1. Basic principles of various imaging tests

Imaging tests play a crucial role in modern medical diagnostics, with different technologies having their own unique principles and application scenarios. PET scanning, or positron emission tomography, is a metabolic activity-based imaging technique that detects the uptake of glucose by cells by injecting tracers containing radioisotopes such as FDG. Because cancer cells are often metabolized, PET scans have unique advantages in tumor diagnosis.

CT (computed tomography) uses X-rays to penetrate the human body and produce images through differences in the absorption of X-rays by different tissues. Its advantage is that it can clearly display bones, lungs and other structures, and the scanning speed is fast, making it suitable for emergency situations. MRI (magnetic resonance imaging) uses strong magnetic fields and radio waves to stimulate hydrogen atoms in the human body to generate signals, and the resolution of soft tissues (such as the brain and muscles) is very high.

Ultrasonography is often used in fields such as obstetrics, cardiovascular, and other fields because it uses high-frequency sound waves to image the reflective properties of tissues and can be observed in real-time without radiation. Each of the four technologies has its own strengths and requires a comprehensive consideration of clinical selection.pet scan 作用

2. Advantages and disadvantages of various imaging tests

PET scans are highly sensitive, can detect small lesions early, and can cover the whole body in a single scan, but they are expensive (about HK$15,000 to HK$20,000 per scan in private hospitals in Hong Kong) and have the problem of radiation exposure. CT is the most prevalent and the scans are fast (usually within a few minutes), but there are also radiation problems, and iodine-containing contrast agents can cause allergic reactions.

MRI does not irradiate and has excellent soft tissue contrast, but it is contraindicated in patients with a long examination time (30-60 minutes) or with metal implants implanted in the body. Ultrasonography is non-invasive, non-radiation, and portable, but image quality is highly dependent on the author's experience, and deep tissue resolution is limited.

• PET scan:Benefits: Metabolic information, full body scan. Disadvantages: high cost, radiation
• CT.Advantages: Fast and clear bone structure. Cons: High radiation dose

3. PET Scan vs. CT

The main difference between PET scan and CT is that it focuses on "function" and "structure". PET reflects cellular activity through tracer metabolism and CT can detect early tumors that are difficult to identify (such as small metastases from lymphoma). According to data from Hong Kong Sanatorium & Hospital, the combined application of PET-CT has improved the accuracy of tumor staging to 92%.

CT is suitable for showing anatomy such as fracture details and the degree of vascular stenosis (95% accuracy of coronary CT contrast). In clinical practice, PET-CT is often used to obtain functional and structural information simultaneously, but the radiation dose (approximately 15-25 mSv, equivalent to 500 chest X-rays) must be weighed.

IV. Comparison of PET scan and MRI

MRI is superior to PET (up to submillimeters) in terms of spatial resolution, especially when imaging the hippocampus, spinal cord, and other brain microstructures. A study from the Chinese University of Hong Kong noted that MRI has a sensitivity of 90% in diagnosing multiple sclerosis, and that PET has a limited role in this area.pet scan 輻射

The strength of PET lies in quantifying metabolic activity, including the detection of β amyloid deposits in the brains of Alzheimer's patients. Recent studies have shown that PET combined with certain tracers (such as Amyvid) can predict the risk of disease 5 to 10 years ago. The two techniques are very complementary and neurology routinely refers to both outcomes.

5. Selection of clinical application scenarios

In the field of oncology, the guidelines of the Hong Kong Centre for Comprehensive Oncology recommend the following:
- 初診分期 ET-CT
- Treatment response assessment: PET scan every 2-3 chemotherapy sessions
- Postoperative recurrence monitoring: MRI (local) + PET (whole body)

Cardiovascular examination can use CT coronary angiography to assess vascular stenosis and PET myocardial perfusion scan to determine myocardial survival, which is important for revascularization decisions. In neurological diseases such as epilepsy, MRI is required to identify structural abnormalities, and PET helps to identify metabolic changes in epilepsy-inducing foci.

6. Choose the appropriate imaging test according to your specific condition

When choosing an imaging test, consider the following:
1. Nature of the clinical problem (structural abnormality or dysfunction)
2. Special condition of the patient (e.g., radiation sensitivity, claustrophobia)
3. Cost-effectiveness ratio (the waiting time for PET scans in public hospitals in Hong Kong is about 4-6 months)

Physicians should develop an individualized plan based on international guidelines (e.g., NCCN oncology guidelines) and local health care resources. The future trend is to develop low-dose multimodal fusion imaging techniques to further improve diagnostic accuracy.

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