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磁力共振掃描是一種利用磁場及共振頻率的造影技術

Magnetic Resonance Imaging

Ionizing radiation-free

MRI Principle

 

Unlike other imaging modalities (X-ray, CT, PET-CT) that produce images with high energy electromagnetic radiation, Magnetic Resonance Imaging (MRI) scanner uses strong magnetic fields and gradients, and radio waves to generate images with the following uniqueness.

 

  • As an ionizing radiation-free imaging modality, MRI is particularly useful for patients who need regular scanning for close monitoring of disease progression.
  • The basis of MRI is the directional magnetic field generated by many free hydrogen nuclei inside a human body. When a human body is placed in a uniform magnetic field, the free hydrogen nuclei align with the magnetic field and create a net magnetic moment. The response signal is then captured by conductive field coils placed around and possessed to obtain multi-planar 3D MR images for precise diagnosis.

 

Below is the information regarding the uses, safety issues, concerns of undergoing a MRI examination and its advantages and limitations in comparison with other types of imaging modalities.

Superior tissue contrast resolution

MRI Uses

 

Comparing with other imaging modalities, MRI provides superior tissue contrast resolution, and therefore is frequently used to assess areas such as the brain, spine, ligament, joints, muscles, heart, blood vessels, breast, abdomen and pelvis. With regards to the diagnosis of cancer, MRI localises the lesion, differentiates benign from malignant soft tissue tumour of some kinds, and detects metastasis if necessary. Apart from assisting doctors make treatment plan, MRI is also used to monitor tumour progression during the entire course of treatment.

醫生檢查MRI影像

MRI Safety

As a diagnostic rather than treatment modality, an MRI examination does not bring any side effects to the patients in general. Below are some proper precautions taken prior to the examination to avoid any accident.

 

  • Due to the presence of a strong magnetic field, all kinds of metallic objects, including medical implants inside the body could be attracted to the machine, causing injuries or even deaths. Therefore, patients are screened thoroughly and required to remove any metallic foreign bodies that are contraindicated before undergoing an MRI examination.
  • The strong magnetic field could cause medical devices, such as hearing aid and pacemaker to malfunction. Therefore, patients are required to either take off all non-compatible devices or change to compatible ones before entering the examination room.
  • The strong magnetic field could cause radiofrequency heating of human body and associated medical equipment, such as ECG lead, which may lead to skin burns. Therefore, all the equipment is checked to ensure the compatibility.
  • Conventional MRI machine might create acoustic noise in rotation, which ranges from 70dB to 80dB. The harsh and strong sound could bring anxiety to patients, or even cause sudden temporary hearing loss in serious case. Therefore, ear plugs are provided for hearing protection.
  • MRI contrast agents that might be injected for some examinations is generally safe without much side effects. Only for patient who are allergic to the contrast agent might experience mild side effects such as itchy skin and nausea. Less than 1% of patients might suffer from life-threatening condition due to the consequence of serious allergic reaction.
  • Patient with severe kidney failure might develop nephrogenic systemic fibrosis as an adverse reaction to the contrast agents. Death, which might also occur in serious case, is rarely seen.
  • Without having any scientific evidence that shows completely no effect on pregnant women in undergoing an ionizing radiation-free MRI examination, patients who are at 12 weeks pregnant or less are not advised to undergo the examination unless necessary.
  • Breastfeeding patients are advised to allow 24 hours for natural excretion of contrast agent from the body before resuming breastfeeding.
  • Children or patients with claustrophobia might experience stress, anxiety or even breathing difficulty. Should there be any issues, radiographers are there to provide as much assistance as possible and sedative drugs might be given if necessary.

Patient safety tips prior to the examination

Diet

Food intake is not allowed for 4 hours before the procedure for patients undergoing magnetic resonance cholangiopancreatography; some cardiac medications should be suspended for 2 days in advance and caffeinated beverage is not allowed on the day of examination for patients undergoing cardiac MRI.

Health Status

Patients should inform us of any diseases such as renal insufficiency, current medications, history of drug allergy or other health conditions such as pregnancy.

Body Accessory

Patients are reminded to remove all metallic objects such as jewelry (earrings, rings, body piercings), personal valuables, electron devices, and asked to change into a gown before the procedure.

Metallic Implants

Patients are screened for compatibility of metallic implants such as pacemaker, artificial heart valves, dental implants, cochlear implants, neurostimulators, intra-uterine devices before the procedure.

 

Should there be any issues, radiographers are there to provide as much assistance as possible.

Diagnosis of Cancer|Imaging Tests

 

Common tests for diagnosing cancer includes blood test, urine test, imaging test, biopsy and genetic test. Apart from diagnosing cancer, these investigations can also localise the primary site, differentiate the specific cancer cell type, perform cancer staging, identify whether there is metastasis, formulate treatment plan, monitor disease progression and detect recurrence. Below is the information regarding different tests.

 

超聲波

Ultrasound

Applications:

  • Localize the site of primary cancer
  • Guide biopsy
  • Detect pathological change of tissues and vascular flow

Strengths:

  • No radiation is involved
  • Short examination time
  • Real-time monitoring of change is allowed

Limitations:

  • Low penetration power to deep tissues
  • Low spatial resolution
  • Operator-dependent result
電腦掃描

CT

Applications:

  • Localize the site of primary cancer
  • Detect pathological change of tissues
  • Guide biopsy
  • Allow cancer staging
  • Formulate treatment plan
  • Evaluate treatment effectiveness
  • Detect recurrence

Strengths:

  • Short examination time

Limitations:

  • Radiation is unavoidable
  • Relatively low spatial resolution for soft tissue structures such as muscle and nerve
  • Risk of contrast allergy
正電子掃描

PET-CT

Applications:

  • Localize the site of primary cancer
  • Detect pathological change of tissues
  • Guide biopsy
  • Allow cancer staging
  • Predict prognosis
  • Formulate treatment plan
  • Evaluate treatment effectiveness
  • Detect metastasis or recurrence

Strengths:

  • High sensitivity in picking up cancer at early stage
  • High accuracy

Limitations:

  • Radiation is unavoidable
  • Injection of radiotracers is needed
  • Might not be suitable for diabetic patients
磁力共振

MRI

Applications:

  • Localize the site of primary cancer
  • Detect pathological change of tissues
  • Differentiate benign tumor from malignant cancer
  • Formulate treatment plan (i.e. localisation for surgical planning or radiation therapy)
  • Monitor treatment progress
  • Evaluate treatment effectiveness
  • Detect recurrence

Strengths:

  • No radiation is involved
  • Better image resolution for soft tissue structures such as muscle and nerve
  • Multi-planar 2D and 3D images are available
  • Contrast allergy is rare

Limitations:

  • Metallic object is contraindicated in a MRI suite
  • Risk of claustrophobia
  • Long examination time

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