Question

Give an example of a trade_off between imaging speed and SNR in medical imaging

Answer 1

Ans: In medical imaging balance between imaging speed and signal to noise ratio depends on many factors.

I will describe this aspect taking cardia MRI scan as an example, which is more difficult and highly error prone compared to CT and Ultrasound scanning. In cardiac MRI the trade-off between imaging speed and Signal to Noise Ratio is very important because it is performed on beating heart and breathing lungs. Usually this technique is always performed corresponding to the techniques like ECG. To acquire less SNR Morphological single time frame images are commonly acquired by acquiring multiple data samples during mid-diastole when the heart has minimal motion. This will have relatively long temporal window with minimal motion artifacts, resulting in a shorter acquisition time. For functional time resolved data, the trade-off between temporal window and the total acquisition time becomes more apparent and less prone to blurring

Time resolved cardiac image can be acquired in two different modes: prospective or retrospective. In prospective imaging the data acquisition starts at the detection of the ECG-trigger and data is acquired for a predefined number of temporal phases, after which the acquisition is idle until the next trigger occur. Retrospective acquisition, on the other hand, does not require a predefined number of time frames as the data is sorted after it is acquired. Furthermore, retrospective sequences run continuously, without any breaks in data acquisition between cardiac cycles, and the steady state can therefore be preserved. Another noise artefact is due to breathing movements, this can be resolved by breathholds, navigator gating, or pneumatic bellows triggering. This is having drawbacks like patient discomfort and breath hold position is not reproducible for multiple readings.

The utilization of multiple-channel coils generally provides improved image quality, and higher field strength scanners provide higher SNR (signal-to-noise ratio).

Some of the techniques and examples are summerised in the following paragraphs

Practical exam implementations achieve a trade-off between image qualities and scan time. To explain the relationships between the scan parameters and introduce how the trade-off between the image quality and scan time can be optimized. Parallel imaging is an efficient way to reduce scan time, but sacrifices the SNR, and may introduce technique depended artifacts. Figure 1 shows data from accelerated acquisitions using GRAPPA, for which the end-systolic and diastolic phases are shown.

By modifying the number of k-space lines per segments per view the acquisition time can be reduced at the expense of the temporal resolution. In Figure 2, the number of views per segment varies, ranging between 20 and 60. The reconstructed number of cardiac phases was chosen to be the same by applying sliding window to share the data among different cardiac phases.

The flip angle influence both SNR and contrast of the blood and myocardium in the cardiac image. In Figure 3 data were acquire with different flip angles. The measured SNRs of the blood in the left ventricles were 3.9, 5.2, 4.8 and 2.8 for flip angles of 30°, 45°, 60° and 75° respectively, and the corresponding CNRs (contrast-to-noise ratio) between blood and myocardium were 3.9, 5.1, 4.8, and 2.7. It suggests a flip angle of 45°~60° could be chosen to achieve the optimal blood SNR and blood-to-myocardium CNR.

Some applications in cardiac MRI require specific sequences while other applications allow multiple choices. The choice does in these cases affect many aspects of the image quality. Figure 4 show images acquired with TrueFISP and GRE, and illustrates the differences in SNR, contrast, and technique specific artifacts.

Reduced spatial resolution increases the SNR, which could be traded for a shorter acquisition time. Figure 5 shows the trade-off between images acquired with different image spatial resolution.

Partial acquisition can be used to shorten the acquisition time or increase the temporal resolution. In Figure 6 the scan time was reduced by partial phase encodings without visually noticeable image quality

For figures 1-6 you may refer to the following manuscript

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4292905/pdf/nihms641175.pdf