Block/System/Config/Form/Field/Checkbox.php
public function getValues()
    {
        $values = [];
        $ratios = $this->pixelRatios->toOptionArray();
        foreach ($ratios as $value) {
            $values[$value['value']] = $value['label'];
        }
        return $values;
    }
public function getValues()
    {
        $values = [];
        $ratios = $this->pixelRatios->toOptionArray();
        foreach ($ratios as $value) {
            $values[$value['value']] = $value['label'];
        }
        return $values;
    }
Get pixel ratio values

@return array