Development of clinical simultaneous SPECT/MRI

Abstract

There is increasing clinical use of combined positron emission tomography and MRI, but to date there has been no clinical system developed capable of simultaneous single-photon emission computed tomography (SPECT) and MRI. There has been development of preclinical systems, but there are several challenges faced by researchers who are developing a clinical prototype including the need for the system to be compact and stationary with MRI-compatible components. The limited work in this area is described with specific reference to the Integrated SPECT/MRI for Enhanced stratification in Radio-chemo Therapy (INSERT) project, which is at an advanced stage of developing a clinical prototype. Issues of SPECT/MRI compatibility are outlined and the clinical appeal of such a system is discussed, especially in the management of brain tumour treatment.

Figure 1.

A full preclinical ring populated with 10 gamma detection modules. The mechanical structure also supports the cooling distribution tubes and the power and optical communication lines. The overall diameter of the insert is 20 cm.

Figure 2.

The three-side-tilable silicon photomultiplier arrays composing the planar detector field of view in the preclinical case. The dead detection area of the single array has been minimized to increase the amount of luminous signal collected.

Figure 3.

INSERT gamma camera configured for preclinical SPECT. In the violet box, the 36-channel application-specific integrated circuit (ASIC) for signal readout and filtering is depicted. Digitized SPECT signals are transmitted through optical fibres. Temperature is stabilized at 0 °C by the cooling unit (an aluminium version of the unit is depicted). SiPM, silicon photomultiplier.

Figure 4.

Planar irradiation profile for a 5 × 5 cm field of view of the preclinical INSERT detector module. (a) A lead grid of holes (0.5 mm in diameter, 2 mm pitch) is employed to collimate the gamma rays. (b) Experimental result for ^99mTc: the event coordinates were reconstructed using a maximum likelihood method. FWHM, full width at half maximum.

Figure 5.

(a) Schematic diagram of the clinical system design with a partial ring of 20 detectors. The patient aperture of 33 cm accommodates the MRI receiver/transmitter head coil. (b) Schematic of complete SPECT insert in the MRI system.

Figure 6.

Multislit-slat collimator corresponding to three detector units. The collimator consists of slats in the axial direction and an array of short slits with their apertures internal to the collimator surface. The figure shows a central slit (a) for each of the three subsections plus slits that are shared across adjacent detectors (b).

Figure 7.

Magnetic field distortion inside a uniform phantom due to the presence of a collimator block (polyimide/tungsten, ρ = 11.0 g cm⁻³) tested for the clinical SPECT/MRI setup. The left image shows a uniform static magnetic field in the absence of the collimator block. For this setup, a magnetic field dispersion (Δf) of approximately 20 Hz was obtained across the slice. After placing the collimator block in close vicinity to the phantom lower right corner, the static magnetic field is significantly distorted (right) which manifests itself by field dispersion across the slice of Δf ≈ 120 Hz.

Figure 8.

An example of eddy current assessment using a reference free induction decay (FID) (black line) obtained for an agarose phantom and pulsed magnetic field gradients placed along the read, phase and slice direction. For comparison, the object under test [polyimide/tungsten sample (ρ = 11.0 g cm⁻³] was placed in the magnet (resembling its position in the SPECT insert) followed by the acquisition of a test FID (blue and red lines). For assessment of the eddy current time constants, the delay between the pulsed magnetic field gradient and the FID acquisition was varied between 0.3 and 300 ms.

Figure 9.

A data acquisition board mounted inside the electromagnetic compatibility shielding test box for the evaluation of the SPECT/MRI interference due to radiofrequency emission (left). Closed test box being fully shielded (right).

Figure 10.

(a) ^99mTc-labelled pentavalent dimercaptosuccinic acid [^99mTc-DMSA(V)] and gadolinium (Gd)-enhanced gradient-echo three-dimensional (3D) sequence MRI visualizes peripheral, more perfused regions of the tumour to express more transporter proteins of phosphate ions related to energy metabolism. Also, the superior nature of SPECT/MRI with very high resolution and high soft-tissue details/MRI-related functionality of the perfusion data readout is presented. (b) ¹²⁵I-deoxy-uridine and Gd-enhanced gradient-echo 3D sequence MRI visualizes central, less perfused regions of the tumour to express more DNA build-up (nucleoside incorporation). This image was taken synchronously with ^99mTc-DMSA(V) images using an energy window centred at 28 keV.