. Because of its sensitivity to changes in marrow fat, MRI is a sensitive but nonspecific means of detecting osteonecrosis and osteomyelitis.
  7. Because of its enhanced soft tissue resolution, MRI is more sensitive than arthrography or CT in the diagnosis of soft tissue injuries (e.g., meniscal and rotator cuff tears); intraarticular derangements; and spinal cord damage, subluxation, or synovitis.
  8. Open access magnets have been developed which allow interventional procedures to be performed with MRI guidance and there is no doubt that this will revolutionise the operating room of the future. There is a vast potential for MRI in the assessment of disease in the abdomen and pelvis and undoubtedly the role of MRI will continue to expand. However, because of the expense of the equipment and its installation, the provision of scanners cannot keep up with the demands for scanning time and most hospitals have to impose strict guidelines for access


Applications of MRI are many, but amongst the commonly imaged parts are brain, spine and musculoskeletal tissues.               
Contraindications to MRI:
  1. Cardiac pacemakers (absolute contraindication) Q            
  2. Cochlear implants
  3. Intraoccular Metallic foreign bodyQ                
  4. Aneurysmal clips
  5. Prosthetic heart valves                        
  6. Claustrophobia
Extra Edge
Neck lesions above the hyoid bone should be studied first with MRI. Pathologic findings of the neck below the hyoid bone should be primarily imaged with CT scanning. 
CT is the imaging modality of choice for conductive hearing loss. MRI is the imaging modality of choice in adult-onset sensorineural hearing loss. 

MRI Contrast Agents
  1. Gadolinium chelates are most commonly used conrast agents in MRI studies. These compounds are relatively safe without any significant risk of anaphylaxis or allergic reactions unlike iodinated contrasts.
  2. The most common side effects of these agents are mild headache and a metallic taste.
  3. Pregnant patients should not receive gadolinium chelates. Many of the agents are known to be able to cross the blood-placental barrier, and their effect on the fetus is unknown.
  4. Nephrogenic systemic fibrosis is a scleroderma like dermatopathy that is known to precipitate due to use of gadolinium chelated in patients with compromised renal function. Increased dose of gadolinium, repeated dose s and use of linear gadolinium chelates particularly gadodiamide (omniscan) increases the risk of this disease entity.
European guidelines for prevention of nephrogenic systemic fibrosis
  1. Patients at a high risk are those who have chronic kidney disease (CKD) 4 and 5 (GFR <30 mL/min), including those who require dialysis. Patients who have a lower risk are those who have CKD 3 (GFR 30–59 mL/min) and children younger than 1 year old, because of their immature renal function. Patients who have normal renal function are not at risk for NSF.
  2. The following contrast agents are considered to be of highest risk for NSF: gadodiamide, gadopentetate dimeglumine, and gadoversetamide. Serum creatinine (eGFR) levels should always be measured before using these three agents.
  3. The intermediate-risk group includes the following agents: gadobenate dimeglumine ; gadofosveset trisodium (It is a blood pool agent with affinity to albumin. Diagnostic results can be achieved using 50% lower doses than with extracellular Gd-CA. Its biologic halflife is 12 times longer than for extracellular agents  and gadoxetate disodium . The determination of eGFR levels is not mandatory before using these agents.
  4. The low-risk group includes the following agents: gadobutrol, gadoterate meglumine, and gadoteridol. The measurement of eGFR levels before administration is not mandatory. 
  CT Scan MRI
Radiation exposure: Moderate - high radiation None
Principal used for imaging: Uses X-rays for imaging Uses magnets and radio waves to create the images.
Time taken for complete scan: Usually completed within 5 minutes Scanning typically run for about 30 minutes.
Details of soft tissues: Less as compared to MRI Much higher detail in the soft tissues
Details of bony structures: Provides good details about bony structures Less as compared to CT scan
Ability to change the imaging plane without moving the patient: Not present( reconstruction can be performed later on MDCT) MRI machines can produce images in any plane

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