Sliding window dual gradient echo (SW-dGRE): T1 and proton resonance frequency (PRF) calibration for temperature imaging in polyacrylamide gel

Magnetic resonance temperature imaging of laser-induced thermotherapy: assessment of fast sequences in ex vivo porcine liver

Aim: To evaluate magnetic resonance sequences for T1 and proton resonance frequency (PRF) thermometry during laser-induced thermotherapy (LITT) in liver tissue. Materials & methods: During LITT (1064 nm; 30 W; 3-cm diffuser; 2–3 min) in ex vivo porcine liver, temperature was measured (25–70°C) utilizing a fiberoptic thermometer and MRI was performed with a 1.5-T scanner through the following sequences: segmented echo planar imaging (seg-EPI) for the PRF method; fast low-angle shot (FLASH), inversion-recovery turbo FLASH (IRTF), saturation-recovery turbo FLASH (SRTF) and true-fast imaging (TRUFI) for the T1 method. Phase angle and signal amplitude (regarding PRF/T1) was recorded in regions of interest, on images under fiberoptic probe tips. Sequences’ thermal coefficients were determined by calibrating phase angle and signal amplitude against temperature and subsequently validated. Results: Coefficients of -0.0089 ± 0.0003 ppm °C-1 (seg-EPI) and -0.917 ± 0.046, -1.166 ± 0.058, -1.038 ± 0.054 and -1.443 ± 0.118°C-1 (FLASH, IRTF, SRTF and TRUFI, respectively) were obtained. Precisions of 0.71, 1.34, 2.07, 2.44 and 3.21°C and, through Bland–Altman analysis, accuracies of -0.67, 0.79, 1.65, 1.57 and 2.13°C (seg-EPI, FLASH, IRTF, SRTF and TRUFI, respectively) were determined. Conclusion: The PRF method with seg-EPI sequence is preferred for thermometry during LITT owing to higher precision and accuracy. Among T1-method sequences, FLASH showed higher accuracy and robustness.

Multimodal cancer treatment: real time monitoring, optimization, and synergistic effects

The primary objective of this analysis is to provide the theoretical framework for a novel multimodal cancer treatment system emphasizing the use of ultrasound as a synergistic drug release mechanism, real time monitoring by MRI of hyperthermic, pO2, and ultrasound induced released effects. The aim is to provide a cure for the 20% of cancer victims who will die of complications from local solid tumors. Adjuvant therapy usually refers to surgery preceding or following chemotherapy and/or ionizing radiation treatment to decrease the risk of recurrence, but the absolute benefit for survival obtained with adjuvant therapy compared to control is only approximately 6%. Tumor hypoxia represents a primary therapeutic concern, besides multi-drug resistance (MDR), because it can reduce the effectiveness of drugs and radiotherapy; well-oxygenated cells require one-third the dose of hypoxic cells to achieve a given level of cell killing. The era of systemic and indiscriminate chemotherapeutic drug delivery into both healthy and pathologic tissues is near an end. Targeted drug delivery using nanoparticles is emerging as the new vehicle, either as a single treatment option, as part of adjuvant procedures or as a component of a multimodal cancer treatment system. There are more than 100 nanosized liposomes or particles, and conjugated anticancer agents in various stages of preclinical and clinical development. Active targeting can be achieved by site-specific delivery or site-specific triggering. Ultrasound can be utilized as both a site triggering and synergistic mechanism in drug release. The process can be monitored using MRI by a physical process called cavitation. An analysis of low frequency ultrasound exposure in combination with liposomally encapsulated doxorubicin (Caelyx) on Balb/c nude mice inoculated with a WiDr (human colon cancer) tumor cell line provided tumor growth inhibition of 30-40%. Mild hyperthermia causes mean intra-tumor pO2 to increase by 25% and enhances tumor radiosensitization. Hyperthermia causes the extravasation of liposome nanoparticles in deep tumor regions. Ionizing radiation improves the distribution and uptake of drugs. Liposomally encapsulated drugs and ultrasound mediated hyperthermia have been proven to circumvent MDR effects. Hyperthermic effects and pO2 monitoring of bodily fluid have been performed by MRI. It is hypothesized that increased vascularization and subsequent increase in pO2 levels to hypoxic regions, and monitoring of drug release through cavitation, can facilitate optimized real time concomitant or sequential treatments of drug therapy, hyperthermia, ionizing radiation, etc., before or after surgery. An improved therapeutic index with the use of the outlined system seems probable.

MR guided cancer treatment system for an elevated therapeutic index - a macroscopic approach

Adjuvant therapy for cancer usually refers to surgery followed by chemotherapy and/or radiation treatment to decrease the risk of recurrence. But still, the absolute benefit for survival obtained with adjuvant therapy compared with control is only approximately 6%. The objective of this analysis is to formulate a non-invasive multimodal cancer treatment system related to cancer stem cells and hypoxic fractions of solid tumors, emphasizing MRI monitoring and guidance, to elevate the therapeutic index. Tumor hypoxia is a therapeutic concern since it can reduce the effectiveness of drugs and radiotherapy, where well oxygenated cells requiring one third of the dose of hypoxic cells to achieve a given level of cell killing. Cancer stem cells might be the cause of tumor recurrence, sometimes many years after the appearance of the successful treatment of a primary tumor. Thus, the primary objective of such a treatment system will be to provide sufficient selective toxicity to both kill cancer stem cells and cells of hypoxic fractions of the tumor. Active tumor targeting with the use of liposomally encapsulated drugs is the starting point of the treatment procedure. The system facilitates quality assurance means by MR monitoring of drug accumulation and drug release, in real time. Cavitation involves the nucleation, growth and oscillation of gaseous cavities. Selective drug release and/or hyperthermia are achieved by ultrasound induced cavitation well defined to the tumor region. Hyperthermic effects, increased vascularization and subsequent increase in pO2 levels to hypoxic regions, can be monitored by MRI. MRI monitoring of key physiological parameters facilitates optimization related to approximate real time concomitant treatments, including correct timing and various combinations of drug therapy, hyperthermia, ionizing radiation, ablation, other treatment options, before or after surgery. The likelihood of an improved therapeutic index with the use of such a system seems compelling. Further research related to optimal timing, combinations of responses between liposomally encapsulated drug dosage, ultrasound exposure, hyperthermia, pO2 response time, ionizing radiation fractionation and treatment time, have to be conducted.

Treatment monitoring and thermometry for therapeutic focused ultrasound

Therapeutic ultrasound is currently enjoying increasingly widespread clinical use especially for the treatment of cancer of the prostate, liver, kidney, breast, pancreas and bone, as well as for the treatment of uterine fibroids. The optimum method of treatment delivery varies between anatomical sites, but in all cases monitoring of the treatment is crucial if extensive clinical acceptance is to be achieved. Monitoring not only provides the operating clinician with information relating to the effectiveness of treatment, but can also provide an early alert to the onset of adverse effects in normal tissue. This paper reviews invasive and non-invasive monitoring methods that have been applied to assess the extent of treatment during the delivery of therapeutic ultrasound in the laboratory and clinic (follow-up after treatment is not reviewed in detail). The monitoring of temperature and, importantly, the way in which this measurement can be used to estimate the delivered thermal dose, is dealt with as a separate special case. Already therapeutic ultrasound has reached a stage of development where it is possible to attempt real-time feedback during exposure in order to optimize each and every delivery of ultrasound energy. To date, data from MR imaging have shown better agreement with the size of regions of damage than those from diagnostic ultrasound, but novel ultrasonic techniques may redress this balance. Whilst MR currently offers the best method for non-invasive temperature measurement, the ultrasound techniques under development, which could potentially offer more rapid visualisation of results, are discussed.

Magnetic resonance temperature imaging for focused ultrasound surgery: a review

Magnetic resonance temperature imaging (MRTI) is an enabling technology that has recently demonstrated the potential to bring the emerging minimally invasive image-guided thermal therapy procedures, such as radiofrequency, microwave, laser, ultrasound, and cryosurgery, into the clinical setting with a level of safety and efficacy not previously possible. By coupling the wealth of soft tissue contrast mechanisms available with magnetic resonance imaging with its intrinsic temperature sensitivity, magnetic resonance imaging is in a unique position to provide image-guided treatment planning and verification and quantitative or qualitative feedback during treatment delivery, heightening of the control the physician has over the method, and enhancement of the ability to deliver conformal treatments. The basic principles behind MRTI technology and its application to minimally invasive thermal therapy during ultrasound thermal therapy delivery are reviewed in this study.